Solar Battery Size Calculator
Solar Battery Size Calculator
The battery bank an off-grid or backup system needs: from the daily load in watt-hours, the days of autonomy you want, the usable depth of discharge, the bank voltage and the inverter’s efficiency.
Battery bank size
3,000 Wh a day, 1 day of autonomy, 50% usable, 24 V bank, 85% inverter, 150 Ah batteries
Bank capacity from the daily load
- E
- AC energy used in a day, in Wh
- days
- days of autonomy — how long the bank must last without charge
- DoD
- usable share of the rating (0.5 = 50%)
- η
- inverter efficiency, because the AC energy comes through it
- k
- temperature capacity factor, 1.0 at about 25 °C
Worked example
3,000 Wh a day, 1 day of autonomy, 50% usable, 24 V bank, 85% inverter, 150 Ah batteries
Ah = 3,000 ÷ (24 × 0.50 × 0.85 × 1.00) = 294.1 Ah
At 150 Ah each: 2 strings in parallel, 2 in series for 24 V = 4 batteries
Installed bank 300 Ah = 7,200 Wh, of which 3,060 Wh reaches the AC side
That is 1.02 days of the stated load
Bank for 3,000 Wh a day, one day of autonomy, 24 V, 85% inverter
| Chemistry and usable depth | Bank needed | Strings | Total batteries |
|---|---|---|---|
| Lead-acid flat plate, 50% usable | 294 Ah | 2 × 150 Ah in parallel | 4 batteries at 24 V |
| Tubular lead-acid, 60% usable | 245 Ah | 2 × 150 Ah in parallel | 4 batteries at 24 V |
| LFP lithium, 80% usable | 184 Ah | 2 × 150 Ah in parallel | 4 batteries at 24 V |
Sizing a solar battery bank
A battery bank is sized from energy, not from watts. Take the watt-hours the house uses in a day, multiply by the days you want it to last without charging, and divide by everything that stops you getting them back out: the bank voltage, the share of the rating you are willing to use, the inverter’s efficiency, and any temperature derate. For 3,000 Wh a day at 24 V, 50% usable and an 85% inverter, that is 294.1 Ah — 2 strings of 150 Ah batteries, 2 in series for the bank voltage, 4 batteries in all.
Series and parallel. Batteries in series add voltage and keep the amp-hours; in parallel they add amp-hours and keep the voltage. Four 12 V 150 Ah batteries can be a 48 V 150 Ah bank or a 24 V 300 Ah bank — the same 7.2 kWh either way. Prefer series: higher voltage means lower current, thinner cable and smaller losses. More than about four parallel strings is hard to keep balanced, and a weak battery in one string drags the others down.
Depth of discharge is the expensive number. Lead-acid life falls steeply with depth: the convention is to use half of a flat-plate battery and about 60% of a tubular one, and Victron’s handbook notes that ageing advances disproportionately fast beyond roughly 80%. LFP lithium is routinely used to 80%, which is why a lithium bank of the same usable energy is so much smaller. Cold reduces the capacity that is available at all — use your battery’s cold-temperature multiplier in the temperature factor — while heat, the usual condition in India and the Gulf, shortens life rather than capacity. IEEE 1013 is the formal method for lead-acid sizing if you need one.
Autonomy. One day is the normal choice for grid backup: the grid returns, the charger refills the bank. Off-grid, two to three days is usual, because the array must both run the house and refill the bank after a cloudy spell — size that array with the solar panel size calculator, the controller with the solar charge controller calculator, and check the backup hours for a particular load with the inverter backup time calculator.
Frequently asked questions
What size battery bank do I need for 3kWh a day?
About 294 Ah at 24 V with lead-acid used to 50% and an 85% inverter — 4 × 150 Ah batteries. With LFP lithium at 80% usable it drops to 184 Ah.
How do I calculate battery bank size for solar?
Daily watt-hours × days of autonomy ÷ (bank voltage × depth of discharge × inverter efficiency × temperature factor). Round up to whole batteries.
How many days of autonomy should I design for?
One day for grid backup, two to three days for an off-grid house. More autonomy means a bigger bank that spends most of its life full, which is expensive but kind to lead-acid.
Should the bank be 12 V, 24 V or 48 V?
Match it to the inverter and the power: 12 V below about 1 kW, 24 V to a few kW, 48 V above that. Higher voltage means lower current and much easier cabling.
Why does the inverter’s efficiency come into battery sizing?
Because the energy figure is AC energy at the socket. At 85% efficiency, 3,000 Wh of AC needs about 3,529 Wh out of the battery.
Related calculators
References
- IEEE Std 1013-2019. IEEE Recommended Practice for Sizing Lead-Acid Batteries for Stand-Alone Photovoltaic (PV) Systems.
- Vader R. Energy Unlimited. Victron Energy. Charge efficiency “will range in between 80% and 95%”; battery ageing advances “disproportionately fast” once roughly 80% depth of discharge is exceeded.
- Rolls Battery (Surrette). Flooded Battery Capacity & Temperature: cooler operating temperatures prolong cycle life, while low temperatures reduce the available capacity (cold-temperature capacity multiplier table).
- Victron Energy. Phoenix Inverter 1200 VA – 5000 VA datasheet (SinusMax). Continuous output at 25 °C: 1,000 W from the 1,200 VA model and 4,000 W from the 5,000 VA model (VA × 0.8); peak 2,400 W and 10,000 W; maximum efficiency 92–95%; zero-load power 8–35 W.