Solar Battery Size Calculator

How the battery calculation works

Battery capacity is quoted in amp-hours, but what you actually consume is watt-hours. The conversion runs through your system voltage, and the amount you can safely take out runs through depth of discharge:

Amp-hours = (daily Wh × days of backup) ÷ (system volts × depth of discharge)

Depth of discharge is where most online figures quietly mislead. A 100 Ah battery does not give you 100 Ah. LiFePO4 gives about 80% of its rating in regular use; AGM and flooded lead-acid give about 50%. That single difference is why a lithium bank half the rated size often outperforms a lead-acid one.

A worked example

Take a common setup: a 45 W compressor fridge running 10 hours, a 25 W roof fan for 8, 15 W of lights for 4, a 65 W laptop for 4, and 10 W of phone charging for 3. That is 1,000 Wh a day.

With two days of backup at 12V on LiFePO4: 1,000 × 2 ÷ (12 × 0.8) = about 209 Ah. On AGM the same requirement needs 334 Ah — and roughly three times the weight.

Common bank sizes, and what they actually cover

The amp-hour figure is easier to judge against real builds than in isolation. These are LiFePO4 banks at 12V with two days of autonomy:

Bank	Usable energy	Supports about
100 Ah	960 Wh	480 Wh/day. Fridge, fan, lights, phones. Weekend use.
200 Ah	1,920 Wh	960 Wh/day. Adds a laptop and a router. Part-time.
300 Ah	2,880 Wh	1,440 Wh/day. Full-time with Starlink and occasional induction.
400 Ah	3,840 Wh	1,920 Wh/day. Heavy electric cooking, or genuine winter autonomy.

Notice the pattern: the usable daily figure is roughly half the stored energy, because you are carrying two days of it. Asking for three days instead of two raises the bank by 50% for the same appliances, which is usually more expensive than adding the solar to refill a smaller bank faster.

Four things that change the answer

Days of autonomy. This is the single biggest lever, and it is a judgement call rather than a calculation. Two days suits most travel: it covers an overcast day without leaving you anxious. Full-timers in northern winters often want three. If you drive most days and run a DC-DC charger, one can be enough, because the alternator refills the bank whether the sun cooperates or not.

Chemistry. Switching this calculator from LiFePO4 to AGM raises the required amp-hours by 60%, because usable depth falls from 80% to 50%. The weight difference is larger still. A 200 Ah LiFePO4 bank weighs roughly 25 kg; the 320 Ah of AGM needed to match it is well over 80 kg, which is real payload in a van that is already close to its limit.

Cold. The freezing-weather option applies a 0.8 derate, which is the honest planning figure rather than a worst case. The harder constraint is not capacity but charging: LiFePO4 must not be charged below 0 °C at all without an internal heater or a warmed compartment. A bank you cannot recharge is a bank you are slowly emptying.

System voltage. Moving to 24V halves the amp-hour figure for the same stored energy, because amp-hours are energy divided by voltage. The stored watt-hours are identical. This does not make the battery cheaper, but it does make every cable in the system thinner.

Where people get this wrong

Sizing the battery before measuring the load. The daily watt-hour figure drives everything. A bank chosen from a forum recommendation is a guess about someone else's fridge.

Treating the rated capacity as usable. A 100 Ah battery is not 100 Ah of usable energy in any chemistry. This is the most common reason a build that looked adequate on paper runs out overnight.

Buying battery instead of solar. A bigger bank stores more but refills no faster. If you are consistently arriving at morning with an empty bank, the array is undersized, not the battery.

What this calculator does not decide

The bank is only one part of the system. You still need enough solar to refill it, a charge controller sized to the array, and cable and fuses rated for the current. The full calculator sizes all of those together, and the method behind every figure is written out in full.

If you want the reasoning rather than the tool, the amp-hours guide works through the table by daily load, and LiFePO4 vs AGM sizing compares the two chemistries directly.