Solar PV to power electric rads / underfloor

loskie

4KWP good days from march to Sept you could make 25 to 30 kwh

Winter time(Nov to Feb) on a good day 4 to 6. Poor day 1

I think over 365 days my 3.6 in SW Scotland averages 8.5kwh per day
(3100kwh per year)
So you can see the problem when you need heat.

ed110220

Perhaps PVGIS is having a funny five minutes, but if I put the location as Bath and put in 4 kWp it gives a yearly generation of 3570 kWh (sounds about right, possibly slightly pessimistic) with the best month being May at 476 kWh and the worst Dec with 27 kWh and a monthly average of 298.

tunnel

I've just pinned Bath, used 4kWp, south facing, 35Deg pitch and get around the 475(give or take depending where in bath) for May but it comes out at 120 for December with a yearly total of around 3850. Are you using different settings Ed?

ed110220

I left it on its defaults and just changed 1 kW to 4 kW. Do any of its settings take account of terrain perhaps, because there are hills to the north and south that could cause shading when the sun is low. That could account for similar May but much worse Dec.

Martyn1981

ed110220 wrote: »

I left it on its defaults and just changed 1 kW to 4 kW. Do any of its settings take account of terrain perhaps, because there are hills to the north and south that could cause shading when the sun is low. That could account for similar May but much worse Dec.

That certainly sounds like an answer, though I'd have thought a 4kWp south system would do better than that just from reflection onto the panels. 27kWh in December is only about 150W for 6hrs a day.

My 3.58kWp ESE have a 63kWh target for December, with shallow pitch panels (2/3 30d & 1/3 20d), so they don't get an awful lot of direct sunshine in December.

Mart.

tunnel

I doubt it Ed, my WNW generates around 0.5-1kWh a day in winter and that's shaded nearly all day, only getting weak sun very late on.
A hill would have to be keeping the panels in constant shade to generate only 27kWh from a 4kWp south facing system in winter mate.

ed110220

I think it must be a bug in PVGIS. If i just search for 'Bath' it gives 27 kWh in Dec, but if I use my own postcode it gives 107 kWh. We are at the base of a North facing hill so would probably suffer as much winter shading as anywhere in town. Keynsham which is nearby and not a hilly location gives 123 kWh.

pinnks

I am about 35 miles east of Bath. My WSW 3.5kWp system gives about 70kWh in December. The SSE 1.75kWp system not much less at about 65kWp.

ed110220

I've looked into it a bit more and it definitely seems to just be unusual local topography. If you search for 'Bath' it picks a spot that isn't the town centre but near the bottom of a very steep North facing hill. The hill is about 100m tall and extremely steep, locally it is called a cliff. The steps up it are locally known as Jacobs ladder and are steeper than a normal staircase in the steepest part.

Maybe PVGIS doesn't cope well with such an unusual situation but that probably doesn't matter too much as I can't imagine there are that many places like it in inhabited parts of the country.

Move the PVGIS point away a hundred metres or so and it goes back to about 100 kWh in Dec. but even 100 kWh is only about £3.50 worth of gas and that's assuming you don't use any of the PV generation for all the electrically powered things in the home. Very, very roughly the average person might use 1000 - 2000 kWh of gas in December for heating so 100 kWh isn't going to make much of a dent.

Ed

Dave-o

Thanks for all the comments. I need to get reading as not sure I understand much of this! We are in Cornwall so gas heating is not an option. Most people use oil but as we're renovating and almost building most from new, we thought best to try whatever eco features we could afford. So let's say we had 10k for this, is it better to lash in an ashp, or solar panels and electric rads (we'd have a log burner to heat a fairly good chunk of the house) or less solar panels and an oil boiler? Is that an easier question to answer?
Thanks from a novice!