Power banks and power stations: choosing, calculating, using safely

Stored energy is measured in watt-hours (Wh). The calculation is simple: a device's power in watts times hours of use gives watt-hours. A 10-watt device runs for ten hours on a 100 Wh store in theory. In practice less is available, because energy is lost when converting the voltage and in the inverter – the part that turns battery power into socket power. Cold reduces usable capacity further.

Power banks are often rated in milliamp-hours (mAh), referring to the voltage of the cells inside, not the voltage at the USB output. To compare them with consumption, convert: watt-hours = mAh × voltage ÷ 1000. For common lithium-ion cells this nominal voltage is usually about 3.6 to 3.7 volts; a 10,000 mAh power bank therefore holds roughly 36 to 37 Wh. Many manufacturers now print the watt-hours directly on the case.

Power stations have a second important figure: the inverter's continuous output in watts. It determines which devices will start at all. If a device needs more power than the inverter can deliver, it shuts off – no matter how full the battery is.

Start with a list: which devices should run during an outage, for how many hours a day and for how many days? The power in watts is on the rating plate or in the manual. A made-up example: a 5-watt LED lamp that shines for four hours a day needs 20 Wh per day. Calculate this for each device and add them up. Then plan with a clear margin, because the full printed capacity is never usable.

For a phone and a lamp, one or two power banks are usually enough. For devices with a mains plug you need a power station whose continuous output is higher than the draw of the largest device. Also check the outputs: USB, 12 volts or socket. Where possible, supply devices directly via USB or 12 volts rather than through the socket – this saves conversion losses. Switch the inverter off when you do not need it, because it uses energy even when idle.

A battery only helps if it is charged. Store it as the manufacturer says and recharge it regularly, for example whenever the clocks change. When severe weather is forecast, charge it fully. During an outage: communication and light first, comfort later.

Lithium batteries store a lot of energy in a small space. If they are damaged, overheated or charged incorrectly, they can catch fire, and such fires are hard to put out. So charge on a non-flammable surface, not on a bed or sofa, not under cushions and not in full sun. Stop using swollen, damaged or unusually hot batteries and dispose of them properly, for example at a battery collection point.

A power station must never feed into the house wiring, for example through a cable into a wall socket. This endangers line workers on the grid and yourself. Fixed connections to the house wiring are made exclusively by a qualified electrician.

For powered medical devices, do not rely on your own calculations. Clarify with the doctor, care service and device supplier which backup supply is suitable.

Battery packs are ideal for communication and light over a few days and for small loads such as a router, laptop or fan. They produce no exhaust, are quiet and may be used indoors. That makes them particularly interesting for flats where a generator cannot be run.

They are unsuitable for electric heaters, kettles, cookers or hair dryers: these devices need so much power that even large stores would be empty after a short time or would not start them at all. A folding solar panel can recharge a power bank or power station during the day, but the output depends strongly on sun, orientation and season.

Batteries age even when they are not used. Technical literature on batteries describes how capacity and power delivery depend on temperature and age. A store bought a few years ago therefore often holds less than at the beginning. A test from time to time shows you what it can really still do.

The calculation in this entry is deliberately rough and is meant to give a feel for the order of magnitude. The manufacturer's information on charging, storage and permitted devices always takes precedence.