How Much Electricity Will My Solar Panels Produce?

When calculating how much electricity a solar photovoltaic system will produce you have to take into account many different factors. The government have a standard rating system called SAP, solar PV SAP calculations are what an installer will use to estimate the amount of electricity your solar PV system will produce over a year.

This SAP (standard assessment procedure) will only produce a close estimate as there is no way of calculating how much there will be over the coming year. In general the calculations are pretty accurate and will give you a good idea about the performance of your particular system.

General rules and assumptions with solar PV

There are some assumptions that are made about solar panels which have generally held true. These can be used to get a very rough yet often closely accurate figure as to how much electricity any particular system will produce. These general figures are based upon the assumption that the panels are place in an optimal situation facing south, angled at about 30 degrees and have little or no shading.

For every 1kWp installed it should generate 850kWh over a year.

An average home in the UK uses about 3,300kWh of electricity.

The cost of electricity (2011 prices) is about 13 pence per kWh.

The export tariff is about 3.1p (2011 prices) per kWh.

The Feed in tariff pays 43.3p (2011 prices) per kWh.

Using these basic assumptions it makes it easier to get a guide figure as to how a particular system will perform over a year. It is however important to remember there are many different factors that will have an impact on the performance of any system and so no figure will ever be completely accurate.

Another important factor about solar is how you use the electricity generated. In theory a 3.88kWp solar PV system should generate all of the electricity a typical UK home needs over the course of a year. In practice this does happen due to fact that we do not use all of the electricity as it is generated.

Solar only works during the day so at night time you would have to buy in the electricity that you use from your energy company at the rate of 13 pence per kWh. Solar produces more electricity than most people can use during the day in summer and not enough electricity during the day in winter. The excess electricity that you produce is sold back to your energy company at the rate of 3.1 pence per kWh. You can immediately see the imbalance; you pay 10 pence more than the electricity companies.

The data below illustrates how much you may save annually on your electricity bill depending on the size of the array that is installed. It is assumed that your roof is south facing with little or no shading and is on a 30 or 40 degree angle. It is assumed that you will only use half of the electricity generated by the panels. The cost of electricity is estimated at 13 pence per kilowatt hour.

• 1.08kWh, 6 panels, 7.8m2 = 918kWh/year, £59.67
• 1.44 kWh, 8 panels, 10.4m2 = 1,224kWh/year, £79.56
• 1.8 kWh, 10 panels, 13m2 = 1,530kWh/year, £99.45
• 2.16 kWh, 12 panels, 15.6m2 = 1,836kWh/year, £119.34
• 2.52 kWh, 14 panels, 18.2m2 = 2,142kWh/year, £139.23
• 2.88 kWh, 16 panels, 20.8m2 = 2,448kWh/year, £159.12
• 3.24 kWh, 18 panels, 23.4m2 = 2,754kWh/year, £179.01
• 3.6 kWh, 20 panels, 26m2 = 3,060kWh/year, £198.09
• 3.96 kWh, 22 panels, 28.6m2 = 3,366kWh/year, £218.79

Making the most of the solar energy produced

You can get a pretty good idea of how much benefit you will get from having panels on your roof. Anyone getting a free solar PV system will benefit financially due to reduced electricity bills. Some people will benefit to a greater degree depending on their lifestyle and energy usage patterns.

If the house is occupied during the day and you use your electrical appliances such as your television, washing machine and tumble dryer then you will benefit more. Every house uses a background level of electricity from appliances such as fridges and freezers so everyone will benefit from using some of the free electricity produced. If you are at home during the day, try to use the energy then rather than in the evening when the panels are not generating power.

Factors affecting solar PV performance

For the panels to work to their full potential they need to be installed on your roof facing in the right direction. The optimum orientation is due south and on a roof angle of 30 or 40 degrees. As the direction and angle varies from this the panels will not be able to produce as much electricity which is the reason that companies offering free solar panels insist on your roof facing in the right direction.

Some companies will install panels on homes where the roof faces as far as 70 degrees from due south, that's nearly east or west facing. At this orientation on a 30 or 40 degree sloping roof the panels could lose up to as much as 10 percent of their potential. Any further than 70 degrees off south then you might as well angle the panels horizontally facing directly up into the sky.

There are new panels that are being tested all the time in the UK climate to see if they perform more efficiently. As these are used by the free installers then their criteria for getting a free installation will become less strict. This means that homes not facing in an optimal direction, have smaller roofs or suffer from a degree of shading could in the future qualify for free panels.