If you size an off-grid system on average daily consumption, it will work beautifully for eight months of the year and fail in the ninth. The battery is not sized for the average day. It is sized for the worst week.
Why the Average Lies
A site that consumes 40 kWh a day in July may consume 22 kWh a day in December — but December also produces 30% of the solar yield. The load goes up as the array goes down. Sizing on the annual average hides both effects.
| Month | Daily load | Daily PV yield | Net |
|---|---|---|---|
| July | 40 kWh | 48 kWh | +8 kWh |
| October | 32 kWh | 26 kWh | −6 kWh |
| December | 26 kWh | 12 kWh | −14 kWh |
An annual-average design sees 36 kWh load against 32 kWh average yield and concludes the array is adequate. It is adequate — for about seven months.
The Four Numbers You Actually Need
- Daily kWh in the lowest-irradiance month — not the annual average
- Peak kW — because a pump starting can pull three times its running watts
- Hours of autonomy required — how many consecutive dark days must the site survive
- Irradiance for that month — peak sun hours per day, not annual kWh/m²
The Sizing Arithmetic
Battery kWh = daily kWh × autonomy days ÷ depth of discharge.
For 26 kWh/day, two days of autonomy and 90% usable depth:
26 × 2 ÷ 0.9 = 57.8 kWh — round to 60 kWh.
Then size the array to refill that battery in one good day, not to match the average load:
Array kWp = battery kWh ÷ (peak sun hours × 0.75), where 0.75 accounts for temperature derating, soiling and inverter losses.
For 60 kWh and 3.5 peak sun hours in December: 60 ÷ (3.5 × 0.75) = 22.9 kWp.
The Mistake That Costs the Most
Oversizing the array and undersizing the battery. The array produces more than the battery can absorb, the charge controller hits absorption voltage by mid-morning, and the rest of the day’s production is thrown away. The customer paid for panels that do nothing after 11 a.m.
A useful sanity check: the array-to-battery ratio should sit between 1.2 and 1.5 kWp per 10 kWh of storage in temperate latitudes, lower in the tropics. If your design is at 3 kWp per 10 kWh, either the battery is too small or the array is too big.
Where the Generator Fits
A generator should be the third line of defence, not the second. Battery first, solar second, generator third. If the generator runs more than 200 hours a year on a system that was sold as solar-hybrid, the design was wrong — not the weather.
Size the generator to cover the peak load plus battery charging, and set the start trigger at 30% state of charge with a two-hour minimum run time. Short cycling kills generators faster than load does.
What to Send Us
- Daily kWh for the worst month
- Peak kW and the largest motor starting current
- Site latitude and, if available, monthly irradiance data
- Autonomy requirement in days
- Whether a generator will be retained
With those five numbers we will return a sized configuration, a bill of materials and a one-page justification of the arithmetic.
Send Us Your Off-Grid Load Profile
Daily kWh for the worst month, peak kW, irradiance if available.