Solar with battery backup keeps your lights, fans and other chosen loads running through a power cut and stores the sun’s energy for the evening. The battery is the part that needs the most thought, because it decides how long the backup lasts and how much the system costs.
This guide shows how to work out the battery size and what to look at in the battery itself.
Key takeaways
- Backup time depends on the battery’s usable energy and the power of the loads.
- Not all of the battery’s rated capacity is usable.
- Lithium (LiFePO4) and lead-acid batteries differ in cost, life and care.
- Batteries need a cool, ventilated place and a proper protection device.
Working out the battery size
Start with the loads you want to back up. Add their power in watts, multiply by the hours of backup you want, and you have the energy in watt-hours. As an example, 400 W of loads for 5 hours is 2,000 Wh, or 2 kWh.
Then allow for two things. A battery should not be emptied completely, so only part of its rated capacity is usable, and the inverter and the wiring lose a little energy. The battery you buy must therefore be larger than the figure from the sum.
Lithium or lead-acid
- Lithium iron phosphate (LiFePO4) batteries allow a larger usable share of their capacity, are lighter and usually last longer, but cost more at the start.
- Lead-acid batteries (including AGM and gel) cost less at the start, but need a larger bank for the same usable energy and are replaced sooner.
The datasheet gives the usable capacity, the number of cycles and the warranty; compare those, not only the price.
Where and how to install
Keep the battery in a cool, dry and ventilated place away from direct sun, and protect it with the breaker or fuse recommended by the manufacturer. Heat shortens battery life, so a hot room or a closed cupboard is a poor choice.
Cables should be of the right size and well connected, and the battery should be fixed so that it cannot fall or be knocked.
Looking after the battery
Check the display or the monitoring app from time to time for alarms, keep the area clean and dry, and ask for a battery check if the backup time becomes noticeably shorter.
When to call us
If you want backup from your solar system, we can size the battery for your loads and quote the complete system. Start from the quote page, read about residential solar, or see the batteries we supply.
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