- A plug-in kit produces most at midday, which on many TOU plans is an off-peak or mid-peak period.
- The value of your solar is the price of the hour it replaces, not your average price.
- Shifting flexible loads to midday captures solar; a battery can move solar into expensive evening hours.
- On plans with a big evening peak (for example 4-9 pm), a battery can improve the economics considerably.
If your utility charges different prices at different times of day, a simple “kWh × average price” savings estimate can be misleading. Here is how to do it properly.
In this article
Match production to prices
A south-facing kit produces roughly 70% of its daily energy between 10 am and 3 pm. On a typical TOU plan:
| Period | Example price | Solar production share |
|---|---|---|
| Off-peak (night) | Low | 0% |
| Mid-peak / off-peak (morning-midday) | Medium | ~75% |
| Peak (e.g. 4-9 pm) | High | ~5-15% (west-facing panels do better) |
So the average value of your solar kWh is close to the midday price. Use that price in the calculator rather than your bill’s average.
West-facing panels can win on TOU
A west-facing kit produces less in total but more in late afternoon, overlapping the start of peak periods. On plans with a large peak premium, west can match or beat south in dollars. See east or west facing balcony solar.
Batteries: where TOU changes the math
A battery stores midday surplus and discharges during the peak. Its value = kWh shifted per day × (peak price − the value that surplus would otherwise have had, often zero) × days per year. Example: 1.5 kWh shifted on 250 days at a $0.45 peak price is about $170 a year, which can make a $800-1,000 battery pay back in five to six years. Without TOU, the same battery earns much less. Battery payback.
Practical tips
- Run dishwashers, laundry and EV or e-bike charging between 10 am and 3 pm.
- Pre-cool your home in the early afternoon with solar before peak.
- Check whether your plan has a cheap “super off-peak” midday window; some UK and US tariffs do, and then daytime solar is worth less.
Worked example: California TOU
Imagine a plan with 30 cents midday and 50 cents from 4-9 pm. A south-facing 800 W kit produces 760 kWh a year, of which 70% is used at home, almost all at midday: 532 kWh × $0.30 ≈ $160. A west-facing kit produces 600 kWh, of which 80% is used at home, with 25% of that during the peak: (480 × 0.75 × $0.30) + (480 × 0.25 × $0.50) ≈ $108 + $60 = $168. The west-facing kit makes less energy but earns slightly more.
Frequently asked questions
How do I find my TOU periods?
They are on your bill and your utility’s rate plan page. Note weekday vs weekend differences and seasonal changes.
Should I switch to a TOU plan because I have solar?
Only if your evening use is low or you have a battery. Otherwise a flat rate may be better. Compare with your utility’s rate tool.
Does TOU matter in the UK?
Some UK tariffs have cheap overnight or midday periods. If your midday rate is cheap, daytime solar saves less per kWh.
Sources
- Time-of-use rate plans, PG&E.
- PVWatts Calculator, National Laboratory of the Rockies (formerly NREL).
- Electricity rates by state (EIA data, June 2026), Choose Energy.
Sources checked in September 2026. Laws, prices and product details change; if you spot something out of date, tell us.