Five-minute Guide to… Solar PV
Solar PVS are still worth considering — but you’ll need to hurry if you want to benefit from Fits payments, says energy efficiency expert Tim Pullen
Solar PVS are still worth considering — but you’ll need to hurry if you want to benefit from Fits payments, says expert Tim Pullen
It’s official — Feed-in Tariffs (Fits) will end in 2019. If you want to invest in renewable electricity generation, and hope to do so through using the Fits scheme, then you have 14 months to do it — and for most homeowners, solar PV is the only viable option.
What is Solar PV?
There are three types of photovoltaic (PV) cell: monocrystalline, polycrystalline and amorphous crystals. The monocrystalline cell is made from a single silicon crystal, polycrystalline is made from many crystals and amorphous cells from thousands of bits of crystal: Monocrystalline cells are the most efficient and consequently the most expensive as a result. Polycrystalline cells are a little less efficient and less expensive. Amorphous cells, used in watches and calculators, are very inefficient and very cheap and with efficiency ratings
of around 7.5%, they offer no practical solution at domestic or commercial scale. The best monocrystalline cells, by comparison, are achieving efficiencies of around 20%.
How Does it Work?
A through PV cell a silicon generates crystal. electricity Because by of allowing its atomic light structure, to pass an electronic charge naturally moves around inside the crystal in a random manner. Arsenic and boron are added to the silicon and these impurities cause negative and positive molecules to join and encourage the flow of an electric charge in a single direction. Bombarding the molecules with sunlight causes electrons to be released, increasing the electric current which can then be harvested. If you want to know more details of how they work and how they are made, then Richard J Komp’s Practical Photovoltaics is a good resource. Solar PV cells have been produced on a large scale for over 40 years and have benefited from a significant amount of research and development, which continues today.
What do you Need?
A typical domestic PV system will be something below 4kw maximum capacity, comprise 12 to 15 panels (depending on the potential output of each panel) and cover an area of around 20m2. Confusion often arises around this figure of 4kw. The correct notation is 4kwp, the ‘p’ being ‘peak’. This means that when the conditions are perfect (which may be midday on 21 June, with a cloudless sky, for a system orientated due south) then the system will produce 4kw of electricity. In all other conditions that system will produce less. Believe it or not, the amount of sunlight reaching the UK is broadly the same year-on-year. It has been recorded for many years and we therefore know what a PV cell is likely to produce in any year. We also know how that figure varies across the country. A 1kw system will produce around 850kwh of electricity in the most southern reaches of the UK, dropping to 750kwh in the north. For ease of calculation, it is reasonable to assume 800kwh per year per 1kw, so a 4kwp array can be expected to produce around 3,200kwh per year.
For comparison, the ‘average’ UK household will use about 6,000kwh per year.
The Importance of Inverters
An inverter converts the variable direct current (DC) output of a PV array to a regulated alternating current (AC) that can be used in the home or exported to the grid. It provides protection measures to prevent export to the grid if there is work being done on it, and protection to the house. There are three types of inverter available for domestic installations: String Inverter PV panels which are held in series, or ‘strings’ (similar to Christmas tree lights). A PV array may have a single string, or up to five strings, one for each row of panels. These are fine where there is no danger of shading or failure of any single panel. If that happens then the whole string will fail (similar to a single lamp failing on the Christmas tree lights). Micro-inverter Sometimes called an edge-inverter; as the name suggests, these are very small and each panel will have its own micro-inverter. In this case, any problem with a single panel will not affect the rest of the array. Battery Inverter These are bi-directional in that they act as both the inverter and battery charger. They are typically connected to the grid, which means the battery can be charged when the PV is not operating and allows PVproduced electricity to be exported to the grid when the battery is fully charged. The type and quality of the inverter is at least as important as the quality of the PV array itself. Most good quality PV systems will offer micro-inverters now, and unless the site is perfect in terms of orientation and shading then these are the ones to go for. If a battery system is installed then, by default, it has to be a battery inverter. As far as assessing quality, the normal rules apply. The cheapest PV system is unlikely to come with the best quality inverter. The inverter is the heart of the system; it controls the panels and will determine how well, or poorly, it works. And it is still true that the inverter is likely to fail before the PV panels.
Is Planning Consent Required?
A change in legislation in 2008 allowed the installation of PV panels to be deemed Permitted Development, and therefore planning permission is not needed, unless the building is listed, viewed by a listed building or in a conservation area. If the house is in a conservation area it is worth asking the local authority, as permission will sometimes be granted.
What Tariffs are Available?
There are actually two tariffs: The Feed-in Tariff, which is paid for each kwh of electricity generated, regardless of what happens to that electricity — the current tariff is 3.93p/kwh (and will be next updated on 1 April 2018). The Export Tariff, which is a minimum figure paid for each kwh of electricity exported to the grid; that currently stands at 5.03p/kwh. The exact export tariff figure is difficult to calculate, as it will be affected by the purchasing company’s attitude to standing charges, admin charges and the like.
How Much Do PV Systems Cost?
The ray installation) £3,000, (typical price plus of for will a 3.8kwp installation. a be domestic below arBudget nected to £4,500 the grid. to That £5,000 3.8kwp for an system installed is likely system, to produce cona little over 3,000kwh of electricity and generate £119 in Fits income per year. It is reasonable to expect to use 50% of the electricity produced in the house. If you are paying 15p/kwh to buy electricity, that 50% then has a value of £225 per year. The other 50% qualifies for export tariff and has a value of £67 per year. So in this case, the total value of the system is some £411 per year. But if 100% of the electricity is used in the home then the value could rise to £569 per year.
Are Solar PV Systems Worth the Investment?
In 2010 the Feed-in Tariff for PV was 44.19p/kwh — compared to the 3.93p/kwh being paid now. A lot of people argued that the rate in 2010 was too high and that it used up too much of the budget available, at the expense of other technologies. The reality is that it kick-started an otherwise dormant technology and there are now over one million PV installations in the UK. The massive drop in the capital cost of PV systems since 2010 means that it is still, just about, possible to justify the installation in financial terms. Some would argue that this is really only true if 100% of the electricity produced is used in the home — in which case you’ll need a battery.
“The massive drop in the capital cost of PV systems since 2010 means that it is still, just about, possible to justify the installation in financial terms”
A Popular Choice with Self-builders and Renovators Self-builders Lindsay and Justin Stead ( opposite) and Andy Bayliss and Rachael Alexander ( above) all opted for solar PV systems for their new homes. Solar-powered homes in the Ty Solar eco-village ( left) in Wales are aimed at first-time buyers.