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Boiler stove with gas combi boiler central heating and hot water
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I wonder if it might be practical to fit a solar thermal installation on your south-facing (front) roof and some solar PV panels on the east and west roofs (if they're large enough).
Solar thermal was popular 15+ years ago when PV panels were much less efficient and much more expensive than they are now.
I don't have specific figures, but you can imagine in 2005 if you wanted a 4Kw solar PV array then you might need 20 x 200W panels and pay nearly £20,000. By comparison a solar thermal system large enough to be useful for heating domestic hot water (only) might have cost well under half that. Fast forward to today and you only need 10 x 400W panels to achieve the same peak power output, and might pay £5000 to £6000 for it. And the solar thermal system (partly because not especially popular now) might still cost you £7000.
The advantage of solar PV is the flexibility. So for example on a sunny day in mid to late March - a time of year when many of us still have some requirement for space heating - by lunchtime our over-sized solar PV array will already have heated the domestic hot water tank to its maximum temperature, it will have powered an entire "intensive" dishwasher run, powered the other less power hungry appliances and lighting etc., and now already be delivering 3kW to drive the electric underfloor heating - all for free. By contrast with all that, a solar thermal system would (in almost all cases) only be heating the domestic hot water tank.
Some have also questioned the long-term reliability of solar thermal systems (the possibility for a minor fault to cause major damage) but I don't have any figures or even anecdotal evidence on that.
Anyway the thing to bear in mind when designing your replacement heating and hot water arrangements is, if you later wanted to make use of solar PV power to heat your domestic hot water tank, then instead of having an immersion heater only at the top of the tank (quite common), you would ideally want one immersion heater at the top and one at the bottom, or at least a bottom-mounted immersion heater. The reasoning is that a top-mounted immersion heater will generally only heat the uppermost portion of the tank, and will reach maximum temperature very rapidly therefore reducing the amount of energy that can be used.
2.72kWp PV facing SSW installed Jan 2012. 11 x 247w panels, 2.5kw inverter. 29MWh generated, long-term average 2.6 Os.