Battery backup calculator
Work out the lithium batteries and inverter you need to keep your essentials running through a power cut.
What should stay on in a power cut?
Add each appliance you want to keep running. The wattages are typical, and you can adjust them in the last step.
How long should the backup last?
Choose the longest power cut you want to cover. Tell us about any inverter or batteries you already have.
Backup time
How long the batteries should carry everything you picked.
Existing inverter
Optional. The battery voltage is on the inverter's rating plate, beside "DC input".
Existing batteries
If you already have our lithium batteries, see how long they will run this load.
Energy by appliance
All three batteries for this load
| Battery | System | Units | Stored |
|---|
How this was sized
Adjust watts, hours and assumptions
Check the label on each appliance and correct any wattage that differs. Set shorter hours for anything you only run for part of the time.
This is an estimate to plan with. We check your inverter, wiring and site before we quote.
How the answer is worked out
Four steps, using the figures printed on our battery datasheets.
- Add up the load Watts multiplied by quantity, for everything you want running.
- Allow for motors starting A fridge, pump or air conditioner draws about three times its running watts for a moment when it starts. The inverter is sized for everything running plus the largest motor starting.
- Work out the energy Watts multiplied by hours, for each appliance, plus 10% for inverter losses.
- Count the batteries Each battery gives up to 95% of its rated energy, less its own losses, and can deliver half its capacity per hour. We use whichever needs more batteries, the energy or the power.
Fridges and air conditioners switch on and off, but we count them as running the whole time, so the result leans to the safe side. Battery figures come from the datasheets for the 12.8V, 25.6V and 51.2V models.
Common questions
How many batteries do I need for 4 hours of backup?
It depends on the load. A load of 300 W for 4 hours uses 1.2 kWh. After losses that takes two 12.8V 100Ah batteries on a 12 V inverter, or one 25.6V 100Ah on a 24 V inverter.
Can I run an air conditioner on batteries?
Yes, but it sets the size of everything else. A 1.5 ton air conditioner drawing about 1,700 W needs a 48 V inverter of about 6.4 kVA and two 51.2V 100Ah batteries, which run it for close to 5 hours. Inverter-type air conditioners start more gently, so lower the start-up surge for them in the last step.
Why does it ask for more batteries than the energy needs?
Each battery has a limit on how fast it can deliver power: 640 W for the 12.8V 100Ah, 1,280 W for the 25.6V and 2,560 W for the 51.2V. A heavy load for a short time can need more batteries for power than for energy.
How long do the batteries take to recharge?
At the recommended charge rate, about 4 hours from empty for the 12.8V and 25.6V models and about 5 hours for the 51.2V, if your charger or solar array can supply that much power.