Worked example
A 3 kWh daily critical load with 1 day of autonomy on lead-acid batteries needs about 7 kWh of rated battery capacity.
Size your battery bank
Solar Battery Backup Calculator
Battery capacity for night-time or cloudy-day backup
Recommended battery capacity
7 kWh
Assumes a 50% usable depth of discharge for lead-acid batteries and 90% round-trip efficiency.
Battery Backup Sizing vs. Grid-Tied Solar Sizing
These are two different sizing questions, easy to conflate. Sizing your solar panels (see our panel size calculator) is about how many units you generate over a year to offset your bill — a grid-tied system with no battery at all still does this job fine, using net metering to bank surplus daytime generation. Sizing a battery is a separate question: how much energy you can store to keep specific loads running when the grid — and the sun — aren't available. A well-designed hybrid system does both, but they're calculated independently.
Battery Types Available in India
| Tubular lead-acid | Lithium (LiFePO4) | |
|---|---|---|
| Upfront cost | Lower | Higher |
| Usable depth of discharge | ~50% | ~90% |
| Typical lifespan | Shorter, more charge cycles degrade it faster | Longer, holds capacity over more cycles |
| Maintenance | Periodic water top-ups needed | Maintenance-free |
| Weight & footprint | Heavier, bulkier per kWh | Lighter, more compact per kWh |
How Many Hours of Backup Do You Actually Need?
Start from what you actually want to keep running, not your whole house. A common approach: list your critical loads (lights, fans, fridge, router, phone charging), add up their wattage, and multiply by how many hours you want them covered to get your daily load in kWh — the input the calculator above uses.
Typical "essentials only" combo
A few LED lights + 2 ceiling fans + a fridge + a router/phone charging together draw roughly 250–350W continuously — over a 4-hour outage that's about 1–1.4 kWh of daily load, before accounting for battery losses. Use the exact wattages from your own appliances for a real figure; our inverter backup time calculator works the other direction — telling you how long a battery you already own will last for a given load.
Related calculators
Frequently asked questions
Why does lithium need less rated capacity than lead-acid for the same job?
Lead-acid batteries are conventionally limited to about 50% usable depth of discharge for reasonable lifespan, while lithium (LiFePO4) batteries can be safely used to around 90% — so a lithium battery needs a smaller rated capacity to deliver the same usable energy.
What should I count as "critical load"?
The essentials you actually want covered during low-sun periods — typically lights, fans, fridge, router and phone charging — rather than your whole household load including AC, which is usually left off a backup circuit.
Does this calculator size the solar panels too?
No — this sizes only the battery bank. For the panel/system size itself, see our solar panel size calculator.
Why size for more than 1 day of autonomy?
A string of cloudy or rainy days can mean your panels don't fully recharge the battery overnight. Extra autonomy days act as a buffer so backup power doesn't run out during a low-generation stretch.
How is battery backup sizing different from grid-tied solar sizing?
Grid-tied sizing (see our panel size calculator) is about how much of your annual consumption solar generation offsets — it works fine with no battery at all if you have reliable grid supply and net metering. Battery backup sizing is a separate question: how much stored energy you need to ride out an outage, sized around your critical loads and how long you want them covered, independent of your panel size.
Tubular lead-acid or lithium — which should I choose?
Lead-acid costs less upfront but needs periodic maintenance (water top-ups) and typically has a shorter usable lifespan. Lithium (LiFePO4) costs more upfront but is maintenance-free, generally lasts longer, and gives more usable capacity per rated kWh thanks to its higher depth of discharge. For many new installations lithium's total cost of ownership is increasingly competitive despite the higher sticker price.
Does a hybrid inverter matter for battery backup?
Yes — a hybrid inverter is what allows automatic switching between grid, solar and battery power. A plain grid-tied (non-hybrid) inverter shuts down completely during a grid outage as a safety measure, regardless of how much battery capacity you have, so battery backup requires a hybrid or off-grid-capable inverter.