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Charge times for common setups
With an MPPT controller and 4 peak sun hours. Each cell shows the days needed and, in brackets, the hours of full midday sun.
| Battery | 100 W of panels | 200 W | 400 W |
|---|---|---|---|
| 100 Ah 12 V lithium, 20% to 100% | 3.1 days (12.5 h) | 1.6 days (6.3 h) | under a day (3.1 h) |
| 100 Ah 12 V AGM, 50% to 100% | 2.3 days (9.3 h) | 1.2 days (4.6 h) | under a day (2.3 h) |
| 1,000 Wh power station, 0% to 100% | 3.3 days (13.0 h) | 1.6 days (6.5 h) | under a day (3.3 h) |
| 300 Ah 12 V lithium, 10% to 100% | 10.6 days (42.2 h) | 5.3 days (21.1 h) | 2.6 days (10.6 h) |
Doubling the panels roughly halves the time, but a lead-acid battery needs extra hours at the end of charging that this estimate only partly includes. Real charging of the last 10–20% is often slower than the table suggests.
How the charge time is calculated
- Energy to put in (Wh) = capacity × (target % − start %) ÷ charging efficiency. Efficiency is 95% for lithium and 80% for lead-acid.
- Real panel output (W) = panel watts × 0.85 for heat, dust, wiring and panel angle × controller efficiency (95% MPPT, 75% PWM).
- Hours of full sun = energy to put in ÷ real panel output. One peak sun hour is one hour of full midday sunlight.
- Days = energy to put in ÷ (real output × peak sun hours − energy your devices use each day). If your devices use more than the panels make, the battery never charges.
- Panels for one day = (energy to put in + energy used) ÷ (peak sun hours × 0.85 × controller efficiency).
- Charge current (A) = real panel output ÷ system voltage. Compare it with the maximum your battery accepts.
The 0.85 is a little higher than the 0.75 used in the solar and battery calculator. That one also covers loads, so it is deliberately cautious. This one only looks at charging.
Using it anywhere in the world
- Ah, Wh, V and W are the same everywhere, and nothing needs converting.
- Choose the sun level for the season you are charging in. Winter in northern Europe can be well under 2.5 peak sun hours.
- Numbers follow your own region’s format.
Why charging takes longer than panel watts suggest
- Dividing battery watt-hours by panel watts. Panels rarely deliver their rated watts, so a real output of 70–80% is normal even in good sun.
- Counting daylight hours. Only the hours near midday deliver full power. That is why the calculator uses peak sun hours.
- Ignoring loads. A fridge and a router can use a large share of a small panel’s daily energy.
- A PWM controller on a high-voltage panel. It can waste a fifth or more of the panel’s power.
- Shade. Shading even one cell of a panel can cut the output sharply.
Common questions
How long does a 200 W solar panel take to charge a 100 Ah battery?
A 200 W panel with an MPPT controller charges a 100 Ah, 12 V lithium battery from 20% to 100% in about 6.3 hours of full sun, which is about 1.6 days at 4 peak sun hours. A lead-acid battery that starts at 50% charge takes a little less, as it needs less energy but charges less efficiently.
How many solar panels do I need to charge a battery in one day?
To refill that 100 Ah lithium battery from 20% in a single day with 4 peak sun hours you need about 320 W of panels. If your devices also use power during the day, add their daily watt-hours in the calculator and the panel size goes up.
What if my devices run while the battery charges?
Then only the surplus charges the battery. With 300 Wh of daily use, the same 200 W panel needs about 2.9 days instead of 1.6 days. If your devices use more than the panels produce in a day, the battery will not charge at all.
Is MPPT really faster than PWM?
Usually yes. A PWM controller pulls the panel’s voltage down to the battery’s, which wastes part of the panel’s power, while an MPPT controller converts the excess voltage into extra current. The calculator assumes 95% for MPPT and 75% for PWM. The gap is smallest when the panel and battery voltages are well matched.
What happens on a cloudy day?
Output falls to a fraction of its clear-sky value, and heavy overcast can cut it a lot. Use the Low setting for cloudy periods, or multiply the days by the amount you expect output to drop. A larger battery and a larger array both help you get through poor weather.
Can I charge a lead-acid battery as quickly as a lithium one?
Not usually. Lead-acid batteries accept less current, often around 20–30% of their capacity per hour, and they charge slowly through the last 20%. The calculator warns when your panels could push more current than a lead-acid battery typically accepts. Check your battery’s datasheet for its limit.
Related calculators
To size a whole system, use the solar and battery calculator. To see how long a charged battery will last, try the battery runtime calculator or the power station runtime calculator. For cables and fuses, use the wire gauge and fuse calculator. For home backup planning, see the power outage calculator.
Charging while you drive? See the alternator charge time calculator, and for covering a whole bill, the bill to solar calculator.