How to work out what you need to run in a power cut

Three numbers matter: daily energy, peak simultaneous draw, and the surge when motors start.

Planning backup means deciding what must keep running and then working out what that requires. Most people account for the first number and are caught by the third.

Step-by-step

1. List what must stay on. Be ruthless — everything you add costs capacity.
2. Enter each item's power and hours per day.
3. Read daily energy, peak load and surge.

Surge is what catches people

Anything with a motor draws far more starting than running. A fridge compressor can pull three to five times its running figure for a second or two. A well pump or air conditioner is worse.

An inverter that cannot supply that surge will trip, and it will trip every time the fridge starts — which is to say constantly. Sizing on running load alone produces a system that fails in ordinary use rather than under stress.

Deciding what is essential

Genuinely essential is usually short: refrigeration, some lighting, phone charging, internet, and any medical equipment. Heating or cooling is where the energy goes, and including it multiplies the system required.

For a short outage, keeping food cold and phones charged is normally enough. For extended outages in a cold climate, heating becomes a safety question rather than a comfort one, and the system has to grow accordingly.

Duration

Multiply daily energy by the days of autonomy you want. Two days is a reasonable target for most grid outages. Anything longer usually needs a generator as well as a battery, because the arithmetic gets expensive quickly.

Frequently asked questions

What is surge and why does it matter?

The brief spike when a motor starts, often three to five times running power. An inverter that cannot supply it will trip every time the fridge cycles.

What should I put on backup?

Refrigeration, some lighting, communications and any medical equipment. Heating and cooling dominate the energy budget and should be included only if genuinely necessary.

How many days of backup should I plan for?

Two days covers most outages. Beyond that a generator alongside the battery is usually more economical than sizing storage for it.