The sizing chain
Four steps, and skipping either of the middle two is why people buy a battery that dies half way through the night:
Daily load × days of backup = usable energy. Then usable ÷ (depth of discharge × inverter efficiency) = nominal capacity — the number on the spec sheet.
Backing up 10 kWh a day for two days needs 20 kWh of usable energy. With an 80% lithium depth of discharge and a 90% efficient inverter, that means buying 27.8 kWh of nominal capacity. The same job on lead-acid at 50% usable needs 44.4 kWh — which is why lithium's higher price per kWh is misleading until you compare usable kWh.
Depth of discharge is not a suggestion
Running a battery flat shortens its life sharply, so manufacturers rate a usable fraction. Lithium iron phosphate typically allows 80% to 90%; lead-acid and AGM are closer to 50% if you want a reasonable cycle life. A 10 kWh lead-acid bank and a 10 kWh lithium bank do not store the same amount of usable energy.
Amp-hours and system voltage
Batteries are often sold in amp-hours, which only means something alongside a voltage: Ah = kWh × 1,000 ÷ volts. The same 27.8 kWh bank is 2,315 Ah at 12 V, 1,157 Ah at 24 V, or 579 Ah at 48 V.
Higher voltage is nearly always better for anything beyond a small system, because current falls for the same power — which means thinner cable and far less loss. A 5 kW load draws over 400 A at 12 V and around 100 A at 48 V. Voltage drop on the battery-to-inverter run is a real design constraint at low voltages.
Startup surge and load type
Anything with a motor draws several times its running watts for a second or two while it gets moving. A fridge rated 150 W can pull around 750 W the instant the compressor starts. That spike is far too brief to matter for battery capacity, but it is exactly what trips an undersized inverter.
The load type sets a typical multiplier, which you can override from the nameplate if you know the real figure:
- Resistive — ×1. Kettles, irons, water heaters. A heating element draws full power immediately, so there is no surge at all.
- Electronic — ×1.5. TVs, laptops, routers, LED lighting.
- Small motor — ×3. Desk and stand fans, blenders.
- Motor — ×4. Water pumps, washing machines, vacuum cleaners.
- Compressor — ×5. Fridges, freezers, air conditioners.
Only one appliance realistically starts at any given instant, so this calculator adds the single largest startup draw on top of everything already running, rather than stacking every surge together — which would oversize the inverter absurdly. The practical failure this catches is an inverter that handles your continuous load perfectly well but shuts down every time the fridge kicks in.
Sizing honestly
- Measure, don't guess. Nameplate watts are peak draw, not average. A fridge rated 150 W might average 40 W over a day because the compressor cycles.
- Leave headroom. Cold weather reduces capacity, and batteries age. Adding 10% to 20% is normal practice.
- Check surge, not just energy. Pumps, compressors and power tools draw several times their running watts at startup. That is an inverter rating question rather than a battery capacity one, but it will stop your system working just as effectively.
- Two days of autonomy is a lot. If the panels recharge daily, one day plus a margin is often enough — and roughly halves the cost.