Off-grid solar system calculator
Size a complete off-grid solar system from your appliance list: battery bank, solar panels, charge controller, inverter, main fuse and battery cable in one place.
Results
| Battery bank needed (rated) | 8.17 kWh |
|---|---|
| Battery bank build | 2S4P: 8 × 12.8 V 100 Ah (25.6 V, 400 Ah, 10.24 kWh) |
| Daily energy from the battery | 3,677 Wh/day |
| Minimum solar array | 1,226 W |
| Solar panels | 4 × 400 W (1,600 W) |
| MPPT charge controller | 100 A at 24 V (needs ≥ 83.3 A) |
| Inverter (continuous) | 3,000 W, surge ≥ 2,929 W |
| Battery current at full inverter output | 138.9 A |
| Main battery fuse | 175 A |
| Battery-to-inverter cable | 2/0 AWG copper |
- Wire panels in series strings within your controller's voltage limit (check cold-weather Voc with the solar string calculator). Use a battery monitor (shunt) to track state of charge.
About this calculator
An off-grid solar system is a chain of parts that all have to agree with each other. The appliances you run set the daily energy and the inverter size. The daily energy and the number of cloudy days you want to ride out set the battery bank. The daily energy and your winter sunshine set the solar array. The array and the battery voltage set the charge controller, and the inverter's battery current sets the main fuse and cable. Change one link, and the others move.
This calculator runs that whole chain at once from a single appliance list, using the same formulas as the individual calculators on this site. Use it to get a balanced first design and a realistic parts list, then refine each component with the step-by-step calculators linked below. Everything is a stated assumption you can edit, and the worked example shows each number.
How to use it
- List every appliance with its running watts, starting watts (for motors and compressors), hours per day and quantity. For fridges, use the 24-hour average, not the compressor's running draw.
- Pick the system voltage. 12 V suits small vans and cabins, 24 V mid-size systems, and 48 V anything with a 3,000 W-plus inverter.
- Choose the battery chemistry and the size of one 12 V battery. The calculator works out how many to wire in series and parallel.
- Set days of autonomy and the peak sun hours for the worst month you plan to live on the system, not the annual average.
- Enter your panel wattage and battery-to-inverter cable length, then read the parts list. Check each part with its dedicated calculator before buying.
The formula
Daily battery energy = Σ(running W × hours × qty) ÷ inverter efficiency + idle W × idle hours.
Battery bank (rated Wh) = daily battery energy × days of autonomy ÷ usable depth of discharge (90% LiFePO4, 50% lead-acid). Batteries in series = system V ÷ 12. Strings in parallel = rated Wh ÷ bank V ÷ Ah per battery, rounded up.
Solar array = daily battery energy ÷ (peak sun hours × (1 − losses)), rounded up to whole panels.
MPPT controller = array W ÷ system V × 1.25, rounded up to a standard size.
Inverter = continuous load × 1.25, rounded up to a standard size. Surge = continuous load + the largest (starting − running) difference.
Battery current = inverter rating ÷ (system V × efficiency). Main fuse = 125% of that, rounded up. Cable: the thicker of the gauge whose NEC 75 °C ampacity covers the fuse and the gauge that keeps voltage drop within your limit over the round trip.
Worked example (default values)
Inputs
| Appliances and daily hours of use | Refrigerator (full size, average draw): 60 W, 24 h/day; Wi-Fi router + modem: 15 W, 24 h/day; LED light bulb: 9 W × 6, 5 h/day; Laptop: 60 W, 4 h/day; TV (50-inch LED): 100 W, 3 h/day; Microwave (1000 W cooking): 1500 W, 0.25 h/day |
|---|---|
| System (battery bank) voltage | 24 V |
| Battery chemistry | LiFePO4 |
| Capacity of one 12 V battery | 100 Ah |
| Days of autonomy (no sun) | 2 days |
| Inverter efficiency | 90 % |
| Inverter idle draw | 15 W |
| Hours per day the inverter is on | 24 h |
| Peak sun hours (worst month you'll use it) | 4 h/day |
| Solar system losses | 25 % |
| Wattage of one solar panel | 400 W |
| Battery-to-inverter cable length (one way) | 5 ft |
| Max voltage drop on that cable | 2 % |
Results
| Battery bank needed (rated) | 8.17 kWh |
|---|---|
| Battery bank build | 2S4P: 8 × 12.8 V 100 Ah (25.6 V, 400 Ah, 10.24 kWh) |
| Daily energy from the battery | 3,677 Wh/day |
| Minimum solar array | 1,226 W |
| Solar panels | 4 × 400 W (1,600 W) |
| MPPT charge controller | 100 A at 24 V (needs ≥ 83.3 A) |
| Inverter (continuous) | 3,000 W, surge ≥ 2,929 W |
| Battery current at full inverter output | 138.9 A |
| Main battery fuse | 175 A |
| Battery-to-inverter cable | 2/0 AWG copper |
Tips and common mistakes
Design for winter, or plan a backup. In much of the US, December gives half the peak sun hours of June. Sizing the array for winter can double it. Many full-time off-grid homes size for spring and fall and run a generator for a few weeks in midwinter instead. Try both numbers here.
Days of autonomy are expensive. Each extra day adds another full day of battery. Two days of LiFePO4 is a common balance for homes with a generator. Cabins used on weekends can often get by with one.
Cut the load first. Every watt-hour you remove from the daily list shrinks the battery and the array together. Swapping an old fridge, killing phantom loads and choosing an inverter with a low idle draw are usually cheaper than adding panels.
Size cables and fuses to the inverter, not today's load. Someone will eventually run the inverter at full power. The fuse and cable here are based on the inverter's rating for that reason.
Frequently asked questions
How big a solar system do I need to go off grid?
It depends entirely on your daily energy use and winter sun. With 4 peak sun hours and 25% losses, a frugal cabin using 3-4 kWh a day needs about 1-1.4 kW of panels and 7-9 kWh of LiFePO4 for two days of autonomy. A typical US home using 25-30 kWh a day would need roughly 8-10 kW of panels and 55-67 kWh of battery for the same two days, which is why most off-grid homes cut their usage hard first.
How many batteries do I need for an off-grid system?
Multiply your daily battery energy by the days of autonomy you want and divide by the usable depth of discharge (about 90% for LiFePO4, 50% for lead-acid). For example, 3.7 kWh a day for 2 days needs about 8.2 kWh of rated LiFePO4, or eight 12.8 V 100 Ah batteries wired as a 24 V bank.
Should my off-grid system be 12V, 24V or 48V?
Use 12 V for small systems under about 1,500 W of inverter, 24 V up to about 3,000-4,000 W and 48 V above that. Higher voltage means lower current, so cables, fuses and the charge controller get smaller and cheaper.
Does this replace a professional design?
No. It gives a balanced first estimate. A permanent home system also needs code-compliant overcurrent protection, disconnects, grounding and often a permit and inspection. Have a licensed electrician or solar installer review the final design.
Related calculators
- Solar panel size calculator
Turn daily energy use into the solar array watts and number of panels you need, using peak sun hours and realistic losses. For RVs, vans, cabins and power stations.
- Battery bank series & parallel calculator
Work out the voltage, amp-hours and total energy of a battery bank wired in series, parallel or series-parallel, with wiring tips for matched batteries.
- Inverter size calculator
Size a power inverter from your appliance list: continuous watts, motor start-up surge, the next standard size and DC current from a 12, 24 or 48 V battery.
- Solar charge controller size calculator
Find the amp rating your MPPT or PWM solar charge controller needs from array watts, battery voltage or panel Isc, with the standard 25% safety margin.
More solar calculators
- Solar charging time calculator
- Home solar payback calculator
- Solar string voltage calculator (series & parallel)
Off-grid solar sizing, step by step
- Rough out the whole system (you are here)
- Size the inverter from your loads · Inverter size calculator
- Plan the battery bank · Battery bank series & parallel calculator
- Check battery runtime · Battery bank runtime calculator
- Size the solar array · Solar panel size calculator
- Check panel string voltage · Solar string voltage calculator (series & parallel)
- Choose the charge controller · Solar charge controller size calculator
- Size the DC cables · DC wire size calculator (12 V, 24 V, 48 V)
- Pick the fuses · DC fuse & breaker size calculator
Estimates for planning only, based on the assumptions shown. Real-world performance varies with equipment, temperature and installation. Updated October 19, 2026. See our methodology.