Charge time is energy divided by power: how much you need to put back, over how fast you can put it in. Chemistry and the last-stretch behaviour of the battery add the fine print, but the headline number is a simple division you can do on any battery, charger or solar array.
The formula
Hours = energy to replace ÷ charging power. A 5 kWh battery discharged to 80% needs 4 kWh back; at 95% charge efficiency that is about 4.2 kWh, so a 1,000 W charger takes roughly 4 hours 12 minutes. Lead-acid adds more on top for the slow absorption phase at the end.
Charge efficiency
| Chemistry | Efficiency |
|---|---|
| Lithium (LiFePO₄) | ~95% |
| Lead-acid / AGM | 80–85% |
The difference is energy lost as heat and gassing during the charge — one of the quiet reasons lithium systems need less solar for the same daily cycle.
Battery Charge Time Calculator
Enter capacity, depth of discharge and charging watts for the hours to a full recharge.
Why lead-acid crawls at the end
Lead-acid batteries only accept full current up to about 80% charge; after that the charger drops into a low-current absorption phase that can add 2–4 hours. Skipping it repeatedly causes sulfation and kills the bank early. Lithium takes near-full current almost to the top, which is why it recharges so much faster in practice.
What charging power for solar?
Use a realistic average, not the array’s nameplate — panels deliver their best for only a few midday hours. As a rough rule, an array yields its rated power × your peak sun hours spread across the day; for a quick estimate, use about 70% of array rating for the main charging window. A 1,600 W array in a 4.5-sun-hour location averages well under 1,000 W.
Can I just fit a bigger charger?
Within limits. Batteries have a maximum charge rate — often written as a C-rate: 0.5C means a 100 Ah battery accepts at most 50 A. Lithium commonly takes 0.5–1C, lead-acid more like 0.1–0.2C. Exceeding it heats the battery and shortens its life, so check the datasheet before upsizing the charger.