There are three main types of solar pv panel. If you really want to know they're called monocrystalline (see 1st photo); polycrystalline & amorphous Thin-Film! They vary in levels of efficiency, durability and capital cost.
The choice of panel will also be
influenced by the pitch of the roof on
which they are to be installed and
the roof area available.
Solar PV can be installed free-standing
on the ground or on units that track the
movement of the sun, but these
systems are not the norm in the UK.
A static, optimally positioned (south facing with 30o to 40o tilt) solar pv "array" 
will produce about 1kW per m2 at noon on a sunny day. This is the "peak" output and the power will obviously vary over any given day and seasonally.
It's possible for all three PV types to be retrofitted to an existing roof or to be integrated into a new build or replacement roof. In the latter cases, PV panels come in the shape of tiles, rather than the large panels used for retrofit 
installations.
Solar pv arrays, both panels and tiles, are virtually maintenance free over a 20-25 year lifetime.
Smaller systems will often be standalone with battery storage (requiring occasional maintenance), an invertor for 240V AC needs and various essential failsafes.
Grid connected systems are metered with the potential for generating an income stream as well as electricity! The Government is considering increasing the financial incentives for solar pv systems, including the value of the certificates associated with the generation of green electricity in excess of 1MWh per annum. These are known as ROCs (Renewable Obligation Certificates) and are tradeable.
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If you would like to know more about Solar PV (or Solar Water Heating) or would like to see how we could assist you, please email us at solutions@watt-knots.co.uk with your name and 'phone number and we'll be in touch.
