Daily load schedule (AC side)
- LED lighting: 40 W for 5 h
- Fridge: 60 W, 35% duty, 24 h
- Laptop: 65 W for 4 h
- Water pump: 50 W for 1 h
- Router: 12 W continuous
- Daily energy at the appliances
Electrical · worked example · 3 pages
Off-grid solar and battery sizing
Sizing the battery and PV array for a small off-grid cabin. Power (W) is a rate; energy (Wh) is what batteries store and loads consume — the schedule below turns each appliance's power and hours into daily energy, then losses, autonomy and the solar resource size the system. Example equipment values; sun hours are site data.
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What this calculation covers
Losses and battery draw
- Inverter efficiency
- Daily energy drawn from the battery
Battery: nominal vs usable
- Days of autonomy required
- Usable depth of discharge (LiFePO4)
- Nominal capacity required
- Chosen bank: two 25.6 V 100 Ah units
- Usable energy in the bank
- Bank meets the autonomy target
- Actual days of autonomy
Solar array
- Peak sun hours, design month
- System derate (wiring, dust, MPPT)
- Array power to replace a day's draw
- Chosen array: two 230 W panels
- Energy a design-month day generates
- A normal day recharges a normal day
A stretch of poor weather
- Generation on a 2-sun-hour day
- Days the bank bridges that weather
Results summary
- The cabin uses about 1.30 kWh a day at the plugs — 1.45 kWh from the battery through the inverter. Two 25.6 V 100 Ah LiFePO4 units hold 5.12 kWh nominal, 4.10 kWh usable: 2.8 days of real autonomy. A 460 W array regenerates a day's use in 4.5 sun hours with margin; in 2-sun-hour weather the net drain is ~0.76 kWh/day and the bank alone bridges about 5 days.
Try changing…
- Add a 900 Wh/day freezer — the array must grow past 700 W and autonomy nearly halves.
- Allow only 50% DoD (lead-acid habit) — the same bank offers 1.8 days; capacity must rise.
- Plan for PSH = 3.0 — the required array grows a half; panels are the cheap fix, batteries are not.