Free engineering tool
Solar battery autonomy & rainy-day calculator
Will your solar street light stay on through the rainy season? Enter the battery, the load and the panel, then the longest rainy spell at your site. The calculator shows how many days the battery lasts, day by day, and what it would take to cover the whole spell.
Why “days of autonomy” on a datasheet is not enough
Datasheets often quote autonomy as battery capacity divided by one night of load, as if the panel produced nothing for days. In reality a panel still collects some energy under cloud, and a battery is rarely full at the start of a storm. What matters for the buyer is simple: on the longest rainy spell of the year, does the light stay on, and how long does it take to recover afterwards?
The calculator answers that with a day-by-day energy balance: each rainy day, the battery loses the night load minus the small rainy-day charge; each sunny day afterwards, it gains the surplus of the panel. Usable capacity takes the depth of discharge and cold-weather derating into account.
Worked example
A 40 W light, 5 h at 100% and 7 h at 30%, uses 40 × 7.1 ÷ 0.9 ≈ 316 Wh a night. A 150 Wp panel with 4.5 sun hours and an MPPT controller collects 486 Wh on a sunny day and about 73 Wh on a rainy day (15%). The net drain is about 243 Wh per rainy day. A 100 Ah, 12.8 V LiFePO4 battery holds 1,152 Wh usable, so it covers about 4.7 rainy days: enough for a 4-day spell, while a 5-day spell needs about 106 Ah. After the spell, the 170 Wh sunny-day surplus refills the battery in about 6 days.
Questions about this calculation
How many days of autonomy does a solar street light need?
Most suppliers design for 2–3 nights without sun, which is enough in dry climates. In monsoon and rainy-season climates, look at the longest run of heavily overcast days in local weather records; 4–5 days is common in Southeast Asia and coastal West Africa. Smart dimming that lowers output when the battery runs low extends autonomy further.
What depth of discharge should I use?
For LiFePO4 batteries, 80–90% usable capacity is normal and they tolerate thousands of cycles. For lead-acid or gel batteries, keep to about 50% to avoid a short life. This is why lithium systems need a much smaller nominal Ah rating for the same autonomy.
Does a solar panel charge on rainy days?
Yes, but little: typically 10–25% of a clear day under heavy overcast, sometimes less in a storm. The calculator lets you set this value; use a lower number for very dark rainy seasons.
Why does cold weather matter?
Battery capacity drops in the cold, and LiFePO4 batteries must not be charged below 0 °C without heating. For sites with freezing nights, choose the derating option and ask for a low-temperature battery or a buried battery box.
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- Solar vs grid street lighting cost calculatorTotal cost of ownership over 5–25 years: trenching, cabling and electricity vs. solar street lights.
- High bay lighting calculatorHow many LED high bays a warehouse, factory or sports hall needs, with layout and W/m².
- LED retrofit energy savings & payback calculatorEnergy, money and CO2 saved by replacing sodium, metal halide or fluorescent lights with LED.
Next step
Ask our engineers to check your design
Calculators give planning figures. Send us the site location, drawings and quantity, and we return a photometric or solar sizing check and a quotation for the complete set.
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