Solar Battery Calculator

A solar battery calculator sizes a bank from three numbers: daily watt-hours × days of autonomy, divided by the share of the pack you can safely use. Use 2,000 Wh a day, want two days without useful sun, hold the pack to 80%, and you need a 5,000 Wh bank — 417 amp hours at 12V.

The formula, term by term

Four inputs, and every one of them is a decision rather than a lookup. The arithmetic is trivial. Choosing the inputs honestly is the work.

• Daily use, in watt-hours. The energy that leaves the battery over 24 hours, which is not the wattage printed on your appliances. Add up each load as watts × the hours it genuinely runs. A 150W fridge compressor that cycles on for a third of the day is about 1,200 Wh, not 3,600 — the refrigerator wattage guide works that one through.
• Days of autonomy. How long the bank has to carry the load with no useful charge coming in. One day is a fair-weather number, two is the working default for a van or a boat, and three or more is what a cabin in a cloudy winter needs.
• Usable depth of discharge. The share of the nameplate capacity you are willing to take out. Lithium iron phosphate is commonly specified at 80–90%. Flooded lead-acid should be held to 50% if you want it to last more than a couple of seasons.
• System voltage. 12V for a van or a single-battery setup, 24V for larger mobile installs, 48V for cabins and house-scale storage. Voltage does not change the energy you need. It only changes the amp-hour number that same energy is quoted in.

Run those through Ah = (daily Wh × days) ÷ (volts × depth of discharge) and you have a bank size. Everything after that is deciding how much you do not trust it.

Battery bank size quick-reference table

Every row assumes two days of autonomy and 80% usable depth of discharge, with daily use measured as energy drawn out of the battery. The bank column is nameplate capacity; the three amp-hour columns are that same bank expressed at each common system voltage.

The table rescales cleanly, so you do not need a second one. For a single day of autonomy, halve every result. For three days, multiply by 1.5. To hold a lead-acid bank at 50% instead of 80%, multiply by 1.6. And read the voltage columns carefully: the same 5,000 Wh bank is 417 Ah at 12V or 104 Ah at 48V, which is why an amp-hour figure quoted without its voltage tells you nothing at all.

Why the battery number comes first

Battery capacity is the first domino in a solar design. The array, the charge controller, the cable and the inverter are all sized downstream of it, so an error here does not stay put — it propagates into every other line of the build.

It is also the number the market quotes in three incompatible units. Power stations are sold in watt-hours, RV and marine batteries in amp hours, and home storage in kilowatt-hours, which is how the same bank ends up looking like three different products. The watt-hours to amp hours calculator moves between the first two, and dividing watt-hours by 1,000 gets you the third.

Two examples of how the answer lands in practice. A van running lights, a water pump, a 12V fridge and laptop charging at roughly 1,000 Wh a day wants 208 Ah at 12V for two days; two 100Ah lithium batteries give 2,400 Wh nameplate and 1,920 Wh usable, about 4% short of the 2,000 Wh target, which is either close enough or a good reason to buy the third. A cabin at 4,000 Wh a day with three days of autonomy needs 15,000 Wh, or 313 Ah at 48V, which is clearly a rack system rather than anything portable.

Then the array has to put it all back. As a rough rule, divide daily watt-hours by your peak sun hours and then by about 0.75 for controller, wiring and heat losses: 2,000 Wh a day in four peak sun hours needs roughly 670W of panel, and closer to double that through a northern winter. The solar sizing guide works the panel side properly, including the tilt that moves those sun hours around.

Below about 3,000 Wh, a packaged unit usually beats a self-built bank once you have priced the cable, fusing and controller — browse solar generators, or check your figure against what size power station you need. Above that, and for any permanent home or RV install, a 48V bank is cheaper per watt-hour and far kinder to your cable budget, because four times the voltage is a quarter of the current.

Depth of discharge, chemistry by chemistry

Depth of discharge moves the answer more than any other input, and chemistry sets it rather than preference. The table below sizes one fixed load — 2,000 Wh a day for two days, so 4,000 Wh of energy you actually have to be able to spend — at each realistic setting.

The lead-acid row is the clearest argument for lithium anyone has written. To store the same 4,000 Wh you can actually spend, a flooded bank has to be 60% larger and correspondingly heavier than the lithium equivalent, and it will not accept charge fast enough to refill itself in the four useful hours of a winter day.

Do not read the 90% row as free capacity, either. That rating assumes a moderate temperature and a moderate discharge current, and the battery management system often cuts off before the datasheet number does. Sizing at 80% and being pleasantly surprised is much the cheaper error.

What this solar battery calculator leaves out

The formula gives you a clean starting figure. Four real-world losses sit outside it, and together they are worth 20–40% on a typical build.

• Inverter losses. If you measured your daily figure at an AC outlet, the battery has to supply more than that. At 90% inverter efficiency a 2,000 Wh AC day is about 2,222 Wh out of the battery, which pushes the two-day 12V answer from 417 Ah to roughly 463 Ah. DC loads skip the tax entirely, which is exactly why DC-native van builds get away with smaller banks.
• Idle consumption. An inverter left switched on with nothing plugged into it still draws something like 5–20W, or 120–480 Wh a day. On a small system that is a quarter of the entire budget, spent on nothing. Switch it off, or put it on the load list.
• Temperature. Usable capacity falls as it gets colder, and most lithium battery management systems block charging below freezing outright. For a bank living in an unheated garage or a van in winter, add about 20% or specify a self-heating pack.
• Age. A pack that holds its rated capacity in year one will not in year eight. Build the headroom in now, because adding a mismatched battery to an aged bank later is worse than buying one size up today.

One thing the capacity figure says nothing about is the inverter rating. Capacity answers “how long”; continuous watts answer “how much at once”, and the two are independent of each other. A 5,000 Wh bank behind a 1,000W inverter still cannot start a well pump. Size the battery on daily energy, and size the inverter on your largest simultaneous load plus whatever it surges to at startup.

Frequently asked questions