Skip to content

Sizing a system

Sizing a system: start with a measured load list

Every off-grid system is designed from one number: the energy consumed in a day. Everything else — battery, inverter, panels, cable — follows from it, and a system designed without it is a guess with a receipt.

By

Sizing is arithmetic, and the only difficult part is getting honest inputs.

List what runs, not what is owned

Go through a real day. Each device, and the hours it genuinely runs. The list is always shorter than expected, and the surprises are the things that run continuously rather than the things that draw a lot briefly.

Measure the two or three that dominate

A plug-in energy meter on a mains appliance, or a clamp meter on a low-voltage one, for a couple of days. Fridges and heating appliances dominate every list, and they are the ones estimated worst. The output is a daily watt-hour figure, which is the number the whole design rests on.

Turn the total into a battery

Daily watt-hours multiplied by the number of days you want to survive without charging, divided by the usable fraction of the battery rather than its nominal capacity. Usable is what the chemistry and the management system allow, and it is smaller than the label.

Then size the inverter separately

The inverter is sized by the largest simultaneous load in watts, not by the daily total, and it needs headroom for the starting surge of anything with a motor. A big battery behind a small inverter starts nothing; a big inverter on a small battery trips.

Finally size the charging

Solar for the days at rest, a DC-DC charger for the days driving, a mains charger for a hookup. Size the solar against the daily total in the worst month you will use the system, and accept that the darkest week is covered by a second source rather than by more roof.

What people ask before they order

Why not just use the labels?

Labels give a maximum rating, and most appliances never draw it continuously. A fridge multiplied by twenty-four hours gives a figure several times too large, which is how people buy a battery bank they never needed.

How long should I measure for?

Long enough to cover the cycling of the largest load, which for a fridge means a couple of days in conditions resembling real use. Shorter measurements catch the compressor running or not running, and neither is representative.

Last reviewed 16 September 2026