The timing mismatch
Your panels produce most between roughly 10 am and 4 pm. Your house demands most cooling from about 4 pm until well after dark, when the structure is fully heat-soaked and the family is home. That gap — production falling as demand climbs — is where evening grid purchases come from.
Under newer net-billing structures, the credit you earn for midday export and the price you pay for evening import are not the same number. Exporting cheap and importing expensive is how a solar home ends up with a bill that surprises its owner.
To fill in: the rate and bill figures in this article are illustrative arithmetic with the assumptions stated inline. Check them against current PG&E / SCE residential rates before publishing, and update the date on the article when you do.
Reducing the evening load is the lever
If the problem is evening consumption, the fix is evening consumption. There are only three general approaches:
- Store it — a battery, which is effective and expensive.
- Shift it — pre-cool during production hours so the compressor runs less later.
- Avoid it — cool with something that costs a fraction of a compressor to run.
A whole house fan is the third. Around 150 watts on low versus 3,500 watts of compressor, in exactly the hours your panels have stopped producing.
The attic is doing you no favours either
Most residential solar sits on a roof above an attic that reaches 140–150°F on a Fresno afternoon. That attic keeps radiating heat down into your ceilings for hours after sunset, which is a large part of why the evening AC load is so stubborn.
Attic ventilation attacks that directly. Lower attic temperature means less radiant gain through the ceiling, which means a shorter, later evening cooling load — on top of whatever the whole house fan itself displaces. There is also a secondary benefit specific to solar: panel output falls as cell temperature rises, and a cooler roof deck is a marginally more productive array.
A realistic sequence
- Pull your interval data. Look at hourly usage on a hot week. Confirm where the evening consumption actually sits before spending anything.
- Fix attic venting. Cheapest, improves everything downstream.
- Add the whole house fan. Displaces the evening and overnight compressor hours your panels cannot cover.
- Then consider a battery. Sizing a battery after you have cut the evening load is considerably cheaper than sizing one before.
That last point is the one solar owners tend to appreciate most. Every kilowatt-hour you stop needing after sunset is a kilowatt-hour of battery you do not have to buy.