Battery Bank Sizing Calculator
Find out how large your solar battery bank needs to be. Enter daily consumption, autonomy days, system voltage and depth of discharge to get the required Ah, usable kWh, and nominal bank size instantly.
Last updated: May 2026
Enter daily consumption, autonomy days, battery voltage and depth of discharge above.
Battery Ah = kWh/day × days × 1000 ÷ (V × DoD fraction) · nominal kWh = Ah × V ÷ 1000
How to size a solar battery bank
Battery bank sizing comes down to two questions: how much energy do you need per day, and how many days must the bank cover without solar input? Multiply those together for your total usable energy requirement. Then divide by the usable fraction of the battery's rated capacity, which is set by the depth of discharge limit. A 10 kWh bank rated at 85% DoD delivers 8.5 kWh usable. Size to that exactly and you hit the DoD limit every cycle, shortening battery life. Add 15 to 30% headroom for winter production shortfalls, temperature derating, and load growth.
Why voltage affects Ah but not kWh
Ah (ampere-hours) is a voltage-dependent quantity. A 48 V system at 200 Ah stores exactly the same energy as a 24 V system at 400 Ah - both hold 9.6 kWh nominal. The 48 V system draws half the current for the same load, which means thinner cable, lower resistive losses, and a smaller charge controller. Higher voltage is almost always preferred for systems above 1 kW continuous load. Small cabins running LED lighting and a 12 V fridge do fine at 12 V.
Depth of discharge by chemistry
| Chemistry | Typical max DoD | Cycle life at that DoD |
|---|---|---|
| LiFePO4 (lithium iron phosphate) | 80 to 90% | 3,000 to 6,000 cycles |
| NMC (lithium nickel manganese cobalt) | 75 to 85% | 1,000 to 3,000 cycles |
| Flooded lead-acid | 50% | 300 to 600 cycles |
| AGM (absorbed glass mat) | 50 to 60% | 400 to 700 cycles |
| Gel lead-acid | 50 to 70% | 400 to 600 cycles |
Where the battery bank sits in the sizing chain
The battery bank is sized from the numbers above it and it sets the numbers below it. You cannot choose a bank sensibly until the daily load and the worst-case sun are known, and once the bank exists its voltage and capacity decide what the charge controller and inverter must handle. The order that keeps a solar plan honest:
- Add up the daily load first. List each appliance, its wattage, and its daily run time; the sum in Wh is the figure every later calculation depends on. The off-grid cabin sizing guide provides a room-by-room appliance worksheet if you have not done this yet.
- Find your worst-case sun. A December or January figure from your latitude, not a full-year mean, is what keeps a bank from running flat in the hardest month. The peak sun hours reference lists both the seasonal low and the annual figure for your region.
- Size the array to refill the bank. Panel area and irradiance determine how many kWh reach the bank each short winter day. The solar panel output calculator converts those two inputs into a daily generation figure you can compare directly with your load.
- You are here: size the battery bank. Daily load times autonomy days, divided by usable depth of discharge, gives the Ah you need at your chosen voltage. The result above is rated capacity before headroom; add 15 to 30% for winter and temperature.
- Match the charge controller to the array and bank. Controller current depends on array watts and bank voltage. The charge controller calculator sizes it with a cold-weather margin.
- Size the inverter to the loads, not the bank. Continuous watts plus surge for motor starts. The inverter sizing calculator handles both.
- Check the economics last. Feed the completed design into the solar payback calculator to see the break-even point and decide whether the numbers justify the cost or whether the array needs to shrink.
A note from my own off-grid wiring: the number that catches people out is autonomy days, not the daily load. On my camper's 12 V setup the daily draw is easy to add up, but it is the run of grey winter days with almost no charge coming in that decides the bank size. The bank voltage stays at 12 V only because the loads are small; once a setup grows past roughly a kilowatt I move to 24 V or 48 V, where thinner cable and a smaller controller quickly pay for the change.
Frequently Asked Questions
What does depth of discharge mean for a battery bank?
Depth of discharge (DoD) is the percentage of a battery's total rated capacity that you use before recharging. A 100 Ah battery at 50% DoD still holds 50 Ah unused; at 85% DoD it retains 15 Ah. The limit protects cycle life: lead-acid batteries lose capacity rapidly if regularly discharged below 50%, while LiFePO4 cells tolerate 80 to 90% routinely. In the calculator the DoD percentage is divided by 100 to become a fraction and then divides the total energy requirement. A higher DoD means a smaller, cheaper bank but more stress on each cycle.
How many autonomy days should I plan for?
Autonomy days depend on your setup type and location. Grid-tied hybrid systems rarely need more than 1 day - the grid is the backup. Off-grid systems in temperate climates with reliable summer sun typically plan 2 to 3 days. Those in northern latitudes or monsoon regions should plan 4 to 5 days to cover winter. Autonomy beyond 5 days pushes bank size and cost to a point where adding a small backup generator is usually cheaper. A practical starting point: 2 days for hybrid, 3 days for off-grid in a moderate climate.
Should I use a 12 V, 24 V or 48 V system?
For small loads under 500 W (camping trailer, small shed), 12 V is straightforward and integrates directly with 12 V appliances. For typical residential loads of 1 to 3 kW, 24 V halves the current vs 12 V and reduces cable losses noticeably. For systems above 3 kW or with long cable runs between components, 48 V is the standard choice: current is one-quarter of the equivalent 12 V figure, charge controllers and inverters are more efficient, and cost per kWh of bank capacity is lower. Most modern all-in-one inverter-chargers are designed around 48 V.
Why does mixing old and new batteries in a bank cause problems?
Batteries in a bank are wired in parallel or series-parallel, which forces them to share current during charge and discharge. A degraded cell with higher internal resistance charges to a different voltage than a fresh cell at the same state of charge. The charge controller sees the bank average and cuts off before the weak cell is full or after the strong cell is overcharged. Within months the new cells degrade to match the weakest ones. The only safe approach is to replace the entire bank as a set, or to use a battery management system that can balance cells independently.
How does temperature affect usable battery capacity?
Cold temperatures reduce usable capacity significantly. A flooded lead-acid battery at 0°C delivers roughly 80% of its rated capacity; at -20°C around 50%. LiFePO4 is more tolerant but still loses 10 to 20% at 0°C, and most BMS units refuse to charge below -5 to -10°C to prevent lithium plating. For systems that must operate in winter, upsize the bank by 20% if temperatures near 0°C are expected regularly, or insulate the battery enclosure. Heat above 40°C accelerates ageing in all chemistries.
Methodology and sources
This tool sizes a solar battery bank from the energy balance behind every storage system: the usable energy a bank can deliver equals its nominal capacity multiplied by the system voltage and the allowed depth of discharge. It rearranges that relationship to find the rated capacity (in ampere-hours) needed to cover your daily load across the chosen autonomy days.
- Method: usable energy = daily kWh × autonomy days; required capacity (Ah) = daily kWh × 1000 × days / (V × DoD fraction); nominal energy (kWh) = Ah × V / 1000; battery count = required Ah / single-battery Ah, rounded up. The DoD percentage is divided by 100 to a fraction before use.
- Standards and sources: standard battery-energy physics (watt-hours = amp-hours × volts) and arithmetic; no formal wiring standard is implemented. The depth-of-discharge and cycle-life figures in the reference table follow common manufacturer datasheet ranges (LiFePO4 80 to 90%, NMC 75 to 85%, flooded lead-acid 50%).
- Assumptions and limits: the result is the rated capacity needed to meet the usable energy target at the stated DoD only; it does not add headroom. Real banks need extra margin for winter shortfalls, temperature derating (cold cuts usable capacity 10 to 50% depending on chemistry), inverter and wiring losses, and load growth. Plan 15 to 30% headroom and confirm the chosen DoD against your battery's own datasheet.
Reviewed and maintained by Rick Oosterling, who builds and wires 12 V, solar and EV systems hands-on. Last reviewed: June 2026. This is a planning aid, not a substitute for a qualified professional or your local wiring and building code. Battery banks store dangerous amounts of energy: verify fusing, conductor sizing and installation against current regulations and the equipment manufacturer's instructions before building.