Already have a battery? This battery backup time calculator tells you how long it will run your fans, lights or AC during load shedding, and how many sunny hours your solar panels need to charge it again. It works for tubular, lead-acid and lithium batteries.
Battery Backup Time Calculator
Aap ki battery kitni der chalegi aur solar se kitni der mein charge hogi?
Lithium: 90% usable; tubular: 50% to protect battery life. Includes inverter losses (about 10–15%). Charging assumes full sun and that the panels are not also running other loads.
Battery backup time formula
Backup time (hours) = Battery voltage × Ah × number of batteries × usable % × inverter efficiency ÷ load (watts)
- Usable %: about 90% for lithium (LiFePO4) and 50% for tubular or lead-acid, to avoid damaging the battery.
- Inverter efficiency: about 85–90%.
Two 12V 200Ah tubular batteries store 4.8 kWh. Using 50% and allowing for losses gives about 2 kWh of usable energy. With a 500 W load (fans and lights), that’s about 4 hours of backup.
Backup time for common batteries
| Battery setup | 300 W load | 500 W load | 1,000 W load |
|---|---|---|---|
| 1 × 12V 200Ah tubular | about 3.4 h | about 2 h | about 1 h |
| 2 × 12V 200Ah tubular | about 6.8 h | about 4.1 h | about 2 h |
| 5.12 kWh lithium (51.2V 100Ah) | about 13.8 h | about 8.3 h | about 4.1 h |
| 10.24 kWh lithium | about 27.6 h | about 16.6 h | about 8.3 h |
How long does solar take to charge a battery?
Charging time is roughly the energy you need to put back divided by about 75% of your panel wattage, to allow for heat, cable and charging losses. For example, 1,170 W of panels (two 585W plates) refills two 12V 200Ah tubular batteries from 50% in about 2.7 sunny hours, if nothing else is running.
If your home also runs on solar during the day, only the surplus charges the battery, so charging takes longer. Add 1–2 extra panels for every battery bank if you depend on backup every evening.
Planning a new battery? Use the battery backup calculator to size one from your load and outage hours.
Frequently asked questions
How long will a 200Ah battery last?
A 12V 200Ah tubular battery gives about 1 kWh of usable energy, so roughly 2 hours at 500 W or 3.4 hours at 300 W.
How long does a 5 kWh lithium battery last?
About 4.1 kWh is usable after losses, which runs a 500 W load for around 8 hours or a 1,000 W load for around 4 hours.
How many panels do I need to charge two 200Ah batteries?
Two 585W panels can recharge two 12V 200Ah tubular batteries from half charge in about 3 sunny hours, if they are not also powering your home.
Prices and rules change often. This page was last reviewed on 12 November 2026. Spotted an outdated rate? Tell us and we'll update it.