Step 1 — Build a real load audit
Start with every load, its power in watts, its hours per day, and whether it is AC or DC. Guessing here produces a system that fails seasonally. The kind of load matters as much as the total: a fridge compressor cycling on is a very different demand from an LED light, even at the same daily energy.
| Load | Power | Hours per day | Daily energy |
|---|---|---|---|
| LED lighting, 8 points | 7 W each | 5 h | 0.28 kWh |
| Ceiling fans, 3 | 45 W each | 8 h | 1.08 kWh |
| Refrigerator, 150 L DC | 60 W average | 10 h equivalent | 0.60 kWh |
| Phone and laptop charging | 65 W | 4 h | 0.26 kWh |
| Water pump, 0.5 kW | 500 W | 0.5 h | 0.25 kWh |
| Television, 32" | 50 W | 4 h | 0.20 kWh |
| Total daily energy | 2.67 kWh |
Step 2 — Size the battery bank
Divide daily energy by the usable depth of discharge, then add autonomy days. LiFePO4 is normally specified at 80% depth of discharge; lead-acid should not be taken below 50% if you want more than 500 cycles.
Formula: (daily kWh × autonomy days) ÷ usable depth of discharge = required capacity. For 2.67 kWh and two days of autonomy on LiFePO4, that is (2.67 × 2) ÷ 0.8 = 6.7 kWh.
Round up to the nearest available block. In this example a 7 kWh LiFePO4 bank, typically 24 V at 280 Ah, covers the requirement with headroom for winter.
| Chemistry | Typical usable DoD | Cycle life at that DoD | Notes |
|---|---|---|---|
| LiFePO4 | 80% | 3,500–6,000 | Preferred for daily cycling and high ambient temperature |
| Lead-acid, tubular gel | 50% | 1,200–1,800 | Lower upfront cost, heavier, more ventilation needed |
| Lead-acid, AGM | 40% | 500–800 | Only economic for very low-cycle backup |
Step 3 — Size the solar array
Array size depends on where the system is installed, because peak sun hours vary from about 2.5 in northern Europe to 6.5 in the Gulf. The formula is: daily energy × system loss factor ÷ peak sun hours.
Use a loss factor of 1.3 to account for wiring, converter, temperature and soiling losses. For 2.67 kWh and a site with 4.0 peak sun hours: (2.67 × 1.3) ÷ 4.0 = 0.87 kWp. Add 20% for winter margin in a temperate climate and the practical specification is 1.05 kWp.
Round to a sensible module count — in this case four 300 Wp modules, or six 200 Wp modules as used in the SP-HS300 kit family.
| Region | Peak sun hours | Array multiplier vs 5.0 |
|---|---|---|
| Northern Europe | 2.5–3.0 | 1.7–2.0× |
| Central Europe | 3.2–3.8 | 1.3–1.6× |
| Mediterranean, Southern US | 4.5–5.2 | 1.0–1.1× |
| Gulf, North Africa, Sahel | 5.5–6.5 | 0.8–0.9× |
| Equatorial Africa, SE Asia | 4.0–4.8 | 1.0–1.25× |
Step 4 — Choose the inverter and controller
Size the inverter on peak simultaneous load, not daily energy. Add the running watts of everything that can be on at once, then add the starting surge of the largest motor — a fridge compressor typically needs 3× its running power for a fraction of a second.
Always use MPPT rather than PWM charge control. The efficiency difference is 15–30% of harvested energy in hot climates, which usually repays the price difference within the first year.
| Item | Value |
|---|---|
| Simultaneous running load | about 300 W |
| Largest motor surge (water pump) | 1,500 W for 1 s |
| Recommended inverter | 1,000 W continuous, 2,000 W peak, pure sine wave |
| Why pure sine | Compressor and fan motors run cooler and last longer |
Five sizing mistakes that show up after installation
- Sizing on daily energy while ignoring motor starting surge, so the inverter trips when the fridge starts.
- Using nameplate panel watts instead of real-world yield, which overstates output by 20–30%.
- Assuming nameplate battery capacity is usable. Only 80% of LiFePO4 and 50% of lead-acid capacity is available.
- Forgetting temperature derating. A panel at 60 °C cell temperature produces roughly 14% less than at 25 °C.
- Ignoring future load growth, then having no room to add a battery or an extra string of modules.
Frequently asked questions
01How do I convert kWh to battery amp-hours?
02Can I expand an off-grid system later?
03How much does an off-grid system cost compared with a generator?
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