Estimate a time-of-use electricity bill from peak and off-peak kWh and rates, compare it with a flat rate, or see the savings from shifting peak use.
Time-of-Use Electricity Cost Formula
This model multiplies usage by the entered rate in each pricing period, adds the fixed charge, and applies the user-entered tax and surcharge percentage to that entire subtotal, including the fixed charge. It does not determine an actual utility’s tax base or tariff:
C = (Kp * Rp + Ko * Ro + Ks * Rs + F) * (1 + T)
In the shift mode, x kWh moves only from peak to off-peak while total usage stays the same. Enter x from zero through the entered peak usage. The modeled savings are:
S = x * (Rp - Ro) * (1 + T)
For a positive peak rate, maximum peak usage under a target bill is found by rearranging the first equation while holding off-peak usage, super off-peak usage, rates, fixed charge, and the modeled percentage charge unchanged:
Kp = [B / (1 + T) - F - Ko * Ro - Ks * Rs] / Rp
Variables:
- C is the estimated monthly electricity bill
- Kp, Ko, and Ks are the kWh used during peak, off-peak, and super off-peak periods
- Rp, Ro, and Rs are the electricity rates per kWh for those periods
- F is the fixed monthly customer charge
- T is the modeled tax and surcharge rate written as a decimal in the formula; enter 5 in the percentage field for 5%
- S is the modeled bill reduction from shifting x kWh from peak to off-peak; a negative value means the bill increases
- B is the target monthly bill
Use usage totals from a bill, smart-meter portal, or utility interval-data download. Match all usage and rates to the same billing period and the plan’s pricing windows; peak hours can vary by season, weekday, holiday, and utility plan. No schedule or tariff rate is supplied by this calculator.
Choose the bill, peak-to-off-peak shift, or maximum-peak-usage result. For the bill and shift modes, peak and off-peak usage and rate fields start blank and require explicit entries; enter 0 for unused periods. Super off-peak is optional: leave both its fields blank to omit that period, or enter both usage and rate, including 0 where appropriate. In maximum-peak mode, enter the known peak rate, non-peak usage and rates, and target bill. All usage, rates, charges, and targets must be nonnegative.
Optional fixed and percentage charges default to zero, meaning no such charges are modeled. A blank optional flat rate omits that comparison; an entered flat rate of 0 is a valid zero-energy-rate scenario, with the same fixed and percentage charges still applied. The all-in rate is total bill divided by total kWh and is unavailable when total usage is zero.
In the budget solve, first compare the target with the bill from non-peak usage and fixed charges, including the modeled percentage charge. If that baseline already exceeds the target, no nonnegative peak usage fits; a negative peak answer is not a usable allowance. If peak rate is zero and the baseline fits, peak usage is unbounded within this model, so there is no finite maximum. With a positive peak rate, a target equal to the baseline allows exactly 0 peak kWh.
Calculate updates results manually. When browser storage is available, the last successful input snapshot is saved and validated and recomputed on return. Editing values keeps a marked previous result until Calculate is selected again. Reset clears this calculator’s saved state, blanks usage and rate entries, and restores the documented optional settings.
Time-of-Use Bill Example by Period
The table shows an illustrative month using the entered model assumptions, not an offered utility tariff. In a flat-rate comparison, the calculator uses the same total kWh, fixed charge, and percentage-charge base for both scenarios; actual competing plans may have different charges.
| Period or charge | Usage | Rate | Cost |
|---|---|---|---|
| Peak | 200 kWh | $0.32/kWh | $64.00 |
| Off-peak | 500 kWh | $0.14/kWh | $70.00 |
| Super off-peak | 100 kWh | $0.09/kWh | $9.00 |
| Fixed charge | — | — | $12.00 |
| Subtotal | 800 kWh | — | $155.00 |
| 5% taxes and surcharges | — | — | $7.75 |
| Estimated bill | 800 kWh | — | $162.75 |
The effective all-in rate in this example is $162.75 / 800 = about $0.2034 per kWh, even though none of the listed energy rates equals that amount.
Example Problems
Example 1: Calculate a time-of-use bill.
For an illustrative month, enter 200 peak kWh at $0.32/kWh, 500 off-peak kWh at $0.14/kWh, and 100 super off-peak kWh at $0.09/kWh. With a $12 fixed charge and a modeled 5% charge on energy plus the fixed charge, energy costs total $143. Adding the fixed charge gives $155, and applying 5% gives:
C = $155 * 1.05 = $162.75.
Example 2: Estimate savings from shifting peak use.
The same customer moves 50 kWh from the $0.32 peak period to the $0.14 off-peak period. Before tax, the savings are 50 * ($0.32 – $0.14) = $9.00. Including the 5 percent surcharge, the estimated savings are $9.45 per month.
Frequently Asked Questions
What is a time-of-use electricity rate?
It is a rate design in which the price per kWh changes by time period. Electricity is generally more expensive during designated peak hours and less expensive during off-peak or super off-peak hours.
How do I know how many kWh I used during each period?
Check the usage breakdown on the utility bill or the interval-data section of the utility’s online account. A whole-home monitor can help estimate the timing of individual loads, but the utility meter remains the billing source.
Does moving usage always reduce the bill?
Only when the entered off-peak rate is lower than the entered peak rate in this model. Equal rates give zero savings; moving usage to a higher rate increases the bill and produces negative savings. A zero shift is valid. The shift keeps total kWh and fixed charges unchanged and applies the same modeled percentage charge. Actual demand charges, minimum bills, credits, differing tax bases, seasonal rules, or other tariff provisions require a separate calculation.
