Solar Battery Backup Calculator
Choose what has to stay on and for how long, and this works out the bank you need – then names the cheapest way to build it from each battery type, using real prices from our catalogue. The number it gives you is usable capacity, which is not the number printed on the battery.
What battery backup do you need?
Pick what has to stay on, and for how long.
Pick what you need to run and for how long.
How the sizing works
Three steps, and the third is the one most people skip.
First, your load. Add up what has to keep running – every appliance multiplied by its wattage. A ceiling fan is about 75 watts, a fridge averages around 150 watts across its duty cycle, a 1-ton inverter AC roughly 900. Multiply that total by the hours you need it for, and you have the energy in kilowatt-hours.
Second, the losses. Energy does not come out of a battery for free. Inverter conversion and the battery’s own round-trip losses cost you around 12%, so the bank has to store more than your appliances will consume.Bank size (kWh usable) = (watts × hours ÷ 1000) ÷ 0.88
Third, and this is the step that catches people out: the result is usable capacity, not nameplate capacity. A battery rated at 2.4 kWh does not give you 2.4 kWh.
Usable capacity is not the number on the box
Every battery has a depth of discharge – how far you can safely draw it down before you start destroying it. Draw a lead-acid battery flat and you will not get many cycles out of it.
A 200Ah 12V battery is 2.4 kWh on paper. What you actually get depends entirely on chemistry:
Chemistry | Typical usable depth | From a 2.4 kWh battery |
|---|---|---|
Lithium (LiFePO4) | 90–95% | ~2.2 kWh |
AGM / VRLA | ~50% | ~1.2 kWh |
Tall tubular | ~50% | ~1.2 kWh |
Flat plate lead-acid | ~50% | ~1.2 kWh |
So two batteries with identical nameplate ratings can differ by nearly half in what they actually deliver. This is why comparing batteries on Ah, or on nominal kWh, or on sticker price tells you very little.
Every figure in the calculator above is usable capacity. The depth of discharge is already applied, per battery, from its own specification — not from an assumption.
Why lead-acid looks cheaper, and when it isn’t
On upfront cost per usable kilowatt-hour, lead-acid genuinely wins. That is not marketing – it is what the prices in our catalogue show, and the calculator will tell you so.
But upfront cost is not the whole number, for three reasons.
Cycle life. A lead-acid battery is typically quoted at several hundred to around fifteen hundred cycles. Lithium iron phosphate is usually quoted in the thousands. If you are cycling the bank every day through load shedding, the cheaper battery may need replacing two or three times over the life of the expensive one – and the second replacement is at whatever prices are by then.
Capacity ratings assume a slow discharge. Lead-acid capacity is normally rated over a 20-hour discharge. Pull the same battery hard over four hours to run a fridge and an AC, and you will get meaningfully less out of it than the rating suggests. Lithium is far less sensitive to this. If your bank is carrying heavy appliances, size the lead-acid option up.
Physical count. A 20 kWh bank built from 1 kWh lead-acid batteries is twenty batteries – the floor space, the cabling, the terminal count and the maintenance are all real costs that do not appear on a price list. This is why the calculator will not recommend a bank of more than twelve units, and why large backup requirements come back as lithium only.
What this calculator does not cover
It sizes the energy. It does not design the system, and four things still need checking before you buy.
Surge and starting loads. An air conditioner compressor or a water pump draws several times its running wattage for the first moment it starts. Your bank and your inverter must supply that peak current, not just the average. A bank that is correct on energy can still trip on startup.
Inverter compatibility. The battery has to match your inverter’s voltage – 12V, 24V, 48V or high voltage – and the inverter has to support the charge and discharge currents involved. A lithium rack also needs the inverter to speak its communication protocol, or you lose the battery management integration.
Recharging. A bank you cannot refill is a bank you use once. Your array and your grid charging window together have to put back what you took out, every day, or the bank runs down over a week of heavy load shedding.
Temperature and siting. Both chemistries lose capacity when cold and age faster when hot. Lead-acid needs ventilation. Neither belongs in an unventilated cupboard in a Multan summer.
Next step
Once you know the bank size, the question becomes which battery – and the spread between the cheapest and the most expensive route to the same usable capacity is wider than most buyers expect.