A time-of-use rate is an electricity rate that assigns different prices to scheduled periods. The applicable calendar and account rules define when each price applies. TOU is the common acronym, but the label alone does not establish your hours, prices or eligibility.
For a homeowner reviewing solar financing and savings assumptions, the useful question is how the actual household load and solar production line up with those periods. An annual electricity total cannot reveal that timing relationship.
This glossary explains the definition and arithmetic. The existing South Carolina net-metering guide retains utility-by-utility buying advice. Unbranded numerical examples below are original hypothetical assumptions, not current tariffs, equipment results or Sunburst household measurements.
What does a time-of-use rate change?
It changes the price applied to electricity according to the scheduled period in which the relevant use occurs. The account’s total quantity still matters, but its distribution across periods matters too.
DOE’s Alternative Fuels Data Center glossary defines TOU as an electricity schedule adjusting price according to when electricity is used. That definition does not prescribe one nationwide set of peak hours.
“On-peak,” “off-peak,” “super off-peak” and “shoulder” are period labels a schedule may use. Obtain their actual definitions and prices. Do not import a period label from another utility or assume that all daylight hours have the same value.
| Information | What it tells you |
|---|---|
| Period name | Which scheduled category applies |
| Calendar | When that category begins and ends |
| Energy price | Value applied to the specified chargeable kWh |
| Account conditions | Which customers and uses the schedule covers |
| Other bill provisions | Charges or adjustments beyond this energy calculation |
The rate schedule should connect those items clearly. If a proposal lists “peak price” without the applicable season and hours, it is missing part of the definition needed to assess the estimate.
A lower-priced period can make flexible timing useful under the right conditions. It is not permission to add unnecessary use, a promise of lower total bills or evidence that a particular solar design is economical.
Is TOU the same as dynamic pricing or a demand charge?
No. These concepts concern different parts of electricity pricing. A TOU schedule uses specified time periods. Dynamic pricing can reflect changing market conditions, while a demand charge uses a defined power measurement and billing rule.
DOE’s utility-rate evaluation guidance distinguishes rates changing by set amounts at set times from rates reflecting hour-to-hour wholesale-market movement. It also separates energy, demand and fixed charges. Its site-evaluation framework supplies vocabulary here, not a residential program eligibility determination.
| Concept | Main quantity or rule | Question to ask |
|---|---|---|
| TOU energy pricing | Chargeable kWh assigned to scheduled periods | Which hours and price apply? |
| Dynamic pricing | Price changes under the arrangement’s market-linked rule | How are prices determined and communicated? |
| Demand charge | Billed power measurement, commonly expressed in kW | Which measurement window and peak count? |
| Fixed charge | Account provision independent of the energy subtotal | What amount applies and when? |
Our kilowatt and kilowatt-hour definitions explain why power and energy are different quantities. Reducing an expensive-period energy total does not by itself establish a lower billed power peak.
An account can combine more than one pricing concept. Identify each relevant component rather than treating the presence of TOU as proof that only energy quantities matter.
Which calendar details must be verified?
Verify the applicable months, daily hours and day types, together with the effective date. A price calendar can vary by season and can change when a new tariff takes effect.
Record whether weekends follow weekday periods and how designated holidays are treated. Do not assume Saturday is inexpensive because that was true under another schedule or an older version of the same utility’s program.
The calendar should also make clear how time boundaries are interpreted in the relevant records. When comparing interval data or setting supported automation, confirm the time convention with the source and equipment documentation. A billing schedule and an app clock should not be assumed to match without checking.
| Calendar attribute | Record to retain |
|---|---|
| Utility and schedule | Full account identifier and applicable rate name |
| Effective version | Start date of the controlling document |
| Season | Months assigned to each calendar |
| Day type | Weekday, weekend and holiday treatment |
| Boundaries | Beginning and ending time of each period |
| Record convention | Time zone and interval labeling used by the data source |
Keep this record alongside the forecast. A seasonal graph is difficult to interpret when the analyst has not stated which calendar was used.
If the rate changes during a historical period, identify the versions used for the comparison. Do not apply a current schedule retrospectively and describe the result as the actual historical bill unless that treatment is explicitly part of the analysis.
Worked example: the same energy total at different times
Assume an original fictional two-period rate: $0.30 per chargeable kWh in the higher-priced period and $0.10 per chargeable kWh in the lower-priced period. Assume 20 kWh of chargeable electricity and no other charges or credits in this teaching example.
In the first scenario, 5 kWh falls in the higher-priced period and 15 kWh in the lower-priced period. The energy cost is 5 × $0.30 + 15 × $0.10 = $3.
Reverse the distribution in the second scenario: 15 kWh at the higher price and 5 kWh at the lower price. The result is 15 × $0.30 + 5 × $0.10 = $5.
| Fictional scenario | Higher-priced energy | Lower-priced energy | Energy subtotal |
|---|---|---|---|
| Mostly lower-priced use | 5 kWh | 15 kWh | $3 |
| Mostly higher-priced use | 15 kWh | 5 kWh | $5 |
Both scenarios use 20 kWh. Their energy subtotals differ because the distribution changes. These are prescribed examples, not representative South Carolina consumption patterns.
The energy-weighted average prices are $3 ÷ 20 = $0.15/kWh and $5 ÷ 20 = $0.25/kWh. Averaging the two scheduled prices equally gives $0.20/kWh, which matches neither scenario.
This is why a household’s average TOU energy price needs its energy quantities as well as its period prices. A simple average of tariff numbers cannot replace the timing analysis.
How can TOU be compared with a flat rate?
Compare the applicable quantities and complete account rules under both arrangements. One appealing off-peak price cannot establish which arrangement costs less for the household.
For an original simplified comparison, use the fictional $0.30/$0.10 TOU prices above and an assumed flat energy price of $0.18/kWh. Assume the same 20 kWh and exclude all other provisions solely to isolate the energy calculation.
Let the fraction of energy in the higher-priced period be p. The weighted TOU price is $0.30p + $0.10(1 − p), or $0.10 + $0.20p. It equals the fictional flat price when p = 40%.
| Assumed higher-priced share | Weighted TOU energy price | Comparison with fictional $0.18 flat price |
|---|---|---|
| 25% | $0.15/kWh | Lower in this limited calculation |
| 40% | $0.18/kWh | Equal in this limited calculation |
| 75% | $0.25/kWh | Higher in this limited calculation |
The 40% threshold is not a general TOU recommendation. It belongs to these invented prices and exclusions. Different prices, seasons, charges or eligible quantities change the comparison.
For a real rate-switching decision, ask which other account conditions change and whether the household qualifies. A calculation using unchanged charges should not be presented as complete if the proposed switch changes them.
What does shifting an existing load accomplish?
It can move chargeable energy between price periods while preserving the intended household task. The financial effect depends on actual prices, quantities and any change in energy use or other charges.
For an original example using the fictional two-period prices, assume an existing 4 kWh task moves entirely from the higher-priced period to the lower-priced period. Assume the same defined energy use before and after.
The earlier energy cost is 4 × $0.30 = $1.20. The later cost is 4 × $0.10 = $0.40. The difference is $0.80, before any other change or cost.
That arithmetic does not establish whether the appliance can complete its task in the new window, whether the owner can maintain the routine or whether the equipment’s energy use remains unchanged. Those conditions belong in the scenario.
| Routine question | Why it matters |
|---|---|
| Is the load flexible? | Some household needs occur at a required time |
| Is the equipment available? | A vehicle away from home cannot charge there in that window |
| Can the task finish? | Available time and supported power affect completion |
| Does energy use change? | A different operating method can alter the quantity |
| Is automation supported? | A modeled routine should match actual control capability |
The existing solar panels for EV charging guide retains that buying decision. Use TOU as one input rather than assuming every vehicle or appliance can be moved to the cheapest period without consequences.
Does solar production coincide with the expensive period?
Not automatically. Solar production follows the site’s conditions and system, while price periods follow the tariff calendar. Compare the production and load profiles with that calendar instead of assigning the highest price to every generated unit.
For an original illustration, assume 2 kWh of solar directly replaces otherwise chargeable household purchases in a period priced at the fictional $0.30/kWh. Its simple avoided energy value is $0.60. The same assumed direct contribution in a $0.10/kWh period avoids $0.20.
These are conditional purchase-avoidance comparisons, not total generation values. Solar that leaves as export needs the separate compensation treatment. Our solar export-rate definition explains why a purchase price is not automatically the value paid for exported electricity.
Also distinguish ordinary household load from utility imports. If solar serves a task directly, the task still uses electricity even though the utility supplies less of it. The solar self-consumption definition owns that physical relationship.
A forecast should therefore show generation, household use, imports and exports by the relevant periods. A yearly production total does not establish how much electricity avoids expensive purchases or enters a particular credit category.
Ask what happens under a different realistic routine. An afternoon task, a morning heating requirement or an evening vehicle arrival can change the overlap without changing the roof’s equipment rating.
How can storage move energy between price periods?
Storage can connect an earlier energy source with later load delivery under a supported configuration. The quantities at the input and load-delivery boundaries can differ, and the account rules determine what operations are allowed.
For an original fictional grid-charging comparison, assume 10 kWh of additional chargeable input at $0.10/kWh. Assume the specified storage pathway later delivers 9 kWh to loads that otherwise buy electricity at $0.30/kWh.
The prescribed input costs 10 × $0.10 = $1. The prescribed delivered energy avoids 9 × $0.30 = $2.70 of purchases. Their difference is $1.70, before additional costs, other account provisions or operating constraints.
| Fictional comparison | Amount |
|---|---|
| Additional input energy | 10 kWh |
| Input energy cost | $1 |
| Delivered energy | 9 kWh |
| Avoided purchase value | $2.70 |
| Difference before other costs | $1.70 |
The 9 kWh is an assumed delivered quantity, not a product efficiency claim. This example does not establish grid-charging permission, usable capacity, warranty treatment, degradation, equipment compatibility or investment payback.
If the input is solar that otherwise earns export compensation, forgone export value replaces the simple grid-purchase input assumption. Do not add both costs for the same energy pathway unless the actual scenario establishes them.
The existing battery quote comparison guide develops the equipment and investment decision. Our battery reserve definition explains why an ordinary-use allocation also needs to be reconciled with any stated backup objective.
Which bill provisions can change the result?
A TOU energy subtotal is one part of the account calculation. Other charges, adjustments, credits or minimum provisions can affect the total. Identify them before calling an energy-price difference a final saving.
For a deliberately fictional example, assume an energy reduction of $6 in a period but an added account charge of $8 compared with the alternative arrangement. The complete simplified difference is a $2 increase, not a $6 reduction.
The added charge is invented to illustrate the calculation. It is not a claim about a utility’s TOU program. A proposal should use the actual applicable provisions and explain which change between scenarios.
Keep export and netting rules separate too. Our net-billing definition describes how imports and eligible exports become charges or credits. A TOU purchase schedule does not, by itself, establish that all exchanges offset across every period.
For any demand provision, ask how its power measurement interacts with the schedule. Moving several tasks into the same cheaper energy window can require a separate demand assessment where the account has that charge. Do not infer the result from kWh alone.
Compare full bills under realistic operating scenarios, with each included item named. A graph of off-peak energy prices should not hide the other assumptions needed for the financial conclusion.
A dated South Carolina example: Dominion Rate 5
Dominion Energy South Carolina’s current Rate 5 tariff, checked October 3, 2026, states an effective date for bills rendered on and after the first July 2026 billing cycle. Its calendar illustrates why seasonal and day-type details matter.
| Rate 5 calendar item | Published period |
|---|---|
| Summer months | May–September |
| Summer on-peak | 4 p.m.–8 p.m. |
| Winter months | October–April |
| Winter on-peak | 6 a.m.–9 a.m. |
| Super off-peak year round | 1 a.m.–5 a.m. |
| Additional winter super off-peak | Noon–3 p.m. |
| Off-peak | Hours not designated on-peak or super off-peak |
Dominion’s official TOU update states that from July 1, 2026, weekends and holidays follow the same treatment as weekdays. Older weekday-only assumptions should not be applied to this updated schedule.
This is a dated provider-specific calendar example, not a statewide schedule or an eligibility determination. No numeric current energy price is quoted here. Use the controlling account documents for a financial calculation, including any solar rider and applicable adjustments.
The utility information hub is a starting point for identifying the relevant account discussion. An address, application cohort and account schedule must be verified rather than guessed from the city.
How should an app or timer be checked against the schedule?
Confirm that the intended routine maps to the current tariff calendar and the device’s supported settings. A label such as “cheap charging” does not establish that the underlying hours still match the account.
Record the selected days, start and end times, season changes and any supported automation behavior. Ask how the device handles time changes and whether the setting follows an updated tariff automatically or needs an owner action.
A battery’s operating mode can also pursue a different goal from the timer’s label. Verify whether the supported configuration prioritizes a reserve, a cost objective or another stated behavior and what conditions limit that operation.
For troubleshooting, keep the setting, its timestamp and the relevant consumption record together. Compare behavior during the intended interval, rather than assuming a completed task means it used only the cheaper period.
| Check | Evidence to retain |
|---|---|
| Current tariff calendar | Dated controlling schedule |
| Device schedule | Saved setting and supported instructions |
| Actual operation | Relevant energy or operating record |
| Seasonal transition | Responsibility for any required change |
| Exception | Documented reason the intended routine was not followed |
This is a review checklist, not a recommendation to override protection, comfort requirements or manufacturer controls. The operating routine should remain supported and appropriate to the household’s actual task.
Which records make a TOU proposal reviewable?
Ask for a period-by-period energy calculation with its calendar and data sources attached. It should identify whether the quantities are measured history, modeled future use or assumptions about changed routines.
Use compatible dates, time conventions and measurement boundaries. A production profile shifted against the wrong load hours can make the estimated coincidence misleading. Missing intervals should remain visible instead of being silently interpreted as zero consumption or generation.
Request at least a realistic current-routine case and, where relevant, the proposed changed-routine case. Keep generation and other inputs consistent when isolating the effect of a schedule change. A different household scenario should be identified as such.
For a residential solar installation quote, the calculation should separate direct solar use, imports and eligible exports. For a battery storage installation quote, it should also disclose source, delivered energy, losses, controls and additional costs.
Bring your electricity bill, applicable schedule, available interval records and usual appliance or charging routine to a free solar assessment. The next step is to connect time-of-use prices to realistic household use and the proposed system, rather than rank options by one attractive off-peak number.
Frequently asked questions about time-of-use rates
Are weekends always off-peak?
No universal weekend rule applies. Check the current schedule’s day-type treatment. Dominion’s dated July 2026 South Carolina update specifically treats weekends and holidays like weekdays for the affected periods; another utility or older schedule can differ.
Does a time-of-use rate change how much energy I use?
The schedule changes how eligible energy is priced. Household behavior or controls may change the timing or quantity, but that is a separate scenario. The worked examples preserve total energy to isolate the effect of moving use between prices.
Can solar generation be valued entirely at the peak purchase price?
Not without evidence. Establish when generation serves otherwise chargeable loads and apply the relevant price. Exported energy needs its own credit treatment. A yearly production total does not show that every generated unit avoids a peak purchase.
Is the average of on-peak and off-peak prices my average price?
Not necessarily. Weight the prices by the applicable energy quantities. In the hypothetical example, the same two prices produce $0.15 or $0.25 per kWh depending on timing. An equally weighted average misses that difference.
Does cheap overnight electricity make a battery profitable?
No automatic investment conclusion follows. Evaluate permitted charging, delivered energy after the stated losses, purchases avoided, additional costs and supported operation. The storage example illustrates a conditional energy-value difference, not payback or equipment suitability.
What should I verify before using a TOU savings estimate?
Verify the account’s schedule and effective calendar, energy quantities by period, purchase and export treatment, other charges and assumed routine changes. Keep measured records separate from modeled assumptions and evaluate a realistic scenario rather than a headline price alone.
Sources and methodology
Primary sources were checked October 3, 2026. DOE’s glossary supports the TOU definition, and its utility-rate evaluation page supports the pricing-component distinctions. Dominion’s current Rate 5 PDF and official update support the bounded seasonal and weekend calendar example.
All unbranded prices, quantities, thresholds and comparisons are original hypothetical teaching examples. No numeric current utility price, household saving, battery performance, program eligibility, product partnership or named expert review is claimed.