What this calculator does
Off-grid battery sizing answers a specific question: how much storage do you need so the system keeps running through a stretch of low sun without draining the battery further than it can safely handle. The three things that decide the answer are how much energy is used each day, how many days of cloudy or low-generation weather you want to cover without any solar input at all, and how deeply the battery chemistry can be discharged before it is damaged or its life is shortened.
That last figure, depth of discharge, is the one people most often get wrong by copying a number from an unrelated system. It depends entirely on the battery chemistry, which is why this calculator asks for it directly rather than assuming one.
The formula
Multiply the daily energy load by the number of autonomy days you want covered, then divide by the depth of discharge (entered as a percentage and converted to a decimal). That gives the required capacity in watt-hours. Dividing by the battery bank voltage converts that into amp-hours, the unit most battery datasheets are labelled in.
| Term | Meaning |
|---|---|
| Daily energy load | The total energy your off-grid loads consume in a typical day, in watt-hours. |
| Days of autonomy | The number of consecutive days with little or no solar input the battery needs to cover on its own. |
| Depth of discharge (DoD) | The share of the battery's rated capacity that can be used before it must be recharged, as a percentage. |
| Battery bank voltage | The nominal voltage of the battery bank, used to convert watt-hours into amp-hours. |
The inputs explained
| Field | What to enter |
|---|---|
| Daily energy load (Wh) | Total watt-hours consumed per day by everything the battery needs to run, added up across all appliances and their run times. |
| Days of autonomy | How many days in a row the system should ride through without meaningful solar charging. Two to three days is a common starting point for a home system; a critical load may warrant more. |
| Depth of discharge (%) | Lead-acid batteries are typically not discharged below about 50% depth of discharge, to preserve their working life. Lithium (LiFePO4) batteries can usually be discharged much further, often 80% to 90%, without the same penalty. Enter the figure that matches your own battery chemistry and manufacturer guidance, since this varies by product. |
| Battery bank voltage (V) | The nominal voltage of the battery bank, commonly 12V, 24V or 48V for off-grid systems. |
When to use it
Sizing a new off-grid system
Once daily load and desired autonomy are known, this calculation gives the minimum usable capacity to shop for, which can then be checked against what a given battery bank actually provides at its rated depth of discharge.
Comparing lead-acid against lithium for the same load
Running the same load and autonomy figures through both a 50% and an 85% depth of discharge shows directly how much smaller, and typically lighter, a lithium bank can be for the same usable storage.
Deciding whether to add another day of autonomy
Autonomy days scale the required capacity directly, so it is straightforward to see the extra capacity, and cost, that an additional buffer day of storage adds.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How required capacity changes with days of autonomy at a fixed load
A fixed 3,000 Wh daily load at 50% depth of discharge, across a range of autonomy targets.
| Days of autonomy | Required battery capacity | Equivalent capacity at 12V |
|---|---|---|
| 1 days | 6,000 Wh | 500 Ah |
| 1.5 days | 9,000 Wh | 750 Ah |
| 2 days | 12,000 Wh | 1,000 Ah |
| 3 days | 18,000 Wh | 1,500 Ah |
| 4 days | 24,000 Wh | 2,000 Ah |
| 5 days | 30,000 Wh | 2,500 Ah |
How required capacity changes with depth of discharge at a fixed load and autonomy
A fixed 3,000 Wh daily load over 2 days of autonomy, across a range of depth-of-discharge settings.
| Depth of discharge | Required battery capacity | Equivalent capacity at 12V |
|---|---|---|
| 40% | 15,000 Wh | 1,250 Ah |
| 50% | 12,000 Wh | 1,000 Ah |
| 60% | 10,000 Wh | 833 Ah |
| 70% | 8,571 Wh | 714 Ah |
| 80% | 7,500 Wh | 625 Ah |
| 90% | 6,667 Wh | 556 Ah |
Questions
Why does depth of discharge matter so much?
A battery rated at, say, 200Ah does not actually deliver 200Ah of useful cycling life if it is only ever safe to draw half of that. Sizing on the usable, not the rated, capacity is what keeps the battery within its safe operating range and its expected lifespan.
How many days of autonomy should I plan for?
It depends on your local weather patterns and how critical the load is. A recreational or seasonal setup might accept one day; a home relying on the system through a run of overcast winter days often plans for three or more.
Can I mix this with grid backup instead of pure off-grid autonomy?
Yes. A grid-tied system with battery backup can use a lower autonomy figure, since the grid covers the shortfall, whereas a true off-grid system with no other supply needs to size for the worst realistic stretch of weather.
How is this different from the battery runtime calculator?
This calculator solves for the capacity a battery needs to have, given a target load and autonomy period. The battery runtime calculator does the reverse: given a battery you already own, it tells you how long it will last against a load, which is the more useful question once the hardware is already chosen.
Once the battery is sized, check how long it will actually run a given load with the battery runtime calculator, and size the array feeding it with the solar energy yield calculator.