Battery sizing without the upsell: when 10kWh is right, when you need 20

Battery upsell is rampant in residential solar. A lot of installers size for the largest system the client will buy. We size for the use case. Two use cases dominate: backup-only (essentials during outages) and TOU arbitrage (store cheap, use during peak).

For backup-only: a single 10kWh battery covers refrigerator, lighting, internet, modem, one HVAC zone, and selective outlets for ~36 hours. If you have well water or medical equipment, add 5–10 kWh. Most clients in this bucket don't need 20+ kWh.

For TOU arbitrage: this only makes sense if your utility has a wide peak/off-peak spread (we're at ~$0.32 vs $0.11 in our service area, which makes the math work). Size to cover peak-window usage from grid down to zero. Typical 4-bedroom: 13–17 kWh. Above 20 kWh is rare unless you have EV charging on the peak overlap.

The honest version: most homes get the right answer at 10–15 kWh. If your installer is pushing 27+, ask them to show their work.