Solar chargers: realistic expectations

Solar cells produce electricity when light falls on them. They deliver most when the sun shines as squarely as possible onto the surface. When the sun is low, clouds pass, dirt lies on the surface or the light comes through window glass, output drops sharply. Even shade on a small part of the surface can noticeably reduce the output of the whole panel, because the cells are connected to each other.

The wattage on the label is measured under ideal laboratory conditions. In everyday use – on a balcony, in changing weather – a panel usually delivers considerably less. Anyone who calculates with the printed figure will be disappointed.

Because the output keeps fluctuating, a buffer battery makes sense. Many phones stop charging when a cloud passes briefly. A power bank usually copes better with fluctuating current and later passes it on evenly to the phone.

Place the panel in direct sunlight without shade, outdoors rather than behind a pane if possible, and point it towards the sun. The sun moves across the sky, so re-aim the panel every one to two hours when you are at home.

Allocate the energy. What the panel delivers goes first to light, radio and one emergency phone – not to entertainment. That way the important things stay supplied even on dull days.

Most important of all is a test before an emergency. Try out on a sunny day and on a cloudy day how long your panel takes to charge a power bank noticeably. Then you know what is realistic and do not unpack the panel for the first time in an emergency.

Only the panel belongs in the sun. Keep the phone and power bank in the shade while charging. In direct sun, batteries can overheat and be permanently damaged; a hot battery also charges less well.

Fasten panels on the balcony or in the garden so the wind cannot blow them down – a falling panel can injure people. Do not climb onto roofs to set up a panel.

A small solar panel is a good supplement in multi-day outages, especially in the summer half of the year, and for small devices such as a phone, headlamp and radio. It is quiet, needs no fuel and produces no exhaust.

It is not a reliable solution for life-sustaining medical devices. Nor are small panels enough for heating, cooking or a fridge. In winter the sun in central Europe is much weaker and shines for a shorter time – exactly when an outage is most stressful because of the cold. So plan for the worst case.

Plug-in balcony solar systems normally feed into the household grid and switch off automatically when the grid fails. Without their own storage and suitable technology, they therefore provide no power during an outage. Whether and how that can be changed is for a qualified installer or the manufacturer to clarify – not a DIY solution.

Small folding panels were originally developed for hiking and camping. They are practical there because you are outdoors during the day anyway. At home during an outage they need more attention: someone has to aim the panel and keep an eye on it.

Devices with a built-in mini panel, such as solar radios or solar lights, usually charge only slowly. They are a nice extra, but no replacement for a stock of charged power banks and batteries.