Solar Battery Bank Size Calculator

How to use the solar battery bank size calculator

Enter the daily electricity the bank has to supply in kilowatt-hours, the days of autonomy you want, the system voltage the bank runs at, and the usable depth of discharge, and the tool returns the bank size in amp-hours plus the same capacity expressed in watt-hours and kilowatt-hours. Amp-hours leads because that is how batteries are sold, labelled and wired together. The watt-hour figure is the one to trust when comparing options, because it is independent of system voltage and therefore the only fair basis for judging two different bank architectures against each other. Both numbers describe gross nameplate capacity, already scaled up so that the slice you actually use covers your energy need.

Start with the daily usage, and be careful about which loads you count. This is the energy the battery must carry, not the energy your solar array produces and not necessarily your whole household consumption. For a grid-tied backup you might only be sizing for a fridge, a furnace fan, a well pump and some lighting, which could be two or three kilowatt-hours a day. For a full off-grid cabin it is everything, which is where figures around 10 kWh a day come from. If you do not have a number yet, add up each load wattage multiplied by the hours it runs, then divide by 1,000. The battery watt-hours calculator handles the reverse direction when you want to check what a specific battery holds.

Days of autonomy is the input people underestimate, and it is a straight multiplier on the answer. It is how long the bank must carry the loads with no meaningful charge arriving: a run of overcast days when the array barely produces, or a grid outage you have to ride out entirely on stored energy. One day is the common default for grid-tied backup where an outage is measured in hours. Off-grid systems in cloudier climates are frequently sized for two or three, because a single genuinely dark stretch can otherwise empty the bank and leave you with nothing. Every extra day multiplies the bank proportionally, so moving from one day to three triples both the cost and the physical space the batteries occupy.

Depth of discharge is the other big lever and it is set entirely by chemistry. Lead-acid banks are conventionally sized around 50 percent usable, and Trojan recommends discharging its deep-cycle batteries only 20 to 50 percent of rated capacity for optimum life. LiFePO4 is designed for far deeper cycling, with Battle Born publishing 3,000 to 5,000 cycles at 80 percent depth of discharge. Because you divide by that fraction, the same energy need produces a bank roughly 1.6 times larger on lead-acid than on lithium. System voltage, meanwhile, changes only the amp-hours and not the energy: a 48 V bank holds the same watt-hours as a 12 V one using a quarter of the amp-hours, which is why larger systems run at 48 V and move the same power at lower current.

The formula

The bank has to hold a day of energy multiplied by your days of autonomy, then be scaled up so that the fraction you are willing to use covers that need, then converted into amp-hours at whatever voltage the bank runs. Daily kilowatt-hours are multiplied by 1,000 first, so the units line up with the volts and amp-hours batteries are actually rated in rather than the kilowatt-hours your bills are.

Two of the three terms are straight multipliers and behave predictably: double the autonomy and the bank doubles, halve the depth of discharge and the bank doubles again. The voltage term is different in character because it changes only how the same energy is counted. Watt-hours stay put while amp-hours scale inversely with voltage, which is the entire reason large systems move to 48 V.

needed Wh = daily kWh × 1000 × days of autonomy
bank Wh = needed Wh ÷ usable depth of discharge
bank Ah = bank Wh ÷ system voltage
10 × 1000 × 1 ÷ 0.80 ÷ 48 = 260 Ah

Worked example with the defaults: 10 kWh of daily use, one day of autonomy, 80 percent usable, at 48 V. The energy needed is 10 × 1,000 × 1 = 10,000 Wh. Dividing by 0.80 gives 12,500 Wh, or 12.5 kWh, of gross bank capacity, because you only intend to draw four-fifths of it. Dividing 12,500 by 48 V gives 260.4 Ah, so a 48 V bank of roughly 260 Ah covers one cloudy day of a 10 kWh load. The same 12,500 Wh is 520.8 Ah at 24 V and 1,041.7 Ah at 12 V.

Move each lever separately to see its weight. Going from one day of autonomy to two doubles the bank to 25 kWh and about 521 Ah at 48 V. Switching from 80 percent lithium to 50 percent lead-acid at one day of autonomy raises the bank from 12,500 Wh to 20,000 Wh, about 417 Ah at 48 V, a 60 percent increase in capacity you have to buy and house for identical delivered energy. Do both at once, three days of autonomy on lead-acid, and the 10 kWh daily load needs 60,000 Wh of bank, which is 1,250 Ah at 48 V.

Several real-world effects sit outside this arithmetic. Round-trip charging is not free, so more energy has to go in than comes out, and the shortfall is larger on lead-acid than lithium. Cold reduces available capacity on every chemistry, and lithium battery management systems restrict charging below freezing. Lead-acid also delivers less than its rating when discharged quickly, an effect described by Peukert law. Finally, this sizes storage only: the solar panel output calculator answers how much production you need to refill the bank each day, and a licensed installer should size and sign off anything permanently wired in.

Frequently asked questions