Sizing an off-grid system is about the worst week of the year, not the average day. Here is how to work out the panels, battery and inverter so the lights stay on through the monsoon.
1. Work out daily energy use
List every appliance, its wattage and the hours it runs each day. Add them up to get watt-hours (Wh) per day. Example: a farmhouse using lights, fans, a fridge, a TV and a small pump for one hour may need about 3,000 Wh (3 kWh) a day.
2. Size the panels for the weaker months
Our 4–5 units per kW rule is an annual average. For off-grid, use a lower monsoon figure of about 3 units per kW per day, and add 20–25% for battery and inverter losses. For 3 kWh a day: 3 ÷ 3 × 1.25 ≈ 1.25 kW, so a 1.5 kW array gives a sensible margin.
3. Choose days of autonomy
Autonomy is how many days the battery can run the site with little or no sun. For our region, 1–2 days is typical for homes. For 3 kWh a day with 1.5 days of autonomy, you need 4.5 kWh of usable storage — about 5 kWh of lithium, or 9 kWh of lead-acid.
4. Rate the inverter
The off-grid inverter must cover the highest load running at one time, plus the surge when motors start. A pump or fridge can briefly draw three times its running power.
These are planning figures. A site visit lets us check shade, cable runs and the real loads before we finalise the design.
