What do you need to keep running when the power goes?
Daily energy, peak load and an honest surge figure — plus what to switch off first if the outage runs longer than you planned.
What do you actually need to keep running?
Build a realistic critical-load profile for an outage — the honest version, not the sum of every nameplate.
Every figure below is editable. Presets are examples, not data — the number on your own appliance's label, or a reading from a plug-in meter, beats any of them. Untick Essential for anything you could switch off if the outage runs long.
Why the surge figure is not the sum of every appliance
Through the day
Each load
If the outage runs long
How long would a battery last?
How this is calculated
Energy uses the duty cycle: quantity × watts × hours × dutyCycle. A fridge cycling at 35% over 24 hours consumes 8.4 hours' worth of energy, not 24.
Peak power does not use the duty cycle. When that fridge's compressor is running it draws its full watts, and the inverter has to carry that at the same moment as everything else that happens to be on. Peak is taken from the busiest hour of the schedule you entered.
Surge is the important one. surge = everything running at the peak hour + the largest single (startup − running) difference. Motors start one at a time unless they genuinely coincide. Adding every appliance's surge together — which is what most quick guides tell you to do — produces a figure that would size an inverter two or three times larger than you need, and it is shown above so you can see the gap.
If two motors in your home really do start together — a well pump and an air conditioner on the same circuit, say — set the coincidence control and the second surge will be counted.
Start with what you cannot do without
The instinct is to list everything, and the result is a system nobody can afford. A more useful exercise is to work out what genuinely has to run — refrigeration so food does not spoil, lights so the house is safe to move around, a router if you work from home, a medical device if someone depends on one — and treat everything else as a bonus you can switch on when there is spare capacity.
The shed plan above exists for exactly this. It shows what each optional load is costing you per day, largest first, so if the outage stretches past what you planned for you already know what to turn off and how much time it buys.
Nameplate, or measured?
Measured is better, and it is not difficult. A plug-in energy meter costs very little and will tell you what a fridge actually uses over a day, which is usually well below what its label suggests. Where you have a measured figure, enter it in the Measured kWh/day column and it will be used instead of the watts × hours calculation.
Limitations and safety
This is an energy planning exercise, not an electrical design. It does not tell you which circuits can be backed up, how to transfer them safely, what protection is required, or whether your wiring can carry the load. A transfer switch or interlock is not optional — back-feeding a generator or inverter into utility wiring can kill line workers. Anything involving a medical device deserves a conversation with the supplier about a proper backup plan rather than reliance on an estimate from a web page.