The short answer
Multiply the kWh used in each tariff window by that window’s price, then add the costs. Off-peak share describes energy rather than elapsed hours. For complete tariff comparisons, use whole-household energy and add fixed charges once; a two-period scenario does not reproduce tiers, demand or complex seasonal billing.
Use the windows in your own contract
Moving a task to night-time only changes its price when the tariff gives that time a different rate. Check the named plan, days, seasons and prices.
A real US example shows why timing cannot be generalized: PG&E describes E-TOU-C peak hours as 4–9 p.m. every day, and E-TOU-D as 5–8 p.m. weekdays, with other hours off-peak. Its guidance also describes seasonal and baseline conditions. These windows are not a quote of current prices and are not a universal US schedule.
For UK context, Energy Saving Trust discusses time-of-use tariffs. Great Britain and Northern Ireland households should use their own supplier’s terms.
Weight by energy, rather than elapsed time
For two periods:
Energy cost = peak kWh × peak rate + off-peak kWh × off-peak rate
An illustrative 1.2 kWh task uses 0.8 kWh at £0.35 and 0.4 kWh at £0.15:
| Period | Energy | Illustrative price | Cost |
|---|---|---|---|
| Peak | 0.8 kWh | £0.35/kWh | £0.28 |
| Off-peak | 0.4 kWh | £0.15/kWh | £0.06 |
| Total | 1.2 kWh | — | £0.34 |
The energy-weighted equivalent price is £0.34 ÷ 1.2, about £0.283333/kWh. The simple average of the two prices would give £0.30 for the task and understate this example.
A programme can draw more electricity in one stage. Half its hours in each window does not establish half its energy in each.
Check where a complete cycle falls in the tariff windows
Suppose a completed task uses 2 kWh and 60% of its energy, rather than its hours, falls off-peak. With illustrative $0.40/kWh peak and $0.15/kWh off-peak prices:
| Energy timing assumption | Calculation | Task electricity cost |
|---|---|---|
| All peak | 2 × $0.40 | $0.80 |
| 60% off-peak | 1.2 × $0.15 + 0.8 × $0.40 | $0.50 |
| All off-peak | 2 × $0.15 | $0.30 |
The mixed timing saves $0.30 per task against all-peak operation. It does not deliver the all-off-peak saving of $0.50. These are chosen prices, not PG&E quotes. A timer’s start time alone does not show which window receives the high-energy heating stages.
If you do not have energy-by-period information, treat the share as an explicit assumption and test alternatives. A monitor’s whole-cycle kWh can establish total energy without establishing its time split. Keep the start, finish, programme and available interval readings in a measurement record.
The tariff tool’s percentage applies to whole-household energy. A dishwasher with 60% off-peak energy does not prove that the household’s annual energy has that same share. Combine non-overlapping loads and include the remaining household consumption before comparing complete plans.
Add fixed charges for a whole-home comparison
Use whole-home consumption when comparing complete tariffs. A collection of appliance rows can help explain that energy, but omitted household loads should not disappear from the tariff decision.
For an illustrative 3,000 kWh/year and 365 days:
| Assumed quote | Energy price | Daily fixed charge |
|---|---|---|
| A | £0.30/kWh at all times | £0.50 |
| B | £0.40 peak; £0.15 off-peak | £0.60 |
At 40% off-peak, B uses 1,200 off-peak and 1,800 peak kWh:
| Annual component | A | B |
|---|---|---|
| Energy | £900.00 | £900.00 |
| Fixed charges | £182.50 | £219.00 |
| Total | £1,082.50 | £1,119.00 |
Equal energy cost does not mean equal annual cost. In this scenario, B costs £36.50 more.
Work out the off-peak break-even share
Let p be B’s off-peak fraction from 0 to 1. B’s annual total is:
3,000 × [0.40 × (1 − p) + 0.15 × p] + 365 × 0.60
This simplifies to £1,419 − £750 × p. Matching A’s £1,082.50 requires p = 336.50 ÷ 750, or about 44.87% off-peak.
That threshold belongs only to these illustrative quotes and consumption. The calculator lets you test percentages; it does not promise that a household can shift that much energy.
At 60% off-peak, B totals £969.00: £150 less than its original 40% scenario, and £113.50 less than A.
Respect the comparison’s limits
The tool models two energy-price periods plus daily fixed charges. It does not reproduce tiers, demand charges, complex seasonal billing, export payments, discounts, exit fees or tax calculations.
A real plan such as PG&E’s can have additional conditions. Use compatible effective prices for an explicitly limited scenario; use the utility’s complete bill comparison when those conditions matter. No tool here fetches live or future hourly prices.
For a single appliance’s avoidable energy, an unchanged household fixed charge should not be added as if avoiding a cycle removes it.
Take the next step
Load the tariff example below, or compare your two quotes with whole-home kWh and consistent charge boundaries.
The household planner can total non-overlapping appliance estimates and pass the total into tariff comparison. The scenarios tool can explore chosen energy and price assumptions.
Keep supplier windows, appliance instructions and your routine in view. This arithmetic does not recommend unattended appliance operation.