Solar Battery Sizing Calculator
Size a solar battery bank from daily energy use, days of autonomy, depth of discharge, battery chemistry, inverter efficiency and temperature — amp-hours, kWh, series/parallel layout, how many batteries, real autonomy and the solar array to recharge it.
How it works
Rated Wh = daily Wh × days ÷ inverter efficiency × cold factor ÷ DoD · Ah = rated Wh ÷ bank voltage · series = bank V ÷ battery V, parallel = ⌈Ah ÷ battery Ah⌉ · array W ≈ daily Wh ÷ efficiency ÷ (sun hours × 0.75)
Start with the energy your loads use in a day and multiply by the days the bank must last without sun. Loads that run on AC pass through an inverter that wastes 5–15%, so the battery has to supply a bit more than the appliances use. Cold batteries hold less charge, so a bank that sits at freezing has to be larger — lead-acid loses much more than lithium. Finally, a battery should never be drained completely: dividing by the usable depth of discharge (about 80–90% for lithium, 50% for lead-acid) gives the rated capacity to buy. Dividing watt-hours by the bank voltage turns that into amp-hours. To build the bank, put enough batteries in series to reach the bank voltage and enough of those strings in parallel to reach the amp-hours; rounding up to whole batteries usually gives a little more autonomy than you asked for, which the calculator reports. The array estimate refills one day of use in the worst-month sun hours, allowing 25% for controller, temperature, dirt and wiring losses.
Worked example
A cabin uses 2,000 Wh a day and needs 2 days of autonomy on a 12 V LiFePO4 bank, with a 90%-efficient inverter and 80% depth of discharge. 2,000 × 2 = 4,000 Wh; ÷ 0.9 = 4,444 Wh from the battery; ÷ 0.8 = 5,556 Wh rated; ÷ 12 V = 463 Ah. With 12 V 100 Ah batteries that is 5 in parallel (500 Ah, 6.0 kWh), which actually lasts about 2.2 days. With 5 peak sun hours, about 600 W of panels refill a day's use.
Sources: U.S. Department of Energy — Off-grid or stand-alone renewable energy systems · NREL — PVWatts calculator (peak sun hours for your location)
Frequently asked questions
What depth of discharge should I use?
LiFePO4 and NMC lithium are usually designed for 80–90%, which still gives thousands of cycles. Lead-acid (flooded, AGM, gel) should be sized for about 50% if it cycles daily; going deeper cuts its life sharply. The table under the results shows how much bigger a lead-acid bank must be for the same job.
What are days of autonomy?
The number of days the bank must run your loads with no solar input. One day is enough for grid backup or overnight use in a sunny place; 2–3 days is common off-grid; 5 or more suits critical loads in cloudy winters. Each extra day adds a full day of storage, so a generator is often cheaper than a very large bank.
Should I size in Ah or kWh?
kWh is the fair comparison because it includes the voltage: 100 Ah at 12 V (1.2 kWh) stores a quarter of 100 Ah at 48 V (4.8 kWh). Use amp-hours only when comparing batteries of the same voltage.
Why does temperature matter?
Battery capacity is rated at 25 °C (77 °F). At freezing, lead-acid delivers only about 70% of that and lithium roughly 90%, so a cold bank must be bigger to give the same runtime. Most LiFePO4 batteries also must not be charged below 0 °C unless they have built-in heaters.
Which bank voltage should I choose?
Higher voltage means lower current for the same power, so thinner cables and less loss. A rough rule is 12 V up to about 1 kW of inverter, 24 V up to about 3 kW, and 48 V above that — most whole-home systems are 48 V.
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This calculator is for educational purposes. Double-check important results before acting on them.