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We're investigating fitting solar PV to our large guesthouse in the UK. We can fit around a dozen panels on the south-facing roof (sadly this is one of the smaller roofs). Some installers are advocating higher-efficiency panels (say 18% rather than the more common 14%).

The price of these panels is higher than the increase in efficiency would seem to warrant (say and increase cost of 40% for an energy improvement of 30%). There are other subtle differences in the specs but nothing that even in the installer's figures suggest any significant financial logic in buying the more expensive panels.

It looks like a no-brainer to get the lower-efficiency panels that pay back in the same timeframe, and indeed more profitably; but are we missing something?

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  • There would be a small reduction in installation effort when less panels need to be used.
    – Highly Irregular
    Feb 1, 2015 at 19:42
  • @HighlyIrregular - Good point, though that should work to even out the costs of the options. As it is, all suppliers are quoting 12 panels regardless.
    – Cheeseminer
    Feb 2, 2015 at 12:45
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    Since you're generating 30% more energy with the more efficient panels, it could be a good deal for the future if the payback period is short -- after the payback period, the more efficient panels are generating 30% more power, which means you're earning a better payback from them.
    – Johnny
    Feb 3, 2015 at 2:20
  • Just an aside regarding the "south-facing roof is small"; I read recently that west-facing panels can sometimes make more sense, because we tend to use more energy in the late afternoon than in the middle of the day. (When you consider the feed-in tariff, and the dark winter months in the U.K., probably still best to go with South-facing, though.)
    – Darren Cook
    Feb 5, 2015 at 22:17
  • @DarrenCook good point but unfortunately (or fortunately, as it's why we moved here) the Malvern Hills are immediately to our West! :-)
    – Cheeseminer
    Feb 6, 2015 at 13:33

3 Answers 3

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Rooftops are pretty much the only place where efficiency matters for PV panels, because surface area is typically a constraint there, but almost nowhere else. And the structure of UK feed-in tariffs usually means it's best to go for the maximum ~3.86 kW installation.

However, a 40% cost increase (I'm assuming that's the increase in price of the total system cost; not just the increase in price for the PV modules themselves) for a 30% improvement in yield doesn't look like a good investment, all other things being equal.

So the only question is, are all other things equal? Are the warranties the same on the panels?

Given the economics don't stack up for the higher-powered system, you could try to use that as a bargaining tool, to see if they'll lower the cost of the higher-powered system. Or find a different supplier - I think the hybrid Panasonic (was Sanyo) HIT panels have even higher efficiency, and are MCS accredited, so you could see if you could get a price on those.

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  • Warranty is a good point. In this case the two quotations we have a essentially similar but we're now getting a third where the warranty may be substantially different.
    – Cheeseminer
    Feb 2, 2015 at 12:48
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Expand the effective roof area.

  • A: Extend the slope of the south facing roof with beams that are flat to the south roof but overhang the slope of the north roof. You should be able to do one more row of collectors, and possibly 2.

  • B: Look at alternate mounting points: Can you mount collectors in the position of window awnings.

  • C: Is your exposure such that you can use wall mounted collectors to advantage. (These are less efficient in summer but more efficient in winter.

  • D: Can you use collectors at the base of the walls? I've seen solar air heaters done this way. Limits your use of the garden next to them.

The price of PV is coming down constantly. My advice would be to buy the system that gives you the biggest bang for your buck today. If, in 10 years a different system makes more sense then sell your present system to someone who has lots of room, and put up the more efficient one. Note: This works best if you do most of the installation work yourself. It also requires a modular inverter system.

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  • All good points but not exactly what I was asking. Incidentally, I'd considered 'C' as a way of expanding area but it's tricky as it's a historic building and wall mounting would be too intrusive.
    – Cheeseminer
    Feb 6, 2015 at 13:36
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The maximum rated power is not the only thing to consider.

You should also look at how your panels are going to perform in low light conditions. Cloudy days, early mornings and late afternoons etc. You need to understand if the more expensive panels are going to start producing earlier each morning and until later each evening.

I have some older 250W panels; now you can get up to 295W in the same panel size with the same number of cells. My understanding is that not only will you get a higher peak output per square meter, but you'll get better performance in less than ideal conditions.

PERC panels are a newer advance which I believe is worth investigating. I don't have any so I cannot comment from experience.

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