What Size Solar Battery Do I Need? A South Carolina Guide

The right solar battery size is not a standard number of batteries or a capacity guessed from square footage. It is the system that can support a defined set of loads, meet their continuous and starting power, deliver the usable energy required for a chosen period, and operate under a realistic solar-recharge scenario.

Those are four separate design gates. A battery can store enough energy but lack the power to start a well pump. Another can start the equipment but run out sooner than the homeowner expects. A large advertised capacity can still be a poor fit if the design does not back up the intended circuits, cannot recharge from solar while the grid is down, or is not approved for the proposed configuration.

Battery count should therefore come last. First build the load and outage plan. Then map it to the usable energy, power ratings, controls, installation limits, and expansion rules of an exact system.

The four answers that determine battery size

Use these questions in order:

1. What receives backup? Identify the electrical boundary and the individual circuits or devices that must remain available.
2. How much power is required at once? Add credible simultaneous running loads and check the hardest motor or compressor start.
3. How much usable energy is required? Estimate the load profile over the chosen outage period, not just an average electric bill.
4. How will energy be replenished? Test no-sun, poor-solar, and better-solar conditions, including the array and controls actually available during an outage.

The National Renewable Energy Laboratory identifies critical load and anticipated outage duration as core resilience-sizing inputs in its battery storage resilience guidance. It also notes that a critical-load profile can differ in magnitude, shape, and timing from the home’s ordinary load. That is why last month’s total consumption is useful evidence but not a complete backup design.

Understand the terms before comparing systems

Battery proposals often place several different measurements next to one another. Treating them as interchangeable leads to avoidable mistakes.

Term	What it answers	What it does not answer
Power, in kilowatts (kW)	How fast the system can supply electricity at a moment	How long it can keep supplying that power
Continuous power	The supported ongoing output under specified conditions	Whether it can start every motor or serve every circuit simultaneously
Peak or surge power	A brief output capability under the manufacturer’s stated conditions	Long-duration output or automatic compatibility with a particular compressor or pump
Energy, in kilowatt-hours (kWh)	How much electrical work can be delivered over time	Whether the inverter can supply a high instantaneous load
Nominal or nameplate energy	A manufacturer’s stated total capacity under its definition	The energy the homeowner can necessarily dispatch
Usable energy	The portion described as available under the manufacturer’s stated operating limits	The energy that will reach the loads after every control setting and system loss
State of charge	The battery’s reported stored-energy level at a particular time	A guaranteed runtime
Backup reserve	Energy intentionally held for an outage under a chosen operating mode	A universal protected minimum or proof that all reserved energy is available in every condition
Autonomy	How long a defined load profile can be supported under stated assumptions	A fixed property of the battery independent of loads, weather, and starting charge

NREL’s residential battery model treats power capacity and energy capacity as separate system characteristics. Current equipment data sheets do the same: usable or nominal energy, continuous output, brief peak output, charge power, and configuration limits appear as distinct fields. A proposal should state which field it uses and under what conditions.

Decide what job the battery must perform

Start with an operating goal, because daily energy shifting and outage resilience are not the same problem.

Outage backup

An outage design must specify which loads remain energized, how transfer or isolation occurs, what starts automatically, whether solar operates while the home is islanded, and how the battery behaves as its reserve is reached. The U.S. Department of Energy explains that solar panels alone generally do not provide power during a grid outage. Properly configured inverters, storage, isolation, and controls are required for independent operation.

Daily solar shifting or rate management

A grid-connected operating plan may store midday solar for later use, maintain a reserve, or respond to a utility or rate program. Its useful capacity depends on the applicable tariff, allowed charging and export behavior, control settings, household load, and the owner’s resilience priority. A system optimized to cycle daily may use a different reserve policy from one held primarily for storms.

Both jobs

Many homeowners want both. That creates a tradeoff: energy used before an outage is not stored for the outage unless the control system restores the reserve in time. The proposal should explain how the operating mode changes, how reserve is set, whether storm preparation features are available, and what happens if an outage begins before the battery reaches the intended state of charge.

Do not let a bill-management estimate stand in for a backup-load study. Likewise, do not assume that a battery reserved for outages will deliver the same everyday economic behavior as one cycled for daily use.

Sunburst’s battery storage service covers new solar-plus-storage projects and storage additions, but the exact backup boundary and supported loads still require a site-specific design.

Whole-home backup is an electrical boundary, not a runtime promise

“Whole-home backup” can mean that the backup system is connected at a point capable of energizing most or all of the home’s circuits. It does not prove that every electric load can run at the same time. It also does not state how long those loads can operate.

A whole-home design still needs:

• a credible simultaneous running-load calculation;
• a starting or surge review for motors and compressors;
• an energy profile over the desired outage window;
• an explicit load-shedding or control strategy;
• a solar-recharge model; and
• a written description of what happens at the system’s power, temperature, state-of-charge, or control limits.

Partial-home or critical-load backup can be more predictable. A dedicated backed-up-loads panel or approved load-control arrangement can prioritize refrigeration, selected lights and receptacles, communications, a suitable water source, and other chosen needs while excluding loads that would dominate the power or energy budget. The correct list is personal; a marketing label cannot choose it.

Do not infer the answer from service size. The rating on a service panel defines an electrical limit, not the home’s actual coincident demand during an outage. Do not infer it from floor area either. Two similarly sized homes can have very different HVAC, water heating, cooking, well, pool, vehicle-charging, and medical-device needs.

Build a critical-load inventory from evidence

Create a row for every circuit or device that might operate during an outage. The purpose is not to add every appliance in the home. It is to describe an intentional operating plan.

Inventory field	What to record
Circuit or device	The exact load and the circuit that serves it
Priority	Must remain available, preferred if energy allows, or deliberately shed
Supply	Voltage, phase, and whether dedicated equipment is involved, as documented by a qualified reviewer
Running demand	Measured or documented watts/kW and the evidence source
Starting demand	Motor/compressor starting requirement, duration, and evidence source where relevant
Operating pattern	Continuous, cycling, scheduled, manually controlled, or event-driven
Required time	Expected hours of use within each outage period
Energy	Running power multiplied by operating time, adjusted for a defensible duty cycle where appropriate
Control rule	Which other loads must be off before this one starts or runs
Consequence of loss	Comfort, food, water, communications, property protection, business interruption, or life safety

Evidence can come from utility interval data, existing solar or whole-home monitoring, compatible plug-load meters used according to their instructions, equipment documentation, and measurements by a qualified professional. Bills can establish total consumption over billing periods. They usually cannot reveal which motors start together or how a specific outage load cycles.

Do not open a service panel, inverter, battery enclosure, disconnect, HVAC cabinet, pump control, or hard-wired equipment to gather data. Do not attempt energized measurements. High-voltage, hard-wired, HVAC, well, and service loads belong in a qualified electrical and equipment review. A visible nameplate can help identify a model, but a nameplate value is not automatically the actual running demand or the battery system’s supported starting case.

Use an evidence hierarchy

When values conflict, record the source rather than silently choosing the smallest number:

1. a qualified measurement representative of the intended operating condition;
2. reliable interval or submeter data covering the relevant load and season;
3. current manufacturer documentation for the exact equipment and configuration;
4. the original electrical or mechanical design records; and
5. a clearly labeled conservative assumption to be verified before final design.

This hierarchy prevents an online appliance table from overriding what the actual home does. It also shows the installer which inputs still require verification.

Pass the power gate: running demand and starting events

Continuous power begins with the loads that may operate at the same time, not the sum of every circuit rating and not a monthly average.

Express the operating case as:

required continuous kW = credible simultaneous running loads + applicable system overhead and design margin

The margin should come from the actual design method and equipment requirements, not a universal percentage copied from a generic article. Load controls can reduce the credible simultaneous total by preventing selected devices from operating together. The proposal should name the controlled loads and explain the priority logic.

Then test the most demanding credible starting event:

starting case = running loads already online + verified start requirement of the next motor/compressor + other coincident loads that cannot be shed

Air-conditioning compressors, heat pumps, well pumps, refrigeration equipment, and other motors can draw differently while starting than while running. The relevant question is not merely whether a battery advertises “surge power.” Confirm that the exact inverter, battery configuration, transfer/control equipment, conductors, and load-control design support the equipment’s start magnitude and duration under the manufacturer’s conditions.

Peak ratings may apply only for a stated interval, state of charge, temperature range, voltage, or unit configuration. Two peak values with different test conditions are not directly interchangeable. If a proposal says a battery “can run the AC,” require the HVAC model, verified start data, already-running loads, control sequence, and supported battery configuration behind that statement.

Why load management can be better than simply adding storage

Adding stored energy does not always fix a power problem. If several large loads start together, more kWh may leave the limiting kW unchanged unless the system configuration also increases supported output. Conversely, a load controller that prevents a nonessential heater or vehicle charger from operating during a compressor start can reduce the power requirement without changing the energy capacity.

The homeowner should know which loads are automatically shed, which require manual discipline, how priorities can be changed, and what happens if communications or a controller fails. An undocumented assumption that “you simply will not use those together” is weak design evidence.

Pass the energy gate: calculate kWh over time

Once the power gate is feasible, estimate the energy required during the chosen outage window.

For each load:

load energy in kWh = running kW × operating hours within the scenario

For a cycling device, operating hours should reflect a defensible duty profile for the relevant season and condition. For a scheduled load, state when it will run. Add the individual load-energy values for the scenario, then account for system delivery and control assumptions using the exact design method.

Avoid a single flat average if the timing matters. An overnight outage, a hot afternoon outage, and a cold-weather morning can present different power and energy profiles even when they last the same number of hours. Water use, food storage, communications, HVAC behavior, occupancy, and work-from-home needs may also change across the event.

Start from the intended battery state, not a full-battery fantasy

A runtime estimate should state the starting condition. Ask:

• Is the battery assumed to begin full, at the configured backup reserve, or at a modeled everyday state of charge?
• Does the system have an approved way to charge from the grid, solar, or both under the applicable operating mode?
• Can a forecast or storm mode raise the reserve, and does it require connectivity or owner action?
• What happens if the outage begins during a daily discharge period?
• What energy does the control system retain for equipment protection or restart behavior?

A full-battery calculation can be a useful upper scenario, but it should not be presented as the only expected condition.

Clarify “usable” before doing the math

Ask the bidder to define each capacity term from the current data sheet and control documentation:

• nominal or nameplate energy;
• manufacturer-reported usable energy;
• energy available at the proposed configuration and operating settings;
• any protected minimum state of charge;
• user-selected backup reserve;
• conversion and standby losses included in the model; and
• energy expected to reach the backed-up loads.

Do not automatically subtract a user reserve from “usable energy” and then subtract the same protected amount again. Definitions differ. Some reserve settings hold otherwise usable energy for an outage; some minimum levels protect equipment or support control functions. The written sizing model should show exactly what is counted once, what is unavailable, and what may become available only when the grid fails.

Test solar recharge instead of assuming it

Solar recharge can extend an outage, but “the sun comes out tomorrow” is not a design calculation. The array must be capable of operating in the islanded configuration, and its production must exceed the loads for enough time to replenish the battery.

The South Carolina Energy Office’s battery-backup guidance identifies an inverter, charge control, emergency-load wiring, disconnect/islanding protection, and an interconnection agreement as parts of a solar backup arrangement. The exact system determines whether solar can restart, how it is controlled when production exceeds load and available storage, and what happens when the battery reaches a low state.

Test at least three cases:

No-sun baseline

Assume no helpful solar input over the initial period. This reveals what the stored energy alone can support and prevents the design from depending on favorable weather during the most critical hours.

Poor-solar recovery

Use a defensible low-production condition for the site’s relevant season and hazards. Consider cloud cover, shorter seasonal days, shade, high household load, and the possibility that a storm leaves debris, damage, protective shutdowns, or communications problems. Do not assume the array remains fully available after the event that caused the outage.

Better-solar recovery

Model a more favorable day while respecting the actual islanded power and control limits. This case can show how loads might be scheduled when solar is abundant, but it should not replace the conservative scenarios.

For each case, track the daily balance:

change in stored energy = solar energy accepted by the system − load energy served − modeled system losses

If the balance remains negative, the battery continues to deplete even though solar produces during the day. If it is positive, confirm that the battery can accept the available charge while supporting the loads and that the array is not limited by its islanded operating controls.

For complex or high-consequence needs, scenario software can test different outage start times, durations, and load shapes. NREL’s REopt Lite documentation describes analysis using location, consumption, outage duration, and technology inputs, while NREL’s resilience work in the System Advisor Model evaluates full or critical load across outage cases. A model is decision evidence, not a guarantee of weather, equipment condition, or uninterrupted service.

Treat medical and life-safety loads differently

Do not use a residential battery runtime estimate as the sole plan for powered medical equipment or another load whose interruption creates immediate danger.

The FDA advises people who depend on powered medical devices to review the device instructions, ask the supplier or manufacturer about compatible backup power, notify appropriate utility or emergency contacts where applicable, and maintain a disaster plan. The CDC likewise recommends identifying backup power sources for medical equipment.

For each high-consequence device, document:

• the exact model and manufacturer instructions;
• whether power quality, grounding, transfer time, or an uninterruptible source matters;
• its internal-battery duration and charging requirements, if any;
• the clinical or supplier guidance for an outage;
• a second independent power or relocation option; and
• the point at which the household will leave or seek emergency assistance.

A home battery can be part of the plan. It should not be described as guaranteed life-safety power.

Decide whether to shed load, add energy, or change the backup strategy

If the first model is not feasible, separate the reason before changing equipment.

Failed test	More useful next question
Continuous demand is too high	Which loads can be excluded, scheduled, or automatically controlled? Does the proposed configuration increase supported continuous output?
A motor cannot start	Can a qualified professional verify the start event and approved mitigation? Can other loads be shed during the start?
Stored energy is insufficient	Can the outage window or operating hours be revised? Is more compatible storage practical?
Poor-solar case does not recover	Can loads be reduced between solar windows, or should another energy source be evaluated?
Whole-home boundary creates ambiguity	Would a defined critical-load panel or controlled circuit plan produce a more reliable outcome?
Long-duration, high-load need dominates	Should a generator or coordinated hybrid design be assessed instead of forcing a battery-only promise?

Storage is valuable because it is quiet at the point of use, responds automatically when properly configured, and can work with solar without relying solely on delivered fuel. A generator can serve a different long-duration or high-load role but introduces fuel, exhaust, noise, siting, maintenance, and operating considerations. If the requested load profile remains impractical after prioritization, review the backup generator service rather than accepting an unsupported battery runtime claim.

Plan for expansion without assuming it will be available

“Expandable later” should be verified, not treated as a permanent feature. Battery products, control hardware, firmware, listings, and approved combinations can change. The original design can also consume the available panel, inverter, conductor, siting, or interconnection capacity.

Ask the designer to document:

• the current minimum and maximum supported battery configuration;
• whether added units must match a product generation, age, chemistry, firmware, or controller;
• whether expansion changes continuous or peak power, usable energy, or both;
• available electrical-panel, inverter, gateway, conductor, and breaker capacity;
• remaining compliant installation space and environmental constraints;
• monitoring and communications limits;
• warranty and service responsibility when units are added later; and
• whether another permit, inspection, utility review, or interconnection amendment would be required.

If the home already has solar, expansion also depends on ownership, inverter compatibility, equipment condition, and the approved outage architecture. The existing-solar battery retrofit guide covers those questions. Sizing should not proceed as though compatibility has already been established.

South Carolina code and utility rules can change the feasible design

A technically appealing capacity is not the final system size if the equipment combination, location, electrical design, or operating mode cannot be approved.

The South Carolina Building Codes Council’s current adoption page identifies the state codes in effect and publishes South Carolina modifications. The authority having jurisdiction applies the current requirements to the actual project. Battery location, equipment listing, protection, clearances, disconnects, labeling, structural support, environmental exposure, and aggregate configuration can affect the permitted design. Do not rely on a universal placement rule or permit timeline from an online article.

System certification also matters. UL’s residential energy-storage safety resources distinguish complete-system certification from test methods used to evaluate thermal-runaway behavior. Ask for the listing and approved combination of the complete proposed system, not just a cell or module certification.

The serving utility may require a new or revised interconnection review, single-line diagram, equipment specifications, protection information, export settings, or operating agreement. Dominion Energy South Carolina and Santee Cooper, for example, publish separate technical and generator-interconnection resources. Their procedures are examples of why the actual provider matters, not statewide substitutes for one another.

Use the provider on a current bill, then consult the South Carolina utility directory and the utility’s current storage and interconnection materials. Do not infer the utility from the city. A municipal utility, cooperative, or neighboring investor-owned utility may serve a property that has the same mailing city as another provider’s customer.

What a battery-sizing package should show in writing

Before product count and price drive the conversation, request a sizing schedule that another qualified reviewer could follow. It should include:

1. Operating goal: outage backup, daily shifting, or both.
2. Backup boundary: the backed-up panel, circuits, devices, and explicitly excluded loads.
3. Evidence: bills, interval data, monitoring exports, equipment documents, measurements, and labeled assumptions.
4. Continuous-power case: which loads operate together and what controls prevent other combinations.
5. Starting-power case: the relevant motor or compressor, start requirement, duration, already-running loads, and approved equipment configuration.
6. Energy model: each load’s running demand, duty or schedule, hours, and kWh for each outage scenario.
7. Capacity definitions: nominal, manufacturer-reported usable, protected minimum, user reserve, and modeled delivered energy.
8. Starting state: the state of charge assumed when the grid fails and how daily operating mode affects it.
9. Solar recharge: whether the exact design supports outage recharge and the no-sun, poor-solar, and better-solar cases used.
10. Load management: automatic and manual controls, priority order, failure behavior, and homeowner responsibilities.
11. Outage behavior: transfer, restart, low-state response, recovery after depletion, and owner notification.
12. Expansion: approved future configurations and the electrical, physical, warranty, and approval constraints.
13. Safety and approval: proposed equipment listing, location, AHJ path, utility path, and required commissioning tests.
14. Owner verification: a demonstration plan showing which circuits transfer and how monitoring, reserve, and load control work.

This is the input to a good proposal, not a replacement for a site-specific electrical design. A later quote comparison can evaluate equipment, installation scope, price, warranty, and contract responsibility. The sizing package establishes whether all bidders are solving the same load and outage problem.

If you have the utility records, critical-load list, high-demand equipment details, and outage goal ready, Sunburst can evaluate the property and proposed backup boundary. Explore a battery assessment for your home to turn those inputs into a site-specific design.

A homeowner worksheet before the assessment

Bring the following information without opening or testing electrical equipment:

• current utility bills and any downloadable interval-use data;
• solar and energy-monitoring exports, if available;
• the original solar proposal, drawings, permission-to-operate record, and equipment list;
• visible model information and owner manuals for priority equipment;
• a circuit list marked must-have, preferred, and excluded;
• the desired initial outage window and a longer stress-test window;
• seasonal operating needs, including heating, cooling, water, and work requirements;
• loads that can be scheduled for midday solar or turned off automatically;
• medical-device and emergency plans developed with the relevant provider;
• expected future loads, such as HVAC changes, a well, an electric vehicle, or an addition; and
• the exact serving utility and any current rate or distributed-energy program documents.

Leave blanks where evidence is missing. A blank marked “field verification required” is safer than a confident number borrowed from a different home.

How Sunburst turns a load study into a system

The gates above are exactly the sequence a Sunburst assessment follows. We start from your utility usage history and the list of circuits you care about during an outage, measure or verify the starting behavior of the loads that actually control the answer, and only then propose usable energy and inverter output. Where the honest answer is “this battery cannot carry central air for two days,” we say that in the proposal instead of implying it away — and we price the alternatives, including a standby generator for long storm outages.

South Carolina specifics matter here. Recharge assumptions depend on your array and your utility’s export rules, summer humidity keeps HVAC duty cycles high, and coastal restoration after a hurricane can run for days rather than hours. Our battery storage service covers the design, the permits, the interconnection and the commissioning that proves the sizing was real, all under our lifetime full-system warranty.

Next steps that pair with this guide: choose the backup boundary, decide where the battery goes, and compare quotes on one scope. When you want the numbers run for your address, book a free assessment.

Frequently asked battery-sizing questions

Can I size a battery from my monthly electric bill?

Not by itself. A bill helps establish total energy use over a billing period, but backup sizing needs the loads that will remain active, their timing, simultaneous power, motor starts, desired outage duration, starting state of charge, and solar-recharge scenario. Interval data can add useful load-shape evidence, but it still must be mapped to the proposed backup boundary.

Should battery size match the solar array size?

There is no one-to-one rule. The array’s production profile affects recharge, while the battery’s power and usable energy must fit the selected loads and outage plan. Array size, inverter limits, islanded solar controls, seasonal weather, export settings, and battery charge acceptance all affect the relationship. If the array itself is being redesigned, use a documented solar system sizing process alongside the storage analysis.

How many batteries do I need for whole-home backup?

That cannot be answered responsibly from the phrase “whole home.” Define the backed-up electrical boundary, credible simultaneous demand, hardest supported start, energy across the desired window, load controls, starting charge, and recharge conditions. Only then can the designer map the requirement to the power, usable energy, and approved configuration of a specific system.

Can a battery run central air conditioning or a well pump?

Possibly, but the equipment name and battery capacity alone do not establish it. A qualified design must verify the running load, starting event and duration, other loads already online, voltage and control requirements, proposed inverter/battery configuration, and energy available after the equipment starts. A claim should be tied to those inputs in writing.

How long will a solar battery last during an outage?

Runtime is the usable delivered energy divided across the actual load profile, modified by starting state, reserve behavior, system losses, equipment conditions, and any solar energy the islanded system can accept. Because those inputs change, a single universal runtime is misleading. Request multiple documented scenarios rather than a best-case promise.

Does solar make battery backup unlimited?

No. The array must remain intact and able to operate while islanded; weather, shade, season, controls, load, and battery charge acceptance determine whether daily energy is replenished. A long outage can deplete storage even with daytime solar if the loads consume more than the system can harvest and store.

Is usable kWh the same as advertised kWh?

Not necessarily. Ask whether the advertised value is nominal or usable, what protected minimum is already excluded, how a user reserve works during an outage, and what delivery losses the sizing model includes. Use definitions from the exact current system documentation.

Is partial-home backup undersizing the system?

No. It can be a deliberate resilience design that preserves priority loads and makes power and energy behavior clearer. It is undersized only if it fails the documented goal. A vague whole-home promise can be less useful than a well-defined critical-load system with tested controls.

Should I buy extra capacity for future needs?

Model the actual future load rather than adding a generic percentage. Then verify whether today’s equipment can be expanded later, what power and energy change, whether compatible hardware is expected to be available, and what electrical, siting, code, utility, warranty, and permitting limits apply.

What is the most important number on a battery quote?

There is no single one. At minimum, compare usable delivered energy, continuous output, starting/peak behavior under stated conditions, backed-up circuits, control logic, solar-recharge behavior, reserve assumptions, and the approved equipment configuration. The correct result is a documented system, not an isolated kWh figure.

Sources and methodology

This guide separates power, energy, critical load, and outage duration using current resources from the U.S. Department of Energy, National Renewable Energy Laboratory, South Carolina Energy Office, South Carolina Building Codes Council, UL Solutions, current utility interconnection materials, and federal medical-device preparedness guidance. Manufacturer technical documentation was reviewed only to confirm that capacity, continuous output, peak output, reserve behavior, and configuration limits are product-specific—not to recommend a brand or publish a generic count.

All runtime, power, recharge, expansion, code, and approval outcomes depend on the specific home, loads, equipment, configuration, weather scenario, authority having jurisdiction, and serving utility. The formulas here organize evidence; they do not replace electrical engineering, manufacturer instructions, permitting, interconnection review, commissioning, or an independent emergency plan.