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Solar Power in the Home

By Smithers

Solar power is coming to most homes in the UK over the next twenty years so what should we know about it?  Most people wonder if they can get "off grid" so that they would no longer need to be connected to expensive electricity supplies.  This article will discuss the feasibility of going "off grid" and in doing so will explain the main problems and advantages of solar power.

Solar power is all about converting the power of sunlight into domestic electricity and so it is all about energy.  Electrical energy is measured in Kilowatt Hours (kWh).  One kWh is used when you run a one kilowatt electric heater for an hour or a hundred watt bulb for ten hours.  What sort of solar power system would be needed to deliver the 5 to 10kWh of electricity per day that is used by the average house in the UK?  

In the UK winter each square meter of panel only receives about 1 kWh over the whole day.

The energy supplied by a whole system with a "nominal" 4kW capacity, about 12 panels and covering about 20 square meters of roof is shown below:

A 4kW system such as in the graph above would only supply 3kWh a day at midwinter.  If you want to go "off grid" you will either need more panels or to cut your electricity consumption. 

The location of the panels is not as important as might be imagined.   The figures in the circles on the diagram below are percentages of optimum power for various panel orientations:

A lot of solar energy is scattered by the atmosphere so even north facing roofs can yield energy. Shading is important and if your panels are in shade they will yield a lot less power.

To get fully off-grid you would need a minimum of 10kWh of backup battery. This will cost £2600-£5000 ( see for instance Voltacon Solar or  LPBF48200 battery).

If you adapt your electricity consumption to be off-grid you will have a huge amount of surplus electricity in the summer.  Electricity companies may pay you a "feed in tariff" of about 5p for each kWh exported to the grid if you use their installation but there is now no government sponsored scheme for exporting energy to the grid.   Any such benefits will reduce towards zero as more solar power is generated.

If you are connected to the grid there is no need for a massive backup battery (just the luxury of a small battery that would cost under £1000 for power cuts etc.)  However, now that the grid does not want our energy the connection can be as simple as a switch that lets the house use the electricity main when no solar power is available.

The price of solar panels has fallen over the past ten years. It is now possible to get new, high efficiency panels of about 330 W that are around 1.6 square meters for around £130 each. 12 panels would  be "nominally" rated at 4kW and provide a real 3kWh per day in midwinter and over 10kWh a day in midsummer.  There is little doubt that for most homes getting "off grid" would mean cutting electricity use to 3kWh a day to survive the bleak midwinter.  The 12 solar panels for this system would cost about £1500.

A solar power system involves more than the panels.  The basic components of solar power are solar panels, an inverter, a charger and preferably a backup battery.  A schematic of a solar power system is shown below:

Typical prices are:

4kW Combined charger/inverters are available at around £850 . Ensure that the combined charger/inverter uses Maximum Power Point Tracking Technology (MPPT).

If independent chargers are used each kWh of battery backup would need a 20 Amp MPPT charger at around £30. A 5kWh backup charged as a single unit could cost £500.  The market for MPPT chargers is very fluid and bargains are available.

A power meter will cost about £40.

Panels will cost £1500 and a starter battery from Voltcon (2.4kWh) will cost £780.

The cables, connectors, frames, isolators, relays etc. mean that you would be lucky to get much change from £4000 for a DIY system that delivers 4kW.  This is about £3000 or more less than getting an installer to do the job but you would need to be very competent at both electrical installation and construction.  If you opt for DIY, the Local Authority Building Control Department (not planning) will need to be informed of the work and a qualified professional may be needed to inspect it (discuss this with Building Control).

Commercial 4kW systems start at £5000 without battery backup, with backup £7000 is typical.  Installers are adding £2000 or more for battery backup (including chargers). These prices are Internet quotes so expect another £1000 to be added in real life.

Given that much of the installation work is on roofs the best option may be to use an installer to fit a system without batteries and then fit your own battery backup later and as needed.

Sample suppliers: Contact Solar Panels UKEon Energy, or do your own web search.

Electric Vehicles

Electric Vehicle (EV) charging can use a lot of power.  Even the household plug connection for an EV draws 10 amps and consumes 2.4 kWh for an hour's charge.  Three panels on the garage roof would supply 1 to 3kWh over a day and would need a 5kWh battery if charging took place in the evening.  A garage system would only supply enough power for excursions of 10-20 miles a day.  It would make sense to link any garage solar panels up to the main household solar panels and upgrade the household system to at least 15kWh of battery backup with enough panels to charge it (an 8kW system), this would save duplicating chargers, inverters etc. and could run an electric vehicle.

The best resource for DIY solar power systems is: Guide to the Installation of Photovoltaic Systems

Important Notice: All the advice given above is intended only as an introduction to solar power systems in the home.  All data given above should be independently checked by anyone proceeding with a solar power installation.


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