Solar battery calculator for off-grid and hybrid systems
This solar battery calculator sizes the battery bank for an off-grid or hybrid solar system. List what you run each day (or enter your kWh a day), choose how many cloudy days the batteries must cover, and it gives the capacity in amp-hours at 12, 24 or 48 V, the number of batteries in series and parallel, the bank’s kWh, and a rough figure for the panels needed to refill it.
It is for cabins, farms, homes with long power cuts and anyone building a 12 V camper or shed system. Every input — watts, hours, depth of discharge, efficiency and battery size — is editable, and each step of the sum is shown.
How to use the Solar Battery Size Calculator
- List your appliances with their watts, hours of use a day and quantity, or pick from Add typical appliance. If you already know your daily use, switch to I know my kWh a day.
- Set the days of autonomy — about 1 for a grid-backed hybrid, 2–3 for off-grid.
- Choose the system voltage and battery type; the depth of discharge fills in, and you can change it.
- Enter the voltage and amp-hours of the battery you plan to buy.
- Read the Ah needed, the battery count and the autonomy you actually get, then copy the summary.
Solar Battery Size Calculator features
Days of autonomy
Storage = daily energy × days, so the bank rides through cloudy spells without the panels.
12, 24 or 48 V
Batteries are arranged in series for the voltage and in parallel for capacity, with a check that they fit.
Battery chemistry
Lead-acid at 50 %, tubular at 80 % and LiFePO4 at 90 % depth of discharge as starting points.
Shared appliance list
The same typical watts as the inverter and generator calculators, from BLDC fans to pumps.
Panel and controller check
Rough panel watts to refill a day’s use and the charge controller current.
The solar battery sizing formula
Battery Ah = daily Wh × days of autonomy ÷ (system voltage × depth of discharge × efficiency). The defaults use 3,600 Wh a day — two fans for 8 hours, five LED lights for 6, a TV for 4, a router all day and a fridge whose compressor runs about 10 hours. Two days of autonomy at 24 V, 80 % depth of discharge and 85 % efficiency need 7,200 ÷ (24 × 0.8 × 0.85) = 441 Ah.
With 150 Ah 12 V batteries that is 2 in series (for 24 V) × 3 strings in parallel = 6 batteries, a 10.8 kWh bank that actually covers 2.04 days. At 12 V the same energy needs 882 Ah — six batteries in parallel and twice the current in every cable, which is why larger systems use 24 or 48 V.
Choosing depth of discharge and autonomy
Autonomy is a trade-off: each extra day adds a whole day of batteries that are rarely used. A hybrid system that can fall back on the grid or a generator rarely needs more than one day; remote off-grid homes often plan for two to three, more where winters are cloudy.
| Battery | Usual DoD for sizing | Notes |
|---|---|---|
| Flooded, AGM or gel lead-acid | 50 % | Deeper cycling shortens life sharply; loses capacity in the cold |
| Tubular lead-acid | 70–80 % | Common for home inverters in India |
| LiFePO4 | 80–90 % | Many more cycles, lighter, needs a battery management system |
Panels, controller and inverter
- To refill one day’s use, panels of roughly daily Wh ÷ (peak sun hours × 0.75) are needed — 3,600 ÷ (5 × 0.75) ≈ 960 W. Use the solar panel calculator for location-based sun hours and roof area, and the solar string sizing calculator to wire them within the controller’s voltage limits.
- A charge controller is sized at about panel watts ÷ battery voltage × 1.25 — about 50 A for 960 W on 24 V.
- The inverter must cover the peak watts and motor starting surges, not the daily energy — size it with the inverter size calculator.
Solar Battery Size Calculator FAQ
How do I calculate solar battery size?
Multiply your daily energy in Wh by the days of autonomy, then divide by system voltage × depth of discharge × efficiency. The solar battery calculator does it and turns the Ah into a number of batteries.
How many batteries do I need for an off-grid solar system?
It depends on use and autonomy. For 3.6 kWh a day and two days at 24 V with 150 Ah tubular batteries, six batteries — two in series, three strings in parallel.
What size battery for a 12 V solar system?
Divide daily Wh × days by (12 × DoD × efficiency). A 1 kWh-a-day 12 V cabin with 2 days of autonomy on LiFePO4 at 90 % needs about 2,000 ÷ (12 × 0.9 × 0.85) ≈ 218 Ah.
What are days of autonomy?
The number of days the batteries can run your loads with no solar input, such as during cloudy weather. More days mean a bigger, costlier bank.
Can I mix old and new batteries?
It is best not to: in series or parallel, the weakest battery limits the bank and ages the others. Buy the whole bank at once, same model and age.
Is this accurate enough to buy from?
It is the standard sizing arithmetic with typical efficiencies. Real capacity falls with temperature, discharge rate and age, so check datasheets and have the system installed by a qualified electrician.