How solar panels work

Virtually endless free electricity comes from just over 150 million kilometres. Most Finns' first contact with solar energy can be found in the top corner of their pocket calculator. The cell, covered by a dark film, works wonders: thanks to it, the device adds, subtracts, multiplies and divides without batteries. For many, the next contact is with solar panels, which are now becoming increasingly popular for environmental and economic reasons. But how is the sun's radiant energy converted into electricity?

Aurinkopaneeleita katolla

Solar panels use silicon cells to convert solar radiation into electrical energy

Solar panels are typically made of silicon cells. Their efficiency is about 10–20%. This number indicates how much of the solar radiation is converted into electrical energy by the cell. Silicon cells are made of monocrystalline or polycrystalline silicon. 

Silicon is a semiconductor, which means that it allows electricity to pass through it. This is crucial: the energy of photons from the sun's radiation strips the electrons from the silicon cells, forming electron-hole pairs. In the internal electrical field of the cell, electrons are transferred to the negative electrode and holes to the positive electrode. A load is connected between the electrodes by conductors. This creates a closed circuit through which the electrons pass, and hence an electrical current. 

A solar panel consists of several cells connected in series. This is done to get the desired output power from the panel. 

The efficiency of a panel is affected by the intensity of solar radiation, that is, the power per surface area, the panel's orientation, angle and temperature. For optimal output, panels should be installed facing south at an angle of 42 degrees. Shadows of any kind – from trees and buildings – will reduce the output of solar power. Finland's sometimes cool summers are well suited to panels, as a cool panel produces better than a warm one. This is due to the properties of the semiconductor. 

The inverter converts solar power into usable form

In addition to the panels, an inverter is needed to convert the DC voltage produced by the solar panels into AC voltage for use in the building. 

Technically, an inverter is one of the components of a solar power system, along with panels and mounting brackets. All three are part of the package and are always installed together. 

The inverter is designed to protect the network. Since the panels themselves do not produce electricity that is directly ready for use, the inverter takes care of converting the electricity into a usable form. 

Unlike the panels, the inverter does not enjoy basking in the sunlight. Hence, inverters are most often placed indoors or at least under a canopy. Protection from direct sunlight or rain is therefore the best way to prevent premature failure of the inverter. 

The average lifetime of an inverter is 8–15 years, as the power electronics inside it reach the end of their useful life as they age. If the lifetime of a solar panel system is 25 years, it is likely that the inverters will need to be replaced once. Replacement is usually factored into the break-even period for solar panels. 

Despite the complex-sounding process, the production of solar power is in practice simple.

Frequently asked questions

What can solar power be used for?

The solar power you produce is automatically routed to your own use. Depending on the equipment in your home, you can use solar power for domestic hot water and indoor heating, cooling, cold storage and lighting. In the future, also for charging an electric car. Surplus solar power is automatically distributed to Caruna's distribution network and you can sell it to your electricity supplier. 

Which roof direction is best for solar power?

South is always the best direction with the most light, south-west and south-east are good runners-up. If the roof faces north, panels are rarely sensible to install. If the house faces east-west, panels can be mounted on both sides, or the orientation can be chosen according to whether the electricity demand is more concentrated in the morning or afternoon. 

Does Finland have enough light for solar power production to be viable here?

Yes. Producing solar power in Finland is much more viable than one might at first conclude.

The overall level of radiation in Caruna's operating area is about the same as in northern Germany. It is just not quite as evenly distributed, but more concentrated in the summer months. So although our light season here in the north is slightly shorter, our long summer days and cooler conditions make up for this considerably.

You can see the viability from the investment break-even calculation provided by the surveyor or supplier that comes with the tender. 

Can panels generate electricity if electricity is cut off?

For electrical safety, a solar power station (without a battery) switches off automatically at the beginning of a power cut and back on after the power cut. On the other hand, an interruption in electricity distribution usually occurs at a windy, rainy and dim time when the sun is not shining and panels are not useful.