To help you work out how much electricity your solar PV panel installation can generate each month here’s an example of a 2.5kW solar system.
The 2.5 kWp solar panels, made up of ten 250W panels on the left side of the roof, are mounted on a modern 3 bedroomed house. The installation cost was around £4,500. Normally for a retrofit to an existing house in the UK you would typically work on about 850 kWh per year of electricity being generated for each kiloWatt of installed panels. This would amount to an annual output of about 2,125 kWh.
However this example is an unshaded roof which is aligned south with the roof specifically designed to optimise the benefits from a solar array.
The achieved generation level is about 1,000 kWh per year for each KW of panel which produces about 2,500 kWh per year. This closely matches the annual metered electricity use of the house.
It’s worth understanding the figures available without a smart meter. Both the metered electricity and the solar generation can be measured. However as we don’t know how much solar generated electricity is exported we can’t actually measure the total electricity demand of the house. A reasonable assumption is that the overall daily house demand for electricity is about 10 kWh in mid-winter and around 9kWh in mid-summer. The variation is primarily down to an increase in electric cooker use, lighting and appliances in the winter months.
So let’s look at the data.
The chart shows both metered average daily electricity usage and generation. The green generation line shows the wide variation in average daily generation between the winter and summer months and the red metered line shows the big fall in bills during the summer months. Shorter days, a lower sun and cloudier weather all contribute to the lower solar outputs in winter. In the summer months a lot of energy will be exported whilst in the winter almost all the energy generated may be used.
Solar Power Is Not Enough On It’s Own
One thing the chart clearly illustrates is that when battery storage costs get cheap enough, probably within the next ten years, it would be possible for this particular UK house to go off grid for around six months of the year, with no need for electricity from the national grid. However during the October-March period an additional source of energy would be required to cover between 40-80% of it’s daily needs. Increasing the size of the solar array to a 4 kWp system, if there was sufficient roof space, would allow self-sufficiency in March and October but would still not get close to covering electrical needs during November-February.
Over the next few years policy makers need to build in the impact of solar panel on the generation needs of the entire national grid. In summer months power stations may well be idle for periods of time whereas in the winter they will be operating at close to full capacity. We will need power stations that can respond to being shut down on a frequent basis.