How-to
How to Size a Lithium Battery for Your Home or Business
To size a lithium battery, add up the wattage of everything you need to run, multiply by the hours of backup you want, then divide by the battery voltage to get amp-hours. Add roughly 20% headroom for inverter losses. A 500W load for 4 hours at 25.6V needs about 100Ah.
The four steps
- Add up the wattage of every appliance you need to run at the same time
- Multiply that total by the number of hours of backup you want, giving watt-hours
- Divide the watt-hours by your system voltage to get amp-hours
- Add about 20% for inverter and conversion losses, then round up to the nearest available pack
Step 1: work out your load
Only count what genuinely has to stay on. Sizing for every socket in the building is the most common and most expensive mistake. Typical draws are below as a starting point; check the rating plate on your own equipment.
| Appliance | Typical draw |
|---|---|
| LED light | 9 to 15 W each |
| Ceiling fan | 50 to 75 W each |
| Wi-Fi router | 10 to 20 W |
| Laptop | 50 to 90 W |
| Television | 80 to 150 W |
| Refrigerator (running average) | 100 to 200 W |
| Desktop computer | 150 to 300 W |
| CCTV system (4 to 8 cameras) | 40 to 100 W |
| Billing counter and POS | 150 to 300 W |
Step 2: decide your backup hours
Be realistic about the outages you actually get. Four hours covers most urban Indian load-shedding. Rural sites, telecom towers and cold storage usually need eight hours or more, because a second cut can arrive before the bank has recharged.
Step 3: convert to amp-hours
Divide your watt-hours by the system voltage. The same energy requirement needs fewer amp-hours at a higher voltage, which is why larger installations move to 48V and above rather than stacking low-voltage packs.
Step 4: add headroom
Inverters are not perfectly efficient and cables lose a little along the way. Adding roughly 20% covers both, and leaves the pack operating comfortably rather than at its limit, which extends its life.
Worked examples
| Scenario | Load | Backup | Energy needed | Suitable pack |
|---|---|---|---|---|
| 2BHK home: lights, fans, Wi-Fi, TV | 400 W | 4 hours | About 1.9 kWh with headroom | ZEL 2500 (25.6V 100Ah) or ZP-26.6 combo |
| Small shop: billing, lights, CCTV, fridge | 800 W | 5 hours | About 4.8 kWh with headroom | ZEL 4800 (48V 100Ah) |
| Telecom site: transmission equipment | 1,200 W | 8 hours | About 11.5 kWh with headroom | ZEL 9600 (96V 100Ah) or banked 48V |
Choosing the voltage
Match the battery voltage to your inverter. Higher voltage carries the same power at lower current, which means thinner cable, lower losses and less heat. As a rule of thumb: 12.8V for small home backup, 25.6V for a typical home or small shop, 48V for commercial premises, and 96V or 120V for industrial and high-voltage installations.
Common sizing mistakes
- Sizing for every appliance rather than only the essential loads
- Using the rated amp-hours of a lead-acid quote directly, when only about half of it was ever usable
- Forgetting the surge current of motors and compressors when picking the inverter
- Ignoring recharge time, so the bank never refills between two outages in one day
- Choosing voltage on price rather than on cable runs and current
Frequently asked questions
How do I calculate what size battery I need?
Total the watts of the loads you need to run, multiply by your required backup hours to get watt-hours, divide by the system voltage for amp-hours, then add about 20% for losses. A 500W load for 4 hours at 25.6V works out at roughly 100Ah.
What size battery do I need to run a refrigerator?
A domestic refrigerator averages 100 to 200W once running, though it draws a large surge on start-up. For 8 hours of fridge-only backup, budget roughly 1.2 to 1.6 kWh of usable energy and confirm the inverter can handle the compressor surge.
Should I choose 12V, 24V or 48V?
Match the inverter, and prefer higher voltage as the system grows. Higher voltage moves the same power at lower current, which reduces cable size and losses. 12.8V suits small home backup, 25.6V a typical home, 48V commercial, and 96V or 120V industrial.
Can I add more batteries later?
Yes, though it is best to expand with matched packs of the same age, model and state of health. Mixing significantly older and newer packs on one bus pulls the whole bank down to the weakest unit.
How long does a lithium battery take to recharge?
LiFePO₄ accepts high charge current for most of the cycle, so recharge is considerably faster than lead-acid, which slows to a long absorption stage near the top. Actual time depends on your charger or solar array rather than on the battery.
Published 4 August 2026. Last reviewed 4 August 2026 by the Zeltron Power engineering team.