When the power goes off, a battery only has to carry what you choose to keep running, for as long as the outage usually lasts. Sizing it comes down to two numbers: the load you keep on (in kW) and the hours of backup you want. Everything else follows from those.
Step 1: list what must stay on
Decide which loads are essential during an outage. For a typical home, that list might look like this:
| Load | Typical power |
|---|---|
| LED lights, 8 x 9 W | 72 W |
| Fridge (average draw while cycling) | 100 W |
| Wi-Fi router | 15 W |
| TV | 80 W |
| Phone and laptop charging | 60 W |
| Security lights or CCTV | 40 W |
| Total | about 370 W, call it 0.4 kW |
Use the wattage on your own appliances where you can. Leave out heavy loads such as electric showers, kettles, irons and cookers; backing those up multiplies the battery size for very little benefit.
Step 2: multiply by the hours you need
Energy the battery must deliver is load multiplied by hours. That is the usable capacity. Lithium iron phosphate (LiFePO4) batteries are normally run to about 90% depth of discharge, so the battery you buy (the nominal capacity on the label) is usable capacity divided by 0.9.
| What stays on | Load | 4 hours: usable / nominal | 8 hours: usable / nominal |
|---|---|---|---|
| Home basics (list above) | 0.4 kW | 1.6 / 1.8 kWh | 3.2 / 3.6 kWh |
| Home with water pump and more | 1.0 kW | 4.0 / 4.4 kWh | 8.0 / 8.9 kWh |
| Small office or shop | 1.5 kW | 6.0 / 6.7 kWh | 12.0 / 13.3 kWh |
| Clinic or school block | 3.0 kW | 12.0 / 13.3 kWh | 24.0 / 26.7 kWh |
Lead-acid batteries should only be discharged to about 50% if you want them to last, so they need roughly double the nominal capacity: the 3.2 kWh home example would need about 6.4 kWh of lead-acid.
Step 3: check the inverter, not just the battery
The battery sets how long you can run; the inverter sets how much you can run at once. Size the inverter at least 25% above your essential load, so a 1.5 kW office needs an inverter of about 2 kW or more.
Motors are the trap. Fridges, freezers and water pumps draw several times their running power for a moment when they start. If the inverter cannot supply that surge, it trips even though the battery is full. Check the inverter's surge rating against your largest motor.
Step 4: make sure the battery can recharge
A battery that is not refilled before the next outage is only half a solution. On the grid, it recharges from KPLC when power returns. To recharge from solar instead, the array needs spare output beyond the daytime load.
In Kenya, each kilowatt of panels produces roughly 3.8 kWh a day on average. Allowing for about 8% lost charging and discharging the battery, refilling the 3.2 kWh home battery needs about 0.9 kW of extra solar, and the 12 kWh office battery about 3.4 kW. In the cloudy months output drops, so a battery sized right in February may not fully refill in July. The monthly values for your area are on the solar by county pages.
How many hours should you plan for?
Look at your own outage history rather than a national average: how long your typical outages last and how often they happen. A few practical rules:
- Size for your common outage, not the worst one. Doubling backup hours doubles the battery, and the rare all-day outage is often cheaper to cover with a generator.
- Separate essential circuits. Wiring essentials to their own board stops someone from accidentally running the kettle off the battery.
- Plan for battery life. Lithium batteries last thousands of cycles, but heat shortens their life. Keep them in a cool, ventilated space, out of direct sun.
Size it for your site
- The free solar calculator takes your essential load and backup hours, sizes the battery and inverter, and adds the solar needed to recharge it.
- The battery backup and storage guide covers usable versus nameplate capacity, charge limits and generator integration in more depth.
- On a time-of-use tariff, see what a KPLC unit really costs before deciding whether to charge the battery from the grid at night.