Before you begin
What you need to size backup power
Use the rating plate for medical devices, pumps and boilers if it differs from the preset.
Solar is modelled at Irish December output, because severe outages are most likely then, and on where the panel is actually pointed, because in December that decides most of what it collects.
Reading the answer
Understanding how long your backup will last
Battery size decides how long the unit lasts. The inverter rating decides whether it can start and run the load at all.
A recommendation beyond 5 kWh means you should trim the list or look at a professionally designed hybrid system.
Boundaries
Assumptions and limitations
Working assumptions
- Each load uses a representative wattage, and a representative share of the time it spends actually drawing power.
- Usable AC energy includes approximately 15% conversion loss.
- Portable solar assumes 1.2 equivalent winter sun-hours and 80% system efficiency.
Where to be careful
- A hardwired boiler or pump needs an electrician-designed connection. Never backfeed a house circuit.
- Medical users should register as vulnerable customers and maintain a device-specific emergency plan.
Worked example
The same question, answered end to end
A rural household planning for a two-day winter outage, with a CPAP machine that cannot stop, and no solar panel.
What was entered
- Essentials: fridge-freezer, broadband, phone charging, LED lights and a CPAP or medical device.
- A 48-hour outage to cover.
- No portable solar panel to recharge from.
How it is worked out
Each load contributes its running wattage multiplied by the share of the time it actually draws power — a fridge-freezer cycles, a router does not.
Those averages are added for the continuous demand, and the running wattages are added again undiscounted for the simultaneous peak the inverter must hold.
The largest single start-up surge is added to the peak, because an inverter that cannot start the compressor is useless whatever its continuous rating.
Average watts × 48 hours ÷ 0.85 gives the battery energy needed, the division being the conversion loss.
Each power class is tested against both that energy and the surge; the first class that passes both is the recommendation.
What the tool returns
- Average load
- 143 W
- Simultaneous peak
- 235 W
- Start-up surge
- 715 W
- Battery energy needed
- 8,075 Wh
Common questions
Questions about this tool
Can a portable power station run an electric shower?
No practical portable unit can support a normal Irish electric shower. Heat water another way during an outage.
Why does a well pump need so much surge capacity?
Motors can briefly draw several times their running power when starting. The inverter must survive that peak even though the average energy use is modest.