Step 1 — Add up your daily consumption
Everything starts from watt-hours per day: each appliance's power multiplied by its hours of use. A camper van example:
| Load | Power | Hours/day | Energy |
|---|---|---|---|
| Compressor fridge | 45 W | ~8 h (cycling) | 360 Wh |
| LED lighting | 20 W | 5 h | 100 Wh |
| Laptop + router | 75 W | 6 h | 450 Wh |
| Water pump | 200 W | 0.5 h | 100 Wh |
| Phone charging etc. | — | — | ~190 Wh |
| Total | ≈ 1,200 Wh/day |
Measuring with an energy meter over a few typical days beats any estimate. If your loads run through an inverter, the battery delivers more than the appliances consume — a 90%-efficient inverter turns 1,200 Wh of consumption into about 1,333 Wh drawn from the battery.
Step 2 — Decide your days of autonomy
Autonomy is how long the bank must carry you with no charging at all: cloudy days for solar, outage length for backup. Two days is a common starting point for non-critical solar systems; go longer in dark climates or for loads that must not stop, shorter if you have a generator or shore power as fallback.
Step 3 — Pick a chemistry and its usable depth
You can only plan on using part of a battery's rated capacity. LiFePO4 comfortably gives 80–90% usable depth of discharge; AGM and flooded lead-acid should be planned around 50%, gel around 60%. This is why a lead-acid bank must be nearly twice the nominal size of a LiFePO4 bank for the same job — often cancelling out its lower upfront price.
Step 4 — Run the numbers
The sizing formula is: daily draw from the battery × days of autonomy ÷ usable depth of discharge. With our example numbers — 1,333 Wh/day, 2 days, LiFePO4 at 90% — that gives 2,963 Wh nominal, which at 12 V is about 247 Ah, or three 100 Ah batteries.
The Battery Bank Sizing calculator (pre-filled with this example) does this calculation live, shows the formula it used, and lets you compare autonomy options and chemistries side by side.
Step 5 — Sanity-check the design
Three checks catch most expensive mistakes. First, system voltage: keep battery-side currents manageable (roughly, 12 V up to ~1,000 W of inverter, 24 V to ~3,000 W, 48 V above that) — the Inverter Sizing calculator shows the current your cables and fuses must handle. Second, charging: a bank you cannot refill in a reasonable time just cycles ever lower — the Solar Charge Time calculator tells you what your panels can actually put back per day. Third, runtime: check that the finished bank really carries your key loads long enough with the Battery Runtime calculator.
Common mistakes
Comparing banks by amp-hours across different voltages (compare watt-hours instead); planning lead-acid at 80% depth of discharge (it will age fast); forgetting inverter losses and idle draw; sizing for summer consumption but winter sun; and buying the battery before measuring the loads. Every one of these is cheaper to catch in a calculator than in hardware.
Frequently asked questions
How many days of autonomy do I need?
For solar systems, plan for 1–3 cloudy days depending on how critical the loads are and your climate. For backup systems, use the longest outage you realistically want to bridge.
Is a bigger battery bank always better?
No. An oversized lead-acid bank may never charge fully, which shortens its life, and any oversized bank ties up money. Size for real consumption with modest headroom instead.
Should I choose a 12 V, 24 V or 48 V system?
A common rule of thumb: up to about 1,000 W of inverter load 12 V is fine, up to about 3,000 W use 24 V, and above that 48 V keeps battery-side currents and cable sizes manageable.