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To size a solar battery bank, divide the energy you need to store by the battery voltage and the usable depth of discharge. For a cabin using 5 kWh per day with one day of backup on a 48 V lithium bank, you need about 123 Ah at 48 V, which is one 48 V 100 Ah battery plus a little margin, or more practically two of them.
Solar battery bank calculator
Estimates use the formulas explained in this guide. Plan a complete system with the all-in-one calculator →
The battery sizing formula
Battery capacity (Ah) = Daily Wh × Days of autonomy ÷ (Battery voltage × Depth of discharge)
- Daily Wh: the energy your loads use per day, mainly overnight if you have solar during the day. Use the load calculation worksheet if you do not know it.
- Days of autonomy: how many days the bank must last with little or no sun. One day is common for cabins with a generator; two to three days for full-time off-grid. See days of autonomy.
- Battery voltage: 12, 24 or 48 V. See 12V vs 24V vs 48V.
- Depth of discharge (DoD): how much of the battery you can safely use. We use 0.85 for LiFePO4 and 0.50 for lead-acid/AGM. See depth of discharge explained.
Worked example
A cabin uses 5 kWh per day. The owner wants one day of autonomy on a 48 V LiFePO4 bank.
- 5,000 Wh × 1 day = 5,000 Wh
- 48 V × 0.85 = 40.8
- 5,000 ÷ 40.8 = 123 Ah at 48 V
The same cabin on a 48 V AGM bank needs 5,000 ÷ (48 × 0.50) = 208 Ah at 48 V, which means four 12 V 210 Ah AGM batteries wired in series, weighing well over 200 kg in total.
Quick tables (one day of autonomy)
Total battery energy needed (kWh)
| Daily use | LiFePO4 (85% DoD) | AGM / lead-acid (50% DoD) |
|---|---|---|
| 2 kWh | 2.4 kWh | 4.0 kWh |
| 5 kWh | 5.9 kWh | 10.0 kWh |
| 10 kWh | 11.8 kWh | 20.0 kWh |
| 20 kWh | 23.5 kWh | 40.0 kWh |
Capacity in amp-hours for LiFePO4
| Daily use | 12 V bank | 24 V bank | 48 V bank |
|---|---|---|---|
| 2 kWh | 196 Ah | 98 Ah | 49 Ah |
| 5 kWh | 490 Ah | 245 Ah | 123 Ah |
| 10 kWh | 980 Ah | 490 Ah | 245 Ah |
| 20 kWh | 1,961 Ah | 980 Ah | 490 Ah |
Multiply by 2 or 3 for two or three days of autonomy.
Turning amp-hours into a battery count
- Decide the bank voltage. A 24 V bank built from 12 V batteries needs 2 in series; a 48 V bank needs 4 in series.
- Divide the Ah needed by the Ah of one battery and round up. That gives the number of parallel strings.
- Multiply strings by batteries per string.
Example: 245 Ah at 24 V using 12 V 100 Ah lithium batteries. That is 2 in series × 3 parallel strings = 6 batteries (300 Ah at 24 V). Or, more simply, two 48 V 100 Ah rack batteries for a 48 V system.
Wiring is covered in how to wire batteries in series and parallel.
Do not forget inverter losses
If your loads run on AC through an inverter, add about 10% to the daily Wh, because inverters are typically 90 to 95% efficient and also draw power just sitting idle. A large inverter’s idle draw alone can be 1 kWh per day. See inverter idle power draw.
Check the battery can deliver the power, not just the energy
Capacity (Ah) tells you how long a battery lasts. Its maximum discharge current tells you what it can run. A 12 V 100 Ah lithium battery with a 100 A BMS can only supply about 1,200 W continuously. If you plan to run a 3,000 W inverter at 12 V, you need at least three such batteries in parallel even if your energy needs are small. Read battery C-rating explained.
Lithium or lead-acid?
For daily cycling, LiFePO4 nearly always wins on lifetime cost: it gives roughly 3,000 to 6,000 cycles versus 500 to 1,000 for AGM, and you can use 85% or more of it. Lead-acid can still make sense for rarely used backup banks. See LiFePO4 vs AGM.
Use the solar and battery calculator to size your bank, panels and inverter together.
Worked example: a home backup bank in Toronto
A family in Toronto loses power for 4 to 6 hours several times each winter after ice storms. They want to run the furnace blower, lights, a fridge, a router and a TV during outages, about 600 W on average.
- Energy per outage: 600 W × 6 h = 3.6 kWh
- Add inverter losses: 3.6 ÷ 0.90 = 4.0 kWh
- LiFePO4 capacity: 4.0 ÷ 0.85 = 4.7 kWh, so one 48 V 100 Ah battery (5.12 kWh), or two 24 V 100 Ah batteries in parallel
- Lead-acid (AGM) alternative: 4.0 ÷ 0.50 = 8.0 kWh nameplate, roughly four 12 V 180 Ah batteries in series (48 V, 8.6 kWh)
Solar of about 3 kW recharges it during the day while also running daytime loads. At Toronto’s 3.9 peak sun hours, 3 kW makes about 3 × 3.9 × 0.80 = 9.4 kWh on an average day, less in December.
FAQ
How many batteries do I need to run a house?
An average home using 30 kWh per day would need about 35 kWh of LiFePO4 for one full day off-grid. Most grid-tied homes back up only essential loads with 10 to 15 kWh. See home battery backup size.
Can I add more batteries later?
You can, but adding new lead-acid batteries to an old bank is a bad idea, and lithium batteries should be matched and balanced. See mixing old and new batteries.
Do batteries need to be bigger in winter?
Often yes. Shorter days mean less solar and longer nights, and cold reduces usable capacity, especially for lead-acid.
How many batteries do I need for a 5 kW solar system?
It depends on night-time use, not array size. Many 5 kW homes use 10 to 15 kWh of storage for evening use and backup.
How many 200 Ah batteries do I need?
At 12 V, one 200 Ah LiFePO4 holds about 2.2 kWh usable. For 5 kWh of daily use, you’d need three in parallel (or move to 24 or 48 V).
Related guides
- How Many Batteries Do I Need for a 5,000-Watt Inverter?
- How Many Batteries Do I Need for a 3,000W Inverter?
- How Long Will a 100Ah Battery Last? (Runtime Calculator + Table)
- How Long Do Solar Batteries Last? (Years and Cycles)
Sources and further reading
- Battery University: BU-205 Types of Lithium-ion: how lithium chemistries, including LiFePO4, compare.
- Battery University: BU-409b Charging Lithium Iron Phosphate: LiFePO4 charging voltages and behaviour.
- U.S. Department of Energy: Solar PV System Design Basics: how panels, inverters, controllers and batteries fit together.