Solar battery size calculator

By the SmartSolarSizer team · Published 21 October 2026

How the calculator works

Three numbers do most of the work, and they're the ones that simple backup formulas leave out.

Usable share. You can't empty a battery every night without shortening its life. Lithium iron phosphate handles deep cycles well: Victron rates its LiFePO4 batteries at 2,500 cycles at 80% depth of discharge and 5,000 at 50% (Victron datasheet). We default to 90%. Lead-acid is different. A deep-cycle battery can go past 50% and up to 80% (PVEducation), but 50% is a common design limit to make the bank last, so that's our default.

Efficiency. Energy is lost in the inverter and in the battery. Victron puts LiFePO4 round-trip efficiency at 92%, while lead-acid manages about 70% (PVEducation). On the way out of the battery we allow 90% for lithium and 80% for lead-acid, the same figures our main calculator uses. That's at the hopeful end for lithium: Victron's Phoenix 12/800 and 12/1200 inverters peak at 91% and 92% (Victron Phoenix datasheet), and an inverter rarely runs at its peak all night, so lower the figure a few points for small loads. Change either if your equipment's datasheet says otherwise.

Standby draw. An inverter uses power just by being switched on, even with nothing plugged in. The Phoenix 12/800 draws 6 W and the 12/1200 8 W at zero load (Victron); a larger MultiPlus-II 48/3000 inverter-charger draws 13 W, or 3 W in search mode (spec sheet). Over a 12-hour night, 10 W is 0.12 kWh, which matters for a small battery running a small load. The calculator assumes 10 W. Set it to 0 for 12 V DC loads such as a DC fridge, LED lights or a router run straight from the battery with no inverter, and use the efficiency box for any DC-DC converter.

Two smaller effects only apply to lead-acid, and the calculator includes both. Its capacity is rated over 20 hours, so it gives less when you drain it faster: Trojan's T1275 Plus is a 150 Ah battery over 20 hours but 120 Ah over 5 (Trojan datasheet). Over the 8-hour example above it gives about 86% of its label. We fit that curve to Trojan's 5-hour and 20-hour ratings and stop it at 1 hour, so below about 5 hours it's an estimate beyond the rated points. It also loses about 1% of capacity for every degree below 20 °C (PVEducation).

How long will a 100Ah or 200Ah battery last?

It depends on the load, the chemistry and the voltage, which is why a single "X hours" answer can't be right for every setup. At 12 V, a 100 Ah battery holds 1.2 kWh (1.28 kWh for a 12.8 V lithium one). Here's how long each lasts at a steady load, with the defaults above:

Lithium: 90% usable, 90% efficiency. Lead-acid: 50% usable, 80% efficiency. Both include 10 W of inverter standby. Lead-acid capacity is adjusted for the discharge time (short runtimes are estimates). Real runtimes vary with battery age and temperature.

The lead-acid column falls away faster at high loads. At 1,000 W a 12 V battery is being drained in a couple of hours, well below the 20-hour rating, so it can't give its full label capacity. A 1,000 W load at 12 V also pulls around 93 amps, which is why bigger systems move to 24 or 48 V.

How long will a 200Ah battery run a fridge, TV, fan or AC?

The fridge figure uses its average draw over a day, from its energy label, because the compressor cycles on and off. The AC is the warning case: a 1.5 ton unit at full load would drain a 12.8 V 200 Ah lithium battery in about 1.3 hours, pulling about 140 amps. For load-shedding in Pakistan or India, that's why an AC needs a large 48 V bank, while a fridge, fans and lights run on a couple of batteries. Food in a closed fridge stays safe for about 4 hours without any power at all (USDA), so short cuts don't need a battery for the fridge alone. All rows include 10 W of inverter standby; a 12 V DC fridge needs no inverter and runs a little longer.

How many batteries do I need for a 3kW, 5kW, 10kW or 20kW solar system?

Battery size follows what you run when the sun is down, not the size of the array. A 5 kW system for a home that's empty all day needs a bigger battery than the same system on a home that uses most of its power at noon. Start from your night-time load in the calculator above.

The table gives the upper limit: the battery it would take to store everything the array makes on an average day, in 51.2 V 100 Ah lithium modules (5.12 kWh each, a common size). A 5 kW system in Lahore makes about 22 kWh a day, so storing every bit of it would take 6 modules. Most homes need a fraction of that.

Output from PVGIS (1 kW of panels, 14% system losses). Modules counted at 90% usable and 90% efficiency.

How many batteries for a 2,000–5,000 W or 3, 5 or 6 kVA inverter?

The inverter's rating doesn't decide how much battery you need; your load and hours do. What it does decide is how much current the battery must supply, and how many batteries you wire in series. First, read the inverter's battery voltage on its label. A 24 V inverter needs two 12 V batteries in series, and a 48 V inverter needs four (or four 12.8 V lithium batteries, which make 51.2 V). So if your 5 kVA inverter is a 48 V model, it needs four 12 V batteries, and those four set the voltage, not the backup time. An inverter rated in kVA can deliver fewer watts than its kVA figure, so check the watt rating on the label.

Tubular inverter batteries, common in Pakistan and India, are flooded lead-acid, so use the lead-acid setting in the calculator.

Second, check the current. At full load the inverter pulls this much from the battery:

Assumes 90% inverter efficiency.

At 12 V, anything over 2,000 W means very heavy cables and batteries working near their limits. Check each battery's maximum continuous discharge current on its datasheet and add batteries in parallel until the bank can supply the current as well as the energy.

Series and parallel: getting the voltage and the Ah right

Batteries in series add their voltage and keep the same Ah: two 12 V 200 Ah batteries make 24 V 200 Ah. Batteries in parallel keep the voltage and add the Ah: two of them make 12 V 400 Ah. Either way the energy is the same, 4.8 kWh. A 48 V bank of 12 V 200 Ah batteries is built as strings of four in series, with as many strings in parallel as the energy needs. The calculator shows that arrangement under "Battery bank". Use identical batteries of the same age in one bank.

kWh to Ah

For reference, a Tesla Powerwall 3 is rated at 13.5 kWh (Tesla datasheet), the last row of the table.

Lithium or lead-acid?

Lithium iron phosphate usually costs more upfront, so compare prices per kWh you can actually use: you can use most of a lithium battery, it wastes less energy and it lasts thousands of cycles. For the same 500 W, 8-hour job above, the lead-acid bank needs about 2.3 times the capacity: 6 × 12 V 200 Ah batteries against 2 × 12.8 V 200 Ah lithium ones at 24 V. Lead-acid still makes sense where it's much cheaper locally, for backup that's rarely used, or where batteries can't be kept above freezing in winter for charging: Victron's LiFePO4 batteries discharge down to −20 °C but only charge above +5 °C (Victron). Lead-acid charges in the cold, but Trojan advises keeping it above 60% charge below 0 °C (Trojan).

How many solar panels to charge a 100Ah battery?

Recharging a 12.8 V 100 Ah lithium battery from 20% to full needs about 1.07 kWh: the 1.02 kWh it gave out plus about 4% lost while charging (the charging half of its 92% round-trip efficiency). Compare that with what one panel makes in a day:

One 430 W panel covers it on an average day in all four places. A 200 W panel falls short even in Phoenix (1.00 kWh), so it refills about 90% a day there. In a dark northern month you need more panels, or accept that the battery won't be full every day. Your charge controller has to match the battery voltage and handle the panel's current; see the appliance guides for whole-system examples.

Common questions

Sources

Solar output figures come from the European Commission's PVGIS 5.3, retrieved 20 October 2026.

• Victron Energy LiFePO4 Smart battery datasheet
• Trojan T1275 Plus datasheet
• PVEducation: lead acid batteries
• Victron Phoenix inverter 12/800–12/1200 datasheet
• Victron MultiPlus-II 48/3000 specifications
• Tesla Powerwall 3 datasheet
• USDA FSIS: food safety in emergencies

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