Solar battery size calculator
Size your solar battery bank from daily energy use — enter kWh per day, days of autonomy and battery chemistry, get required kWh, amp-hours and battery count.
Quick answer: Daily kWh × days of autonomy ÷ usable depth = bank size. A home using 10 kWh/day with 1 day of autonomy needs 11.1 kWh of lithium (90% usable — 231 Ah at 48 V) or 20 kWh of lead-acid (50% usable).
Your storage needs
Ballpark, not gospel. Real consumption varies by season and habit — log a week of evening usage before buying, and confirm charge-controller and inverter compatibility with your installer.
Typical daily loads (for estimating your kWh)
| Appliance | Typical draw | Daily use |
|---|---|---|
| LED lights (10 bulbs) | 100 W | 0.5 kWh |
| Refrigerator | 150 W | 1.5–2 kWh |
| TV + set-top box | 120 W | 0.5 kWh |
| Ceiling fans (4) | 300 W | 2–3 kWh |
| Laptop + phones | 100 W | 0.4 kWh |
| 1.5-ton inverter AC (8 h) | 1,200 W | ~10 kWh |
Add up what you actually run during outages — most backup setups cover lights, fans, fridge and devices (3–5 kWh/day), not the air conditioner.
Questions people actually ask
Straight answers, no fluff. Then run your own numbers above.
How do I calculate solar battery bank size?
Multiply your daily kWh use by days of autonomy, then divide by the battery's usable depth (0.9 for lithium, 0.5 for lead-acid). A home using 10 kWh/day with 1 day of autonomy needs 11.1 kWh of lithium bank or 20 kWh of lead-acid.
How do I convert kWh to amp-hours?
Divide watt-hours by system voltage: amp-hours = (kWh × 1000) / volts. An 11.1 kWh bank at 48 V is about 231 Ah; the same bank at 12 V would be 925 Ah.
How many batteries do I need for a 5 kW solar system?
It depends on your daily consumption, not the array size. A 5 kW array making ~21 kWh/day typically pairs with 10–15 kWh of lithium storage for overnight use — enter your actual daily kWh above for an exact number.
What are days of autonomy in battery sizing?
The number of cloudy days your batteries must cover with no solar input. Use 1 day for grid-tied backup, 2–3 days for true off-grid. Each extra day multiplies the bank size — and the cost.
Does lithium or lead-acid change the battery size?
Yes — dramatically. Lithium (LiFePO₄) gives 90% usable depth, so an 11.1 kWh bank covers 10 kWh of use. Lead-acid only gives 50%, so the same use needs a 20 kWh bank: nearly double the batteries for the same backup.
What size battery bank for 10 kWh daily use?
With 1 day of autonomy: 11.1 kWh of lithium (231 Ah at 48 V, about five 12 V 200 Ah units) or 20 kWh of lead-acid. Add a second autonomy day and both numbers double.
Should I size batteries for winter output?
If you rely on solar year-round, yes — winter days are shorter and cloudier. Either size autonomy for the worst month or keep a generator/grid fallback for the 4–6 darkest weeks instead of buying a huge bank you rarely cycle.
How many solar panels recharge my battery bank?
Roughly 2–5 kW of panels per 10 kWh of battery so the bank refills on a normal solar day. An 11 kWh bank pairs well with a 3–5 kW array in average sun.
Sizing the whole backup setup? The solar system calculator sizes the inverter to match this bank. Then the solar cost calculator prices panels plus storage, and the panel output calculator checks what your array actually generates per day.