How Many Solar Batteries Do You Need? Size the Evening

Key Takeaways

• Batteries are sized to the evening, not the house: the kWh between sunset and sunrise is the daily-shifting target.
• The EIA-average home (~29 kWh/day) typically spends 8–15 kWh of it after dark — conveniently, one standard wall unit's usable capacity.
• Backup sizing is a different question: essential-loads × outage-hours, where honesty about "essential" saves thousands.
• Match usable capacity and output kW, not nameplate — the kW-vs-kWh distinction bites hardest in storage.

First, name the job

Battery count is downstream of purpose, which is why our worth-it test comes first. Daily shifting — moving your solar noon into your grid evening — sizes to overnight consumption. Outage backup sizes to essential loads times expected outage length. Off-grid sizes to worst-month autonomy, a stricter discipline with its own guide. Households that buy before naming the job buy someone else's battery.

The daily-shifting calculation

Start from your day: the EIA-average American home consumes about 29 kWh across 24 hours. The battery only needs the dark share — evening cooking, lighting, electronics, overnight HVAC — which for typical patterns lands around 8–15 kWh. Two refinement passes beat every rule of thumb:

1. Read your own curve. Smart-meter portals and solar apps (the monitoring habit from our savings guide) show your actual sunset-to-sunrise total — the number itself.
2. Subtract what you can shift away. Every load moved into daylight — dishwasher, laundry, EV charging in the solar window — shrinks the battery you must buy. Habits are the cheapest kilowatt-hours of storage on the market.

Then buy usable capacity matching that number. A typical wall unit's ~13 kWh usable covers the average evening with margin; big homes, electric heat, or resistive water heating push toward a second unit — or better, toward efficiency first.

The backup calculation

Outage sizing multiplies two honest numbers: essential-load kWh per day × days of autonomy you're buying. The discipline is the word essential — fridge, network, some lights, phone charging, a furnace fan run perhaps 6–8 kWh a day; add central AC or electric heat and you're sizing a power plant. That's why serious backup installs wire an essential-loads subpanel, and why a bidirectional EV — with several wall units' worth of capacity built in — rewrites this particular math for households that own one.

Don't forget the kW side: the battery's output rating must cover simultaneous essential loads (a well pump's surge is the classic trap). Two smaller units sometimes beat one large one purely on combined output.

Assumptions labeled in text; EIA average consumption as baseline. Your meter data supersedes every row.

The buying translation

Convert the target to hardware with three checks: usable kWh at the design depth (nameplate minus reserve), continuous kW against your simultaneous loads, and expansion path — can a second unit join later? That last one turns sizing anxiety into sequencing: buy the evening you have now, measured; add capacity when the meter — not the salesperson — says you're short. Storage is the rare solar component where starting small and growing is engineering, not compromise.