Battery sizing guide
How to size an off-grid battery bank
Start with energy used each day, choose how long you need to run without charging, and account for the battery reserve and conversion losses. Then test the result against difficult weather and load scenarios.
1. Measure daily energy use
For each load, multiply its power in watts by hours used per day, then divide by 1,000 to get kWh. Use measured consumption when possible: a fridge or pump cycles, so its nameplate power alone can overstate daily use. Include inverter standby draw and seasonal loads.
| Load | Example calculation | Daily energy |
|---|---|---|
| Fridge | 60 W × 12 h | 0.72 kWh |
| Lights | 40 W × 5 h | 0.20 kWh |
| Laptop | 65 W × 4 h | 0.26 kWh |
| Router | 10 W × 24 h | 0.24 kWh |
| Water pump | 500 W × 0.5 h | 0.25 kWh |
| Other loads and standby | Measured estimate | 0.73 kWh |
| Total | 2.40 kWh |
2. Convert the daily load into nominal capacity
Choose days of autonomy for the period when solar or other charging may be unavailable. The usable fraction depends on the battery manufacturer’s discharge limit and your chosen reserve; the efficiency factor should match the path from battery to your loads.
Nominal capacity (kWh) = daily load (kWh) × autonomy days ÷ (usable fraction × delivery efficiency)
Worked example: 2.40 kWh/day × 2 days ÷ (0.80 × 0.90) = 6.67 kWh nominal. A 7 kWh bank is a starting estimate, not a final hardware specification.
Daily load
2.40 kWh/day
Autonomy
2 days
Usable fraction
80%
Delivery efficiency
90%
Estimated nominal battery capacity
6.67 kWh
The 80% usable fraction and 90% delivery efficiency are example assumptions, not universal battery values. Check the battery, inverter, temperature limits, current limits, and reserve settings for your actual equipment. Avoid applying inverter losses twice if your measured daily load is already on the battery side.
3. Simulate the difficult days
Enter your loads, battery capacity, state-of-charge limits, solar input, and location in PyMox. Compare a normal day with a low-solar period and a higher-load day. Check the minimum state of charge, time at reserve, and whether charging restores the battery before the next low-production period.
Normal solar day
Check whether charging restores the battery.
Low-solar period
Check the minimum battery state of charge.
Higher-load day
Check how long the battery stays at reserve.
Solar production changes by location, season, orientation, and weather. NREL PVWatts can provide a separate solar estimate, but it models grid-connected PV production and does not prove that an off-grid battery system will survive a particular sequence of cloudy days.