Photovoltaic Solar Power

Photovoltaic solar panels (PVs) use light to generate electricity. They are particularly well suited to sites where a grid connection would be difficult or expensive or that are only used in the summer. For an independent power supply, solar works well with wind as there is a good balance of power throughout the year. For small-scale energy production, solar photovoltaics is the most reliable and the easiest to install.

Photovoltaic systems use photovoltaic cells to convert solar radiation into electricity. The cells consist of multiple layers of semi-conducting silicon-based materials. When light shines on the cells, an electric field is created across the layers, causing electricity to flow. The greater the intensity of sunlight, the greater the flow of electricity.

Solar Cell Solar Panel

A single photovoltaic cell (above left) produces only a tiny amount of energy, so the cells are connected together into a solar panel (above right) in order to boost their power. Solar panels, in turn, can then be connected together in order to build larger and more powerful systems.

Start-up costs used to be high but have been falling by between 20-30% a year for most of the past decade and are now often the cheapest form of electricity production for micro power generation. Photovoltaic panels have some key advantages over other renewables:

Photovoltaic solar panels can be as modest as providing a little power for lighting and conveniences in a beach hut...

The efficiency of photovoltaic solar panels are comparatively low: around 10-15% of the sun’s energy is converted into electricity. They need to be sited in a good position, angled towards the sun in order to get the best productivity and kept away from shade. Energy production is the greatest during the summer.

The capacity of photovoltaic solar panels is measured in watt-peak power (Wp). A 100Wp solar panel would generate 100 watt-hours of electricity per hour in peak conditions, which you would get with the sun directly overhead on a clear day and an ambient temperature of 25°C (77°F). As well as traditionally shaped solar panels, it is possible to get ‘solar tiles’ that look like roof tiles to integrate into an existing roof on a house and flexible solar panels that can bonded on to caravans or boats.

Solar panels generate a DC supply, which is perfect for charging batteries. This current can then be fed into an inverter to provide an AC supply for powering higher voltage devices or feeding onto the grid.

Panels come in a variety of shapes, sizes and specifications, ranging from a 5Wp 12v solar panel that can be used to keep a car battery topped up in winter, charging up a mobile phone or powering an occasional shed light, up to a 300Wp 24v solar panel that would typically be used on a grid-tie project. Smaller panels – typically up to around 150Wp – tend to output a nominal 12v supply, whilst larger panels output a 24v–30v supply. It is also possible to buy a solar panel with a built in inverter, so that the panel itself provides a grid-ready AC supply. These panels are specifically designed for grid-tie applications.

photovoltaic systems can be installed on a building with a roof or wall that faces within 90 degrees of south, so long as nothing overshadows it. photovoltaic systems can also be free-mounted on pole mounts or free-standing structures.

...to providing a home with much of its electrical energy...

Restrictions and issues with solar energy

Solar panels can be extremely heavy, although more recent designs are becoming lighter. If installing solar panels on a roof, it must be strong enough to hold the weight of the panels. In some cases you will need to employ a structural engineer in order to ensure your roof is suitable for installing solar panels.

If the solar panel is in shadow for parts of the day, the output of the system decreases significantly. Even if only part of a panel is in shade, the output of the whole panel will drop. If you have multiple solar panels connected together, a drop in efficiency in one panel will affect the power generation of all the panels: a solar array is only as good as its weakest cell. For this reason, shading must be kept to an absolute minimum in order to avoid significant power loss.

For the same reason, if you have multiple solar panels in your system, they must all face the same direction and be mounted at the same angle. You cannot, for example, have one panel facing south and another facing east in order to capture the sun at different times of the day. If you did this, each solar panel would be restricted by the performance of the other, significantly reducing the overall amount of energy generated.

A further issue with connecting multiple solar panels together is that all the solar panels need to be identical. If you mix and match the solar panels – for instance, connecting a 20Wp and a 50Wp panel together in series – the performance of 50Wp would be reduced to the output of the 20Wp panel.

If you have a mismatch of different panels, or if you want to mount them facing in different directions, split them and treat the two as separate solar energy systems. In some instances, you can connect the mismatched panels together as two sets of parallel-connected solar panels (see below), but in the main, you are best to treat them as separate systems.

For bigger installations, you can have multiple strings (series) of solar panels that are then connected together in parallel. In reality, you would not actually do this. Instead, you would either have each string operating independently of each other. Each string would then either feed into its own solar controller or grid-tie inverter, or would feed into a grid-tie inverter that has multiple inputs for separate strings.

...to creating huge solar farms generating electricity for the National Grid.

Connecting Up Solar

For most applications, a single solar panel will not provide enough energy for your requirements. In this case, multiple solar panels can be connected together, creating a solar array, in order to boost the capacity of the system.

There are two different ways of connecting solar panels together: you can connect them in series (called a string of solar panels) or in parallel.

Calculating solar energy

The amount of solar energy available to you depends on your location. It changes throughout the year from one season to the next. Solar energy is the combination of the hours of sunlight and the intensity of sunlight at your location. This combination of hours and intensity is called solar insolation and the results are expressed either as kilowatt-hours per square metre, spread over the period of a day (kWh/m²/day). One square metre, incidentally, is just under 10 square feet.

A worldwide database of solar insolation has been compiled by NASA, based on weather satellite information from the past twenty years. The database takes into account atmospheric conditions, average cloud cover and surface temperature and are based on sample readings every three hours. Based on this information, you can calculate the amount of solar energy for any location on the planet, broken down on a month-by-month basis. You can then calculate the size of your solar array for producing a given amount of power.

Thankfully, you do not have to do any of this yourself, as we have created a set of online renewable energy calculator tools on this website that do all the calculations for you. This includes providing solar insolation data for the whole world and providing solar energy calculators and solar panel sizing information at the touch of a few buttons. Visit the Solar Calculators pages on the Renewable Resources section of this website to access these calculators.