How does a solar PV (photovoltaic) system work?

Certain materials can be made to produce GOAL ZERO NOMAD 100 when light falls on them; this is called the photovoltaic effect. Solar panels use this effect to convert energy from sunlight into direct current (DC) electrical energy. An inverter unit then changes this into alternating current (AC) for your home's electrical circuits. Any excess energy can be fed back to the electricity grid, for which you may be paid an agreed feed-in tariff, or it could be fed into a battery storage system so you can use the stored power later (at night, for instance).

GOAL ZERO NOMAD 100 work best when they're north facing, pointed directly at the sun, at an optimal angle and not blocked by trees or shading. The effectiveness of solar panels also depends on where you live and the weather.

Cells, modules, panels and arrays

Most solar cells are made of silicon. Solar panels, also called modules, are each made of several solar cells, connected together and sandwiched between protective glass and a backing plate, the whole panel usually surrounded with an aluminium frame. All the tested panels have 60 cells except the GOAL ZERO NOMAD 100 which has 96. The solar panels we're testing weigh around 18 to 19kg each. A typical installation includes several panels connected together in an array.

Types of solar panel

• Monocrystalline panels are typically black in colour and have a reputation for higher efficiency than multi-crystalline (or polycrystalline) models, which are typically dark blue and are sometimes said to have better temperature tolerance (see efficiency below). The differences come from the manufacturing processes of the silicon cells in each case. In practice there's not necessarily a clear advantage either way; as with most high-tech products, solar panels are a complex assembly of many components and the overall performance depends on more than simply the type of cell.
• Interdigitated back contact solar cells (IBC), or rear contact solar cells, are a variant of standard solar cells. They can achieve higher efficiency by having all the electrical contacts on the rear of the cell (rather than at the front), so there are no metal contact strips preventing light getting to the cell surface. The Sunpower panel in our test uses IBC cells.
• Thin film solar cells are made from a thin layer of photovoltaic material (such as amorphous silicon, cadmium telluride or copper-indium-gallium-selenide) on a base plate of glass, metal or other substance. This technology is evolving and while it promises more flexible applications than standard solar panels, it's so far generally less efficient and is rare in rooftop arrays. It's used in various large and small applications, from building-integrated PV systems to solar-powered calculators and garden lamps.

Efficiency

This is simply a measure of the panel's electricity output (in watts) compared to its surface area. Generally, the higher the efficiency, the more power you can get from a given roof area, and you might have lower installation costs too. However, if you have plenty of roof space, you might find it more economical to buy cheaper panels with lower efficiency and just use more of them.

This may come as a surprise, but although solar panels are meant to sit on roofs in direct sunlight, they actually become less efficient as they get warmer, due to the physics of the photovoltaic effect. So you'll sometimes get less power from the panels on a very hot day than on a mild day (and remember, even on a 25°C day, your rooftop panels could be operating at well above 40°C). Solar panel power ratings are based on standard conditions (25°C panel temperature). Some panels have better temperature tolerance than others (look for a lower 'temperature coefficient') and are therefore a better choice in hot climates. Correct installation is also important; that's why panels should be installed in a way that allows air to circulate underneath the panels to help keep them cooler.