How to Size a Home Battery

What you need

The list of loads you want to keep running during an outage, their watts, how long they must run, and the battery voltage — plus the battery backup calculator. Size for the essentials (fridge, lights, router, a phone charger), not for everything in the house: backing up the whole house needs a far bigger — and far more expensive — battery.

Step 1 — List the loads

Add up the watts of everything that must keep running. A fridge is about 150 watts, lights perhaps 200 watts, a router 10 watts — the exact figures are on the appliance labels or in their manuals. Do not guess the watts: a load list built on guesses sizes a battery built on guesses.

Step 2 — Work out the energy

Multiply the load by the hours it must run: watts × hours = watt-hours. The battery backup calculator does it: a 500-watt load for 4 hours needs 500 × 4 = 2,000 watt-hours (2 kWh). That is the energy the battery must deliver — not the energy it stores, which is bigger, because of the next step.

Step 3 — Account for usable capacity

A battery is not fully usable. Two factors cut it down: depth of discharge (how much of the battery you may use — 80% is the usual figure) and efficiency (how much energy is lost in charging and discharging — 90% is usual). Together they give a usable factor of 80% × 90% = 0.72. The required capacity at 12 volts is 2,000 ÷ (12 × 0.72) = 231.48 amp-hours. A 200 Ah battery is not 200 usable amp-hours — it is 200 × 0.72 = 144 usable amp-hours at 12 volts.

Step 4 — Check the backup time

Work the other way round for a battery you already have: a 200 Ah battery at 12 volts gives 200 × 12 × 0.72 ÷ 500 = 3.46 hours for a 500-watt load. If that is shorter than the outages you expect, you need a bigger battery — or a smaller load list. Cutting the load list is usually cheaper than buying more battery.

Step 5 — Plan the installation

Batteries are heavy — a large one can weigh 50 kg or more — so the location must take the weight and be dry, ventilated and out of the way. The inverter and the wiring must match the load, and if the battery charges from solar panels, the solar panel calculator tells you what the panels can supply. The installation itself is electrical work, not DIY.

Worked example

A load of 500 W for 4 hours needs 500 × 4 = 2,000 Wh (2 kWh) of energy. The usable factor is 80% × 90% = 0.72, so the required capacity at 12 V is 2,000 ÷ (12 × 0.72) = 231.48 Ah. A 200 Ah battery lasts 200 × 12 × 0.72 ÷ 500 = 3.46 hours for the same load. Those are exactly the numbers the battery backup calculator gives you.

Common mistakes

• Sizing for the whole house. Backing up everything — including the cooker and the air conditioning — needs a battery several times bigger than backing up the essentials. List the essentials and size for them.
• Forgetting depth of discharge and efficiency. A 200 Ah battery is not 200 usable amp-hours. The usable factor (0.72 in the example) is the difference between a battery that lasts and one that dies an hour early.
• Mismatched voltage. The battery, the inverter and the loads must all agree on the voltage. A 12 V battery does not run a 24 V system.

Safety

Mains wiring and electrical installation must be handled by qualified professionals — a home battery stores a lot of energy, and a wrongly installed system can cause fire or serious electric shock. Never connect a battery system to the mains yourself, and never work on a live battery bank. Some battery types need ventilation while charging — follow the manufacturer's instructions exactly — and keep batteries away from heat, sparks and children. If a battery is swollen, leaking or hot, do not touch it: call a professional.

Try these calculators

• Battery Backup Calculator — capacity and backup time for any load.
• Solar Panel Calculator — output and panels for your roof.
• Electricity Cost Calculator — cost from units, tariff and standing charge.