How much energy a battery stores
Battery energy is voltage multiplied by capacity. For example, a 51.2V 100Ah battery stores 51.2 × 100 = 5,120 Wh, or 5.12 kWh. Always compare batteries in kWh, because Ah alone does not tell you the energy when the voltage differs.
Lithium (LiFePO4) vs lead-acid
- Lithium iron phosphate (LiFePO4): usually a longer cycle life, deeper usable discharge, lighter weight and almost no maintenance, but a higher upfront price.
- Lead-acid: lower upfront price, but heavier, usually shorter-lived and (for flooded types) needs regular maintenance. It is generally not discharged as deeply.
- Datasheets differ by model. Many LiFePO4 batteries are rated for thousands of cycles, while lead-acid is often rated for far fewer. Check the manufacturer's cycle-life and depth-of-discharge figures.
How to size your battery
Multiply your average backup load (kW) by the hours you need, then divide by the usable fraction of the battery.
Example: a 0.5 kW load for 8 hours needs 4 kWh. With about 90% usable capacity (typical for LiFePO4) you need roughly 4.4 kWh, so one 5.12 kWh battery fits. With lead-acid used to about 50%, you would need roughly 8 kWh of battery.
Heavy loads such as air conditioners and pumps drain a battery much faster, so include them in your calculation only if you want to back them up.
Make sure it works with your inverter
- Battery voltage must match what your inverter supports (for example 48V low-voltage or high-voltage).
- Many lithium batteries communicate with the inverter through a BMS. Check that your inverter supports your battery model.
- Do not mix old and new batteries, or different types, in one bank.
Installation and safety
- Install in a ventilated, protected place away from direct sun and moisture, following the manufacturer's IP rating and instructions.
- Use correctly rated breakers and cables.
- Have the system installed and tested by a qualified technician.
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