Home backup: which loads to put on the essential circuit

· 5 min read ·

See how to choose the loads for the essential circuit of a battery system, respect the inverter limit and estimate the hours of backup, in a calculator.

Whole home or essential circuit

During an outage, a battery system can power the whole home or only an essential circuit. Almost always the second option is the more sensible: the battery lasts longer and the inverter does not have to be huge.

Whole home

Everything stays on

  • Nothing changes for the occupants
  • Needs a much larger inverter and battery
  • A shower or oven ruins the backup time
  • Higher cost

Essential circuit

Only what matters

  • The battery lasts much longer
  • Smaller inverter and battery
  • You choose the loads
  • Needs the loads split onto another board
The essential circuit is what usually balances cost and comfort.

How the essential board works

The essential loads sit on a separate board, connected to the inverter’s backup output. While the grid is normal, the inverter passes the energy through; if the grid fails, it isolates the home from the grid and feeds only that board with energy from the battery and the panels.

  1. Grid
  2. grid
  3. Hybrid inverterwith backup output
  4. backup
  5. Essential loads board
  7. Essential loads
When the grid is down, only the essential board stays powered.

Build your circuit

Tick the loads that would sit on the essential circuit. The sum must fit within the inverter’s backup power, and the battery’s usable energy sets how many hours it lasts.

Interactive

Build the essential circuit

Tick the loads that would stay on during an outage. The sum must fit within the inverter’s backup output power, and the battery sets how many hours it lasts.

500 Wpower of the loads switched on
17%of the inverter limitwithin the limit
9 hof battery, at 90% efficiency

The calculation uses running power. Motors (refrigerator, pump, air conditioner) draw more at start-up: check the value in the inverter manual.

What goes in and what stays out

Usually goes in

  • Refrigerator and freezer, so food is not lost
  • LED lighting in the main rooms
  • Router, modem and phone chargers
  • The water pump, if the home depends on it
  • Medical and safety equipment, as a priority

Usually stays out

  • Electric shower, oven and electric hob
  • Large air conditioners, unless the system was built for them
  • Irons, hair dryers and other heaters
  • Loads you rarely use but that would always stay on

Appliances that heat draw a lot in a short time and drain the battery. If one is indispensable, size the system for it from the start.

The effect of each load on the backup time

With 5 kWh usable and 90% efficiency, the energy delivered is 4.5 kWh. Here is how a few extra loads shorten the battery hours:

Battery hours by the loads switched on (5 kWh usable)

Essentials (500 W)9 h
+ water pump (total 1,250 W)3.6 h
+ air conditioner (total 1,900 W)2.4 h

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Only the essential loads board receives the inverter’s backup output; the heavy loads stay on the main board.

Try it: Add the home’s water pump (750 W) to the essential board and redo the sum and the battery hours. Does the power still fit the inverter?

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