What Size Home Battery Do I Need?

Battery sizing has no single answer because batteries do three different jobs, and each implies a different size. Start by deciding which job you're buying.

First: what's it for?

Most systems serve all three, but the binding constraint is usually one of them. Size for that, and treat the others as bonuses.

Sizing for backup

Step 1: Define essential loads. Not everything — the things that matter during an outage:

Loads that change everything: electric resistance heat, central air conditioning, electric water heating, electric range, EV charging. Including these multiplies the requirement dramatically.

Work out your figures with the home electrical load calculator and the appliance wattage reference.

Step 2: Estimate daily energy for those loads — average draw over 24 hours, not peak.

Step 3: Decide autonomy. How many days without grid power?

Step 4: Account for solar recharge. This is the big one. If your system can operate while islanded and keep the solar running, the battery recharges each day — which transforms a one-night battery into an indefinite supply, weather permitting.

Confirm your configuration supports this. Not all do, and it's the single most consequential backup design question. See solar plus battery backup design.

Sizing for solar self-consumption

The useful size is roughly your typical daily surplus, capped by what you'd actually consume later.

Two limits, and the smaller one governs:

• Surplus available — production minus daytime consumption. No point storing more than you generate.
• Evening consumption — no point storing more than you'll use before the next solar day.

Capacity beyond either limit sits idle most days and earns nothing.

Get your production shape from the solar output calculator, and note that surplus varies enormously by season — see solar production by season.

Sizing for rate arbitrage

Size to the energy you consume during peak-rate periods.

The battery charges off-peak (or from solar) and discharges through the peak window. Capacity beyond your peak-period consumption doesn't get used.

Value per cycle is roughly the peak-to-off-peak spread. See time-of-use rates and solar and solar battery payback.

Power vs energy: check both

The most common sizing oversight.

Energy capacity (kWh) — how long it runs your loads.

Power output (kW) — what it can run at once.

Surge rating — what it can start.

A battery with generous capacity but limited power output cannot start a well pump or run central air, regardless of how full it is.

Motor loads are the test case. Pumps, compressors and some appliances draw a large surge at startup that the battery inverter must supply. Same consideration as starting watts vs running watts for generators.

So check three numbers: usable capacity, continuous power output, and surge capability — against your essential loads including their starting demands.

Usable capacity is the number

Nameplate capacity is total cell energy. Usable capacity is what the system will actually deliver, after the BMS reserves headroom at both ends to protect cell life.

Always size against usable capacity, and account for round-trip efficiency — energy out is less than energy in.

See depth of discharge and usable capacity.

The backup reserve tradeoff

If you're using the battery for both daily arbitrage and outage backup, you must decide how much capacity to reserve for outages.

• High reserve → better outage protection, less daily cycling value
• Low reserve → more daily value, less energy available when an outage hits

Most systems let you configure this, and some adjust it automatically around weather forecasts. Decide it deliberately rather than accepting a default.

Modularity

Many systems are modular — you can add capacity later.

That's genuinely useful, because it lets you start with a size matched to your clearest need and expand if it proves insufficient.

Two things to confirm:

• Can modules be added later, and is the inverter sized to support them?
• Will the same modules still be available in a few years?

A worked approach

1. Decide the primary purpose — backup, self-consumption, or arbitrage
2. For backup: list essential loads, estimate daily energy, choose autonomy, confirm solar recharge while islanded
3. For self-consumption: estimate daily surplus and evening consumption, take the smaller
4. For arbitrage: estimate peak-period consumption
5. Check power output and surge against your largest motor loads
6. Convert to usable capacity, accounting for round-trip efficiency
7. Decide the backup reserve if serving multiple purposes
8. Check modularity so you can expand if needed

Run it with the home battery sizing calculator and price it with the home battery cost calculator.

The bottom line

Size for the job you're actually buying: backup sizing starts from essential loads times days of autonomy (transformed if solar can recharge while islanded), self-consumption sizing is capped by your daily surplus or your evening use, and arbitrage sizing matches your peak-period consumption. Check power output and surge rating alongside capacity — a full battery that can't start the well pump isn't sized correctly — and always work in usable capacity, not nameplate.

Size with the home battery sizing calculator, check your loads with the home electrical load calculator, or read how home batteries work.