How we calculate
OffPeakWise turns each utility's published prices into hour-by-hour estimates. This page lists every formula and assumption so you can check our work. Found a mistake? Email info@offpeakwise.com.
Rates and hours
- SRP: totals per kWh from SRP's rate schedules effective with the November 2025 billing cycle (E-23, E-28, E-16, E-29), the monthly service charge tiers and the E-16 demand charges. Last checked 2026-11-03.
- SCE: per-kWh prices for bundled customers as published on SCE's plan pages for rates as of 2026-10-01, the 79¢ daily Base Services Charge, the 9¢ baseline credit and the daily baseline allocations by region. Last checked 2026-11-03.
- Seasons follow calendar months. Utilities bill by billing cycle, so months that straddle a season change will differ slightly.
- Holidays: SRP's six holidays use their observed weekday (Saturday → Friday, Sunday → Monday). SCE's eight holidays move from Sunday to Monday. We are confirming both rules against the tariffs.
Monthly bill formula
- Split your monthly kWh evenly across the days of the month.
- Spread each day's use over 24 hours with the routine you picked (weekday or weekend shape), blended with an air-conditioning shape.
- Add EV charging, spread evenly over the charging window you picked.
- Price each hour at the plan's rate for that season, day type and hour, and add it up.
- SCE time-of-use plans: subtract 9¢ × min(monthly kWh, baseline allocation). SCE Tiered: 30¢ up to the baseline allocation, 39¢ above it. Baseline allocation = daily allocation for your region and season × days in the month.
- Add the fixed charge: SRP's monthly service charge for your home type, or SCE's Base Services Charge × days.
- SRP E-16 only: billing demand = average over weekdays of the highest hourly use between 5 and 10 p.m., × 1.3 to allow for spikier real days; demand charge = billing demand × the season's $/kW.
Taxes, city fees, riders and programs such as SRP's Economy Discount or SCE's CARE and FERA bill discounts are not included.
EV charging
- Energy from the wall = miles × EPA kWh per 100 miles ÷ 100 ÷ 0.9. The 0.9 allows for about 10% lost while charging. Efficiency ratings: U.S. EPA via fueleconomy.gov (Find a Car results and web services), retrieved 2026-11-08.
- The EV cost shown is your estimated bill with the EV minus your bill without it, so baseline credits, tiers and demand charges are included.
- The gas comparison uses the mpg and gas price you enter.
Daily routines (assumptions)
Each routine is a share of a day's non-cooling use by hour. It is blended with an air-conditioning shape: share = (1 − c) × routine + c × cooling, where c is the share of the month's use that goes to cooling. For SRP, c is 0.55 in July–August, 0.45 in other summer months and 0.08 in winter. For SCE, c is 0.45 (hot regions), 0.3 (moderate) or 0.15 (cool) in summer and 0.05 in winter. These are simplifications; your own hourly data from your utility account will always be more accurate.
Weekday routines
| Hour starting | Away 9 to 5 | Home during the day | Night owl |
|---|---|---|---|
| 00:00 | 2.9% | 2.6% | 5.0% |
| 01:00 | 2.6% | 2.3% | 4.2% |
| 02:00 | 2.5% | 2.2% | 3.4% |
| 03:00 | 2.4% | 2.1% | 2.6% |
| 04:00 | 2.5% | 2.2% | 2.3% |
| 05:00 | 3.2% | 2.6% | 2.2% |
| 06:00 | 4.8% | 3.6% | 2.3% |
| 07:00 | 5.2% | 4.4% | 2.8% |
| 08:00 | 4.1% | 4.7% | 3.1% |
| 09:00 | 3.2% | 4.7% | 3.3% |
| 10:00 | 2.9% | 4.6% | 3.5% |
| 11:00 | 2.9% | 4.7% | 3.7% |
| 12:00 | 3.1% | 4.9% | 3.9% |
| 13:00 | 3.1% | 4.7% | 3.9% |
| 14:00 | 3.2% | 4.6% | 3.9% |
| 15:00 | 3.5% | 4.7% | 4.1% |
| 16:00 | 4.3% | 5.0% | 4.5% |
| 17:00 | 6.1% | 5.7% | 5.0% |
| 18:00 | 7.5% | 6.2% | 5.4% |
| 19:00 | 7.7% | 6.1% | 5.8% |
| 20:00 | 7.3% | 5.7% | 6.1% |
| 21:00 | 6.3% | 5.0% | 6.5% |
| 22:00 | 5.0% | 4.0% | 6.6% |
| 23:00 | 3.7% | 3.0% | 6.0% |
Air-conditioning shapes
| Hour starting | SRP | SCE |
|---|---|---|
| 00:00 | 3.4% | 1.7% |
| 01:00 | 3.0% | 1.3% |
| 02:00 | 2.7% | 1.1% |
| 03:00 | 2.5% | 1.0% |
| 04:00 | 2.3% | 1.0% |
| 05:00 | 2.2% | 1.0% |
| 06:00 | 2.2% | 1.2% |
| 07:00 | 2.4% | 1.7% |
| 08:00 | 2.8% | 2.3% |
| 09:00 | 3.3% | 3.1% |
| 10:00 | 3.8% | 4.1% |
| 11:00 | 4.3% | 5.2% |
| 12:00 | 4.8% | 6.2% |
| 13:00 | 5.2% | 7.2% |
| 14:00 | 5.7% | 8.1% |
| 15:00 | 6.1% | 8.7% |
| 16:00 | 6.5% | 8.9% |
| 17:00 | 6.7% | 8.7% |
| 18:00 | 6.6% | 7.9% |
| 19:00 | 6.1% | 6.6% |
| 20:00 | 5.3% | 5.0% |
| 21:00 | 4.6% | 3.5% |
| 22:00 | 4.0% | 2.5% |
| 23:00 | 3.7% | 2.0% |
Live peak status
The "right now" panels read your device's clock and convert it to the utility's time zone (Arizona time for SRP, Pacific time for SCE), then look up the period for that hour, day type and season.
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