Battery Size Calculator
Enter the average power you use, the hours it runs, the battery voltage and chemistry, and how many days of backup you want. The calculator gives the amp hours you need and how many 100 Ah batteries make it up.
- Ah and kWh
- Lithium or lead-acid
- Days of backup
Average power is total watt-hours divided by hours. A fridge rated 150 W may average only 50 W.
Battery size is energy used divided by the share of the battery you can use. Lithium batteries can be run down to about 10% safely. Lead-acid should stop at about half, or they wear out fast. That is why a lead-acid bank needs nearly twice the capacity for the same runtime.
Battery size chart
Ah at 12 V. Search for a daily energy.
| Energy per day | Lithium (Ah) | Lead-acid (Ah) |
|---|---|---|
| 500 Wh | 49 | 88 |
| 1 kWh | 97 | 175 |
| 2 kWh | 195 | 351 |
| 3 kWh | 292 | 526 |
| 5 kWh | 487 | 877 |
| 10 kWh | 975 | 1754 |
Halve the Ah at 24 V and quarter it at 48 V. The stored energy stays the same.
Runtime and capacity
500 W at 12 V. Slide the hours.
The blocks are 100 Ah 12 V batteries. Shading shows the part you can actually use.
How to size a battery
Energy first. Step through it.
Find watt-hours
Watts × hours.
Cold reduces capacity. Lead-acid loses about 20% at 0°C, and lithium should not be charged below freezing.
Lithium, AGM or flooded
Battery chemistry. Compare them.
Light, long life, use 90% of capacity.
Sealed, no maintenance, moderate cost.
Cheapest, needs topping up and ventilation.
Lithium costs more up front but often less per cycle, because it lasts several times longer.
Before buying batteries
System checks. Tick each item.
Tick each item as you go.
Never mix old and new batteries, or different types, in one bank. The weakest one drags the rest down.
What you get
Battery bank size. Each tile explains one result.
Ah needed.
kWh used and stored.
100 Ah blocks.
Load amps.
Usable share.
Series connections raise voltage; parallel strings raise capacity. A 24 V bank of 12 V batteries uses pairs in series.
Frequently Asked Questions
No question matches that search.
How do I calculate battery size?
Watts × hours ÷ volts ÷ usable depth. 300 W for 5 h at 12 V lithium is about 146 Ah.
How long will a 100 Ah battery last?
A 12 V lithium 100 Ah runs 100 W for about 10 hours.
What size battery for 1 kWh a day?
About 100 Ah of 12 V lithium, or 175 Ah of lead-acid.
Is 24 V better than 12 V?
For bigger systems, yes. Current and cable size halve.
How deep can I discharge a lead-acid battery?
About 50% for good life.
What is a kWh in amp hours?
At 12 V, 1 kWh is about 83 Ah.
How many days of backup do I need?
Two to three days for off-grid solar in cloudy climates.
Batteries that last the night
Work out the energy you use, divide by voltage and usable depth, and add a little for losses. Choose the chemistry that suits your budget, and you will have power when you need it.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.