Solar storage battery capacity: how to calculate it
Sizing a solar storage battery is arithmetic, not guesswork. This article works through the calculation in general terms, with examples a buyer can repeat for any system.
Layered pack cross-section — the site's recurring diagram motif. It is a schematic, not a photograph of a product, factory or customer project, and it is not presented as a photograph of this company's factory.
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Step one: the daily load in watt-hours
Add up everything the battery must power overnight, or during a sunless period, in watt-hours. A light drawing 10W for 5 hours needs 50 watt-hours. A refrigerator drawing 100W for 8 hours needs 800 watt-hours. A set of communications equipment drawing 20W continuously needs 480 watt-hours a day. Add the loads together; the total is the daily energy the battery must deliver, and it is the only honest starting point for sizing. A buyer who sizes from a single appliance, or from a round capacity figure, has not yet sized the system.
Losses belong in the same step. Inverters, wiring and the battery itself lose energy, so the battery must store more than the loads consume. A planning allowance of 15% to 20% for losses is a reasonable general starting point, although the exact figure depends on the inverter, the cable runs and the battery's efficiency. A buyer who ignores losses will find the battery empty before the night ends, whatever its label capacity says.
Step two: from watt-hours to amp-hours
Divide the daily energy need by the pack's nominal voltage to get amp-hours. A daily need of 1200 watt-hours at 12V nominal needs 100 amp-hours; the same need at 24V needs 50 amp-hours; at 48V it needs 25 amp-hours. That is why amp-hours alone do not size a system: a 100Ah pack at 12V stores about 1200 watt-hours, while a 100Ah pack at 48V stores about 4800 watt-hours, and the two are not comparable purchases. Always convert to watt-hours before comparing packs at different voltages.
The detail packs on this site illustrate the conversion. A 12V 100Ah pack stores about 1200 watt-hours at 12V nominal, or about 1280 watt-hours at 12.8V nominal for a LiFePO4 pack of that class. A 12V 200Ah pack stores about twice that. A 48V 15Ah pack stores about 720 watt-hours. Those are arithmetic conversions from the listings' voltage and capacity figures, not separate claims, and a buyer can repeat them for any pack on this site.
Step three: depth of discharge and autonomy
Depth of discharge is how far the battery is drained in normal use, and it affects both sizing and life. A pack routinely drained to empty ages faster than one drained to half, and a buyer should size for the depth of discharge the application actually uses rather than the pack's full capacity. If the system needs 1200 watt-hours a night and the pack is drained to 80% depth, the pack must store 1500 watt-hours, not 1200. The cycle-life figures on listings assume stated depths, and a buyer should ask what depth applies to the exact listing's figure.
Autonomy is how many sunless days the pack must bridge. A grid-connected backup system may need one night; a fully off-grid system may need two or three days. Multiply the daily need by the autonomy days, after allowing for depth of discharge and losses, and the result is the pack's required energy. A buyer who sizes for one day and needs three will be disappointed on the second cloudy day, whatever the pack's quality.
Step four: charger and system matching
The charger must match the pack's chemistry and voltage, and the solar charge controller, the inverter and the charger must agree on voltage and chemistry. The 14.6V 40A charger on this site, for example, is made for 12V LiFePO4 packs in 4-series configuration, and its charging-time reference, as listed, is approximately 3 hours for a 12V 100Ah battery and 5.5 hours for a 12V 200Ah battery, with longer times for larger capacities. A buyer should confirm the whole chain — panels, controller, battery, inverter, charger — before ordering, because a pack that matches the load but not the charger will not charge correctly.
Finally, confirm the physical pack against the space and the mounting. A 12V 200Ah pack at 521 x 238 x 217 mm and 23.5 kg needs a planned mounting, not a casual shelf, and a home storage system at higher voltages needs more. Measure the space, allow for cables and ventilation, and confirm the exact listing's dimensions before ordering. A correctly calculated capacity in a pack that does not fit is still a failed order.
Worked example: a small off-grid cabin
A worked example makes the arithmetic concrete. Consider a small off-grid cabin with four loads overnight: LED lighting drawing 30W for 5 hours, a small refrigerator drawing 60W for 8 hours, communications equipment drawing 15W continuously, and a water pump drawing 100W for 30 minutes. The lighting needs 150 watt-hours, the refrigerator 480, the communications 360 and the pump 50, for a daily total of 1040 watt-hours. Allowing 20% for inverter, wiring and battery losses brings the stored-energy need to about 1250 watt-hours for one night.
At 12V nominal, 1250 watt-hours needs about 104 amp-hours; at 12.8V nominal for a LiFePO4 pack, about 98 amp-hours. A 12V 100Ah pack, storing about 1200 watt-hours at 12V nominal, is therefore close for one night but short once depth of discharge is considered: at 80% depth, the pack must store about 1560 watt-hours, needing about 130 amp-hours at 12V, so a single 100Ah pack is undersized for a full night at that depth, and a 200Ah pack, storing about 2400 watt-hours, covers the night with margin. That margin is not waste: it is the difference between a system that works on a cloudy morning and one that cuts out before morning.
For two days of autonomy — the cabin's occupants want power through a sunless day as well — the need doubles to about 2500 watt-hours of stored energy after losses, or about 3120 watt-hours at 80% depth, needing about 260 amp-hours at 12V. Two 12V 200Ah packs in parallel, or a larger 24V or 48V system, would serve, and the choice between them follows from the inverter and charger available at each voltage. A 48V system at the same energy needs only about 65 amp-hours, with thinner cables and lower losses, which is why larger off-grid systems usually move to 48V even though the battery arithmetic is the same.
The charger check closes the example. A 12V 100Ah pack charged by the 14.6V 40A charger on this site takes approximately 3 hours by its reference table, or 2.5 hours by its description, and a 200Ah pack takes about 5.5 hours by the table or 5 hours by the description; a buyer planning the solar array should confirm which figure applies and size the array's daily output to replace the night's consumption plus losses. A system sized this way — load, losses, depth, autonomy, charger — will work; a system sized on a round capacity figure may or may not, and the difference will appear on the first cloudy morning.
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Send one message with your device type, the space the pack must fit, the voltage and capacity you need, the peak current if it matters, and the model description if a page on this site already matches. Add the documents you need — certificates are available upon request — and whether you need a listed model or a custom design. Use the contact page or write to [email protected], or call +86-755-25629920. A specific message like that is what turns a range of hundreds of models into a short, answerable question.