The basic idea
An off-grid system has three parts: solar panels that make power during the day, a lithium battery that stores it for the night, and an inverter that turns it into normal household power and manages everything. Sizing means matching all three to how much you use and how much sun you get.
Step 1: List what you'll run
Multiply each appliance's power by the hours it runs per day to get watt-hours. Here is an example for a one-bedroom home in a hot climate:
| Appliance | Power | Hours / day | Energy / day |
|---|---|---|---|
| LED lighting | 60 W | 5 | 300 Wh |
| Refrigerator | – | – | 1,000 Wh |
| Phones, laptop, router | 100 W | 4 | 400 Wh |
| TV | 80 W | 3 | 240 Wh |
| Water pump | 400 W | 1 | 400 Wh |
| Split air-conditioner | 900 W | 6 | 5,400 Wh |
| Total | about 7.7 kWh |
Step 2: Size the panels
Find the peak sun hours for your site: the number of hours per day of full-strength sun, averaged over the worst season you need to cover. Then allow for losses in wiring, heat and the battery.
Array size (kW) ≈ daily energy (kWh) ÷ (peak sun hours × 0.75)
With 7.7 kWh per day and 5 peak sun hours: 7.7 ÷ (5 × 0.75) ≈ 2.1 kW. Rounding up for cloudy days gives an array of roughly 2.5 to 3 kW.
Step 3: Size the battery
The battery covers the night and cloudy spells. Decide how many days of autonomy you want, then allow for the usable share of the battery.
Battery (kWh) ≈ daily energy × days of autonomy ÷ usable share
For one day of autonomy with a lithium battery at about 90% usable: 7.7 × 1 ÷ 0.9 ≈ 8.6 kWh, so a 10 kWh battery. Two days of autonomy roughly doubles it.
Step 4: Size the inverter
The inverter must handle everything that can run at the same time, plus start-up surges from motors and compressors. In the example, the air-conditioner, fridge, pump and a few small loads together reach about 2 kW, so a 3 to 5 kW hybrid inverter leaves comfortable headroom.
Practical tips
- Size for your worst month, not your average one.
- Add a generator input for long cloudy spells if comfort matters.
- Run heavy loads such as washing machines and pumps in the middle of the day.
- Keep panels clean; dust can noticeably cut output in dry climates.
- Leave space on the roof and in the power room to add capacity later.
How we size your system
Send us your site location and the appliances you plan to run. We'll size the array, battery and inverter for your climate and include them as line items on your quote. The figures above are an example only; your system may be larger or smaller.
