Flat rate
One energy rate, all dayCompare electricity plans
WattReplay: flat-rate vs. time-of-use electricity cost calculator.
Replay your usage against flat and time-of-use rates.
Local file. No account.
$161.40 flat vs. $154.30 TOU, $7.10 apart on 732 kWh.
Comparison workspace
Compare your usage.
Saved on this browser
No cloud sync or encryption. Other people using this browser profile can see these plans.
Example data · 30 synthetic days · Editable rates
Comparison ready. Updates as you edit.
Your wattreplaySame usage. Both plans.
Time-of-use comes out ahead.
$7.10 less (4.4%) for this dataset, without changing when you use electricity.
Time-of-use
Lower estimate
Off-peak + peak energy ratesSee tiered, seasonal, and solar-adjusted estimates
- Tiered-rate estimate
- $148.00
Same imported usage. These are simplified, user-entered scenarios, not verified utility tariffs.
Illustrative example, not your bill. Fixed charges apply once to this dataset. No annualization.
- Consumption
- 732 kWh
- Peak share
- 27.6%
- Highest interval
- 2.4 kW
Coverage check · 720 intervals · 720 hours recorded
Aug 1, 2026, 12:00 AM EDT to Aug 31, 2026, 12:00 AM EDT (end exclusive).
No gaps between the first and last interval. This does not verify a complete billing period.
When your energy is used
All imported kWh, grouped by local hour
Off-peakPeak
Read hourly data as a table
| Hour | Off-peak kWh | Peak kWh |
|---|---|---|
| 12 AM | 12 | 0 |
| 1 AM | 12 | 0 |
| 2 AM | 12 | 0 |
| 3 AM | 12 | 0 |
| 4 AM | 12 | 0 |
| 5 AM | 15 | 0 |
| 6 AM | 24 | 0 |
| 7 AM | 36 | 0 |
| 8 AM | 27 | 0 |
| 9 AM | 21 | 0 |
| 10 AM | 21 | 0 |
| 11 AM | 21 | 0 |
| 12 PM | 27 | 0 |
| 1 PM | 30 | 0 |
| 2 PM | 27 | 0 |
| 3 PM | 30 | 0 |
| 4 PM | 16 | 32 |
| 5 PM | 21 | 42 |
| 6 PM | 24 | 48 |
| 7 PM | 21 | 42 |
| 8 PM | 19 | 38 |
| 9 PM | 45 | 0 |
| 10 PM | 27 | 0 |
| 11 PM | 18 | 0 |
See exactly what makes up the bill
| Component | Flat plan | TOU plan |
|---|---|---|
| Energy | $146.40 | $134.30 |
| Common rider / delivery | $0.00 | $0.00 |
| Fixed charge | $15.00 | $20.00 |
| Entered non-coincident demand | $0.00 | $0.00 |
| Tax rate | 0% | 0% |
| Tax charge | $0.00 | $0.00 |
| Total estimate | $161.40 | $154.30 |
Both plans include the common delivery rate and subtotal tax. TOU demand is based on the highest interval-average kW, not necessarily your utility’s measurement window.
03
What could better timing change?
A separate scenario, not part of the comparison above.
%
Keep my routineShift everything
Scenario TOU estimate
Less than unshifted TOU
40.4 kWh moved from peak to off-peak. Total energy and demand charge stay unchanged. Hypothetical timing, not guaranteed savings.
Nothing is saved until you choose to save. JSON files contain your usage. Review before sharing.
What WattReplay does
WattReplay compares two ways an electric utility could bill the same energy use: one flat rate all day, or a time-of-use (TOU) rate that charges more during a defined peak window and less off-peak. Enter the same usage once, enter both tariffs, and see which one costs less for that dataset.
How the calculation works
Let E be total kWh, P peak kWh, O off-peak kWh, D the highest interval-average kW, and t the subtotal tax rate as a decimal. Flat cost = (E × flat rate + flat fixed charge + E × delivery rate) × (1 + t). TOU cost = (P × peak rate + O × off-peak rate + TOU fixed charge + E × delivery rate + D × demand rate) × (1 + t). Each interval is classified peak or off-peak from the peak window and weekday you enter, then summed.
Worked example
800 kWh total, 200 kWh in the peak window and 600 kWh off-peak. Flat plan: $0.14/kWh, $8 fixed, $0.03/kWh common delivery, 5% tax. TOU plan: $0.24/kWh peak, $0.09/kWh off-peak, $5 fixed, 3 kW highest interval at $4/kW demand, same delivery and tax. Flat = (800 × 0.14 + 8 + 800 × 0.03) × 1.05 = $151.20. TOU = (200 × 0.24 + 600 × 0.09 + 5 + 800 × 0.03 + 3 × 4) × 1.05 = $150.15. TOU is $1.05 cheaper for this same usage.
Assumptions
Only one flat tariff and one two-period TOU tariff are modeled. Fixed charges apply once to the dataset you supply, not once per day. Demand uses the highest interval-average kW in the dataset, which can differ from a utility's own demand window. Seasonal tiers, holidays, export credits, and minimum bills are separate optional inputs, not assumed.
When to use the result
Use this when you have interval or typical-day usage and both tariffs' prices, and want to compare the same energy under each. It is a same-usage estimate, not a bill prediction: it does not account for future usage changes or plan eligibility rules.
Common mistakes
Mixing a partial billing period for one plan with a full period for the other. Leaving out a fixed or demand charge that the tariff actually has. Assuming a low off-peak rate alone makes TOU cheaper without checking how much usage actually falls in the peak window.
Related
Is time-of-use worth it? · Make sense of your meter CSV · What load shifting can change · Full methodology & limitations
Explore more
US average electricity price history & cost calculator · Time-of-use peak windows by timezone
Sources
U.S. Department of Energy: utility-rate evaluation · Con Edison TOU rules and exclusions
Audit every interval Transparent calculations, exportable evidence
720 intervals. Export includes every row, not only this page.
| Minutes | kWh | Peak kWh | Off-peak kWh | |
|---|---|---|---|---|
| 2026-08-01T04:00:00Z | 60 | 0.4 | 0 | 0.4 |
| 2026-08-01T05:00:00Z | 60 | 0.4 | 0 | 0.4 |
| 2026-08-01T06:00:00Z | 60 | 0.4 | 0 | 0.4 |
| 2026-08-01T07:00:00Z | 60 | 0.4 | 0 | 0.4 |
| 2026-08-01T08:00:00Z | 60 | 0.4 | 0 | 0.4 |
| 2026-08-01T09:00:00Z | 60 | 0.5 | 0 | 0.5 |
| 2026-08-01T10:00:00Z | 60 | 0.8 | 0 | 0.8 |
| 2026-08-01T11:00:00Z | 60 | 1.2 | 0 | 1.2 |
| 2026-08-01T12:00:00Z | 60 | 0.9 | 0 | 0.9 |
| 2026-08-01T13:00:00Z | 60 | 0.7 | 0 | 0.7 |
| 2026-08-01T14:00:00Z | 60 | 0.7 | 0 | 0.7 |
| 2026-08-01T15:00:00Z | 60 | 0.7 | 0 | 0.7 |
| 2026-08-01T16:00:00Z | 60 | 0.9 | 0 | 0.9 |
| 2026-08-01T17:00:00Z | 60 | 1 | 0 | 1 |
| 2026-08-01T18:00:00Z | 60 | 0.9 | 0 | 0.9 |
| 2026-08-01T19:00:00Z | 60 | 1 | 0 | 1 |
| 2026-08-01T20:00:00Z | 60 | 1.6 | 0 | 1.6 |
| 2026-08-01T21:00:00Z | 60 | 2.1 | 0 | 2.1 |
| 2026-08-01T22:00:00Z | 60 | 2.4 | 0 | 2.4 |
| 2026-08-01T23:00:00Z | 60 | 2.1 | 0 | 2.1 |
| 2026-08-02T00:00:00Z | 60 | 1.9 | 0 | 1.9 |
| 2026-08-02T01:00:00Z | 60 | 1.5 | 0 | 1.5 |
| 2026-08-02T02:00:00Z | 60 | 0.9 | 0 | 0.9 |
| 2026-08-02T03:00:00Z | 60 | 0.6 | 0 | 0.6 |
| 2026-08-02T04:00:00Z | 60 | 0.4 | 0 | 0.4 |
1–25 of 720
Know what you are comparingBetter questions.
01 / The basicsBetter questions.
Better energy decisions.
Real tariffs can be more complex than two prices. Know where this comparison stops.
Is time-of-use worth it?
Why a low off-peak rate is only part of the story.
02 / Your dataMake sense of your meter CSV
Intervals, offsets, gaps, and one consistent billing period.
03 / Your routineWhat load shifting can change
Separate a cheaper plan from a different habit.
Good questions. Clear answers.
Does a lower off-peak rate always mean TOU saves money?
No. Savings depend on how much of your usage falls inside the peak window, not just the off-peak rate. A cheap off-peak rate does not help if most of your energy is used during peak hours.
What counts as "coverage" in this comparison?
Coverage counts recorded minutes against the gap between your first interval start and last interval end. A gap-free dataset can still omit the beginning or end of an actual bill, so coverage is not a claim of a complete bill.
How is load shifting different from the flat-vs-TOU comparison?
Load shifting is a separate hypothetical: it estimates cost if a percentage of peak usage moved to off-peak, holding total kWh, fixed charges, and measured demand unchanged. It is not part of the same-usage comparison above.
Is my meter data uploaded anywhere?
No. CSV files, JSON plans, and calculations are processed on your device, including inside a Web Worker. This app has no server endpoint that receives your usage or rates.
Why does the tool ask for a UTC offset on every interval?
Peak-window classification depends on local time, and offsets (including daylight saving time changes) must be explicit to sort intervals correctly and avoid silently misclassifying peak and off-peak minutes.
What tariff structures are not modeled?
Only one flat tariff and one two-period TOU tariff are supported by default. Seasonal tiers, holidays, demand ratchets, export credits, net metering, minimum bills, and eligibility rules are separate optional add-ons or are not modeled at all.
The fine print, in plain sight.
- One flat tariff and one two-period TOU tariff. Rates are your assumptions, not verified utility offers.
- No seasonal tiers, holiday calendars, demand ratchets, export credits, net metering, minimum bills, or eligibility rules.
- Energy is assumed uniform within an interval. Demand is the highest interval-average kW, not necessarily the utility measurement window.
- Fixed charges apply once to the imported dataset. No missing energy or annual savings are invented.
- Load shifting is hypothetical. Total energy stays constant and the demand charge is held unchanged.