Solar Base

Backup load calculator

Which circuits go on backup, and how long the battery holds them when the grid goes down.

Circuits
Back upCircuitRunning watts
Fridge and freezer
Lights
NBN and Wi-Fi
TV and chargers
Water pump
Split-system air conditioner
The inverter's continuous backup rating
Load on backup Within the backup output 500 W
Hours from the reserve5.4 hours
Reserve2.7 kWh
Hours from a full battery27.0 hours
Backup output4,500 W spare of 5.0 kW

How it's worked out

Hours = energy available ÷ load. The reserve is the share of the battery held back for an outage, which is what is left if the grid goes down at the end of a long evening; a full battery is the best case.

Running watts are examples: use the appliance's label or a clamp meter. Motors such as pumps and compressors draw several times their running watts as they start. Checked 21 October 2026.

Solar Base keeps every job on one record, from the first enquiry to the meter alteration, and does the portal work for you. See how it works

Agree the backup circuits before the install

A battery's backup output limits what can run at once, and its reserve decides how long. Walking the customer through which circuits matter, and what they cost in hours, saves the call after the first blackout when the air conditioner did not come on.

Hours = usable kWh × reserve ÷ load kW

Worked example

CircuitsFridge, lights, NBN, TV: 500 W
Battery13.5 kWh usable, 20% reserve = 2.7 kWh
2.7 ÷ 0.55.4 hours from the reserve, 27 from full

Questions installers ask

Why plan on the reserve, not a full battery?

An outage can arrive at any time, including after a long evening has drawn the battery down to its reserve. The reserve is the figure the customer can rely on.

What about starting current?

Motors draw several times their running watts for a moment as they start. Check the inverter's surge rating before putting a pump or a compressor on backup.

Can a battery back up the whole house?

Only if the backup output covers everything that might run at once. Most homes put a few essential circuits on backup instead.

A tool does one sum. Solar Base does the whole job.

The STC count, the NMI, the meter alteration and the invoice, worked off one job record, with the last step left to you.

Register interest
5.4 hours
500 W
Show the breakdown