Will solar panels save us money?

QrizB

sazzlelondon said:

QrizB said:

sazzlelondon said:

They have quoted 13x310w panels for 4.03kWp vs 10x400w panel for 4kwp - if thats what you mean? Sorry new the lingo.

Thanks, yes.

I don't know how they can calculate that you'll generate 5667-6800 kWh/yr from a 4.03kWp array - typically the best you'll see is one kWh per year per watt of panel, less if you're in the north of the UK. Your parents don't happen to live in Spain or North Africa, by any chance?

If those quotes are for a 4kWp array they are a *lot* less reasonably priced.

OK, the way it has been explained to me - is as we are West facing based in Surrey the eradiance (not sure how to spell that) is c914 per 1kw of solar. So he is estimating to generate 5,000kw electricity we need 16 x 310w panels generate 4.96kw system of solar. So 4.96kw x 914 = 4533.44 kw a year per system. And if we use micro then there is 20% efficiency creating 5000+ kw. 

Thats how i’ve understood it. 

That sounds like snake oil, sorry.

Let's assume 16 panels is a 5kW system. I've gone to the PVGIS website and picked a spot in Guildford. A 5kW south-facing system on an ideal pitch of roof is predicted to generate 4980kWh/yr, but point that roof west and I get 4180kWh.

And there's no way that micro inverters will magically increase the output by 20%.They might stop you losing 20% if your roof is partially shaded by trees or a chimney, but they can't create electricity from nowhere.

£7300 for a basic 5kWp seems a bit on the pricey side; I would have hoped you could get that size of system for £5000-ish.

Looking at payback periods:

• If you really work at it, you might manage to use 50% of the electricity you generate (value 20p/kWh) and export 50% (value 5p/kWh). 2090 x 20p + 2090 x 5p = £522.50 per year, payback period 14 years.
• More likely you'll use 33% and export 67%. 1400 x 20p + 2780 x 5p = £419 per year, payback 17 years.

Adding a battery will mean you might (might!) be able to use 67% and export 33%, earning £626 pa, but the price goes up so payback now takes 18 years.

Solarchaser

Yeah I have to agree with QrizB,  I assumed the figures you quoted at the start was your kwp of the panels, not projected output.

arty688

If you want to play with the man maths also its worth considering that the electric you save is tax and NI  free , so the saving of £522.2 could be looked at as £670ish gross for basic rate tax payer. You also have to consider where the money to but the PV is coming from.

silverwhistle

arty688 said:

If you want to play with the man maths also its worth considering that the electric you save is tax and NI  free , so the saving of £522.2 could be looked at as £670ish gross for basic rate tax payer. You also have to consider where the money to but the PV is coming from.

Yes, when I bought mine I used my emergency cash ISA which was yielding annually a figure so small it wouldn't have bought a round in the pub. I wouldn't have sold any of my long term ISA investments, although as I'm now getting older my attitude to that has probably changed - there's no point in keeping capital indefinitely.

With the current surge in fuel prices it's nice not to have to worry _too_ much. March to October my hot water is sorted but I get a useful amount of water heating on clear winter days even if I need to top up with the gas boiler.

michaels

QrizB said:

sazzlelondon said:

QrizB said:

sazzlelondon said:

They have quoted 13x310w panels for 4.03kWp vs 10x400w panel for 4kwp - if thats what you mean? Sorry new the lingo.

Thanks, yes.

I don't know how they can calculate that you'll generate 5667-6800 kWh/yr from a 4.03kWp array - typically the best you'll see is one kWh per year per watt of panel, less if you're in the north of the UK. Your parents don't happen to live in Spain or North Africa, by any chance?

If those quotes are for a 4kWp array they are a *lot* less reasonably priced.

OK, the way it has been explained to me - is as we are West facing based in Surrey the eradiance (not sure how to spell that) is c914 per 1kw of solar. So he is estimating to generate 5,000kw electricity we need 16 x 310w panels generate 4.96kw system of solar. So 4.96kw x 914 = 4533.44 kw a year per system. And if we use micro then there is 20% efficiency creating 5000+ kw. 

Thats how i’ve understood it. 

That sounds like snake oil, sorry.

Let's assume 16 panels is a 5kW system. I've gone to the PVGIS website and picked a spot in Guildford. A 5kW south-facing system on an ideal pitch of roof is predicted to generate 4980kWh/yr, but point that roof west and I get 4180kWh.

And there's no way that micro inverters will magically increase the output by 20%.They might stop you losing 20% if your roof is partially shaded by trees or a chimney, but they can't create electricity from nowhere.

£7300 for a basic 5kWp seems a bit on the pricey side; I would have hoped you could get that size of system for £5000-ish.

Looking at payback periods:

• If you really work at it, you might manage to use 50% of the electricity you generate (value 20p/kWh) and export 50% (value 5p/kWh). 2090 x 20p + 2090 x 5p = £522.50 per year, payback period 14 years.
• More likely you'll use 33% and export 67%. 1400 x 20p + 2780 x 5p = £419 per year, payback 17 years.

Adding a battery will mean you might (might!) be able to use 67% and export 33%, earning £626 pa, but the price goes up so payback now takes 18 years.

These numbers look correct to me.  As stated the micro inverters will help make sure you don't get less than  4180 but won't let you get any more than that.

The only bit of extra man-maths you might be able to do is consider whether electricity prices might go up by more than inflation going forward and whether with the batteries you could 'arbitrage' a time of day tariff to reduce your overall unit price.

Heating hot water with electricity you could sell for 5p per unit rather than gas that costs 4p per unit will never make sense.

Solarchaser

If gas is 75% efficient at heating and electric is 100%, id be sticking with the electric tbh