General

Do we have enough sun in the UK?
Yes, most people are suprised at how much. A single pitch of a modestly sized house will receive around 54,000 kWhr of light energy per year. This compares to around 4,000 kWhr of heating demand and a similar amount of electricity use for the same property.
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Solar Heating

Don't solar panels have to face South?
No, anywhere on the South side of East through West works (90 degrees to 270). All duopitch roofs have one side facing this way. Solar PV is more affected by orientation than solar heating.
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How much energy can solar panels provide?
It is easy to size a system to provide around half of a building's needs over the course of a year. For an average sized home, with a hot water demand of around 2,000kWh or 100 litres/day at 60C, a 3 metre square area of solar panel should do this. It is possible to size a solar panel to provide a greater proportion, but increasing areas follow a law of diminishing returns, where each square metre added provides less energy than the previous one.
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See also Case Studies 001 and 002 for real-life data on performance.

How do solar hot water systems work?
A dark coloured surface absorbs the light that strikes it. The light energy is converted to heat, and the surface warms up. Solar panels trap the heat and a liquid flowing through the panel transports it away to where it can be used. Normally this is to some kind of store like a hot water cylinder.
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Why focus on supplying domestic hot water rather than space heating?
Solar energy is seasonal - there's much more in the longer days of summer than in the winter. Space heating demand is also seasonal, but in the opposite way. Hot water use is relatively constant throughout the year, and therefore makes a better target for solar heating. In new houses, levels of insulation have reached the point where hot water is the principal heating load.
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How do you compare the performance of different solar panels?
Two numbers are needed to describe the performance of a solar panel, the zero loss efficiency (the higher the better), and the heat loss coefficient (the lower the better). These are the two numbers that are used in the SAP 2009 calculation to estimate the annual energy output of a solar panel for a given surface area.
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How much does solar panel efficiency matter?
Very little. When the "fuel" is free, the efficiency is irrelevant. What matters is the ratio between the capital cost, and the energy collected (the payback). Of course, the efficiency will effect the amount of energy collected, but not so much as you might think.
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What is the Solar Fraction?
The solar fraction is the proportion of hot water demand met by solar heating over the course of some period (day, month, year).

Do you need to interface the controls with the boiler?
No. You can set up the plumbing so that the boiler and solar systems work in tandem without needing complex and costly control systems that manage both together.