Is a balcony power plant worth it? Yield, self-consumption and the maths

As a rule of thumb, a well-oriented photovoltaic system in Germany generates around 800 to 1,000 kWh per kWp of installed panel output per year. Values tend towards the upper end in the south and the lower end in the north. This rule of thumb applies to a south-facing orientation with a tilt of around 30 to 35 degrees and no shading – in other words, ideal roof conditions.

On a balcony, things often look different. Panels mounted vertically on a railing deliver noticeably less over the year – depending on location, often roughly a quarter to a third less. East- or west-facing balconies also lose a little compared with south, but spread the yield better across the day. Shading from neighbouring buildings, trees or the balcony above can reduce yield considerably.

You only save money with the electricity you use at the moment it's generated. With balcony power plants, surpluses usually flow into the grid without payment. How high your self-consumption share is depends on your base load – fridge, freezer, router, heating pump and standby devices run constantly and absorb some of the solar power during the day.

If you're at home during the day, run the dishwasher and washing machine at midday or work from home, you'll reach much higher shares than a household that's empty during the day. A metering plug or smart meter shows you how high your base load really is.

The following calculation is an example, not a promise. All values are assumptions that you should replace with your own figures:

Vertical mounting without tilt, a north-west-facing balcony or low self-consumption extend the payback period considerably. Conversely, inexpensive kits, high daytime consumption and rising electricity prices shorten it. Factor in the lifetime too: solar panels often have performance warranties of 20 to 30 years, while inverters often last less long. A balcony power plant that has paid for itself after five years keeps on saving afterwards.

More panel output – up to the permitted 2,000 Wp – mainly increases yield in the morning and evening and on cloudy days. That raises self-consumption, because the 800 watts are available for more hours. At the same time, costs rise, and more power is clipped at midday.

A battery makes midday surpluses usable in the evening, increasing self-consumption. However, it often costs more than the balcony power plant itself. Whether it pays off depends on how many kilowatt-hours the battery actually shifts per year. As a rough guide: a battery that shifts 1.5 kWh on each of 250 days a year delivers around 375 kWh of additional self-consumption – at 35 cents per kWh (assumption), that's approx. 131 per year (euros). Divide the battery's extra cost by this amount to see the payback period.

A battery makes most sense with plenty of panel output, a flat that's empty during the day and high evening consumption. With 800 Wp on a north-west-facing balcony, on the other hand, there's often hardly any surplus to store.

A few adjustments help you get more out of your balcony power plant:

It's not just the kit price that counts. Factor in the mount, a feed-in socket installed by a qualified electrician if needed, longer cables and shipping costs. Large panels are often delivered by freight carrier, which can increase the price noticeably. Thanks to the zero VAT rate for photovoltaic components on residential buildings in Germany, no VAT is charged on the components themselves.

On the other hand, there are hardly any running costs: there's no maintenance as such – occasional cleaning and checking the fixings are enough. But expect to replace the inverter once during the panels' lifetime.

Annual yield is distributed very unevenly. From April to September, a balcony power plant generates most of its electricity; in the winter months, often only a small fraction of that. This has two consequences: the largest surpluses arise in summer and flow into the grid without a battery, and in winter the system only covers part of the base load, even with low consumption. Vertically mounted panels balance this out somewhat, as they capture the low winter sun better.

In some cases, payback is questionable:

First: estimate the yield. Take the panel output in kWp and multiply it by a value between 600 and 1,000 kWh – towards the lower or upper end depending on orientation, tilt and shading. Online calculators such as the European Commission's PVGIS provide more precise values for your location if you enter your panels' orientation and tilt.

Second: estimate self-consumption. Use a metering plug or your meter to measure the minimum amount of electricity your home needs during the day. If the base load is 150 to 200 watts, it absorbs a good share of the solar power; if it's well below that, more flows into the grid. Third: multiply the share you use yourself by the unit rate on your electricity bill and divide the purchase cost by the result. That gives you an honest payback period for your situation.

After a few months of operation, you can check the estimate against real figures: the inverter app shows generation, and a metering plug or meter reading shows your consumption. If you compare grid consumption with the same period the previous year, you'll see how much electricity the balcony power plant actually replaces – the picture becomes most meaningful after a full year, regardless of weather fluctuations.