(Solar) efficiency isn’t everything?

To be a truly sustainable, energy independent, energy efficient house I need some way to generate my own electricity. At the moment I can only see two viable options (although I’m open to suggestions!), photovoltaic (PV) solar panels and/or a micro wind turbine.

I was leaning toward trying to do both, but my spirirts were dampened slightly when I learnt how poorly the turbine at 3 acorns had been performing.

So despite the fact that the UK has been on ‘amber alert’ this weekend due to the ‘near gale force’ winds of the storm called St Jude, I think I’m going to have to put my trust in solar panels for the majority of my electricity generation.

I’ve been looking in to solar panels quite a lot recently, and there’s a lot of information out there regarding thin-film solar, mono-crystalline and poly-crystalline, etc.

It seems the important figure to pay attention to is solar module (panel) efficiency (different to the solar cell efficiency, which is a bit higher). The efficiency is how much of the sun’s energy gets converted in to electricity.

At the time of writing this, the most efficient solar panel is SunPower’s X-Series that is 21.5% efficient. If you’ve only just started reading up on solar panels, this seems like a fairly low number, but the thing to bear in mind is that theoretically the most efficient solar panel we can ever produce will only achieve 44% efficiency (as only a certain range of light within the spectrum is used in PV).

Of course you’ll see news stories of record breaking solar cells of 44.7% efficiency, but these are CPV, or Concentrated Photovoltaic cells that use a concentrator to effectively shine more than 1 sun’s worth of light on to the panels. So it’s not quite the same.

Sounds good right? Sign me up, I’ll take some solar panels and a concentrator please. But unfortunately they aren’t available for residential use, and the cost is prohibitive anyway.

But it’s not important. 21.5% is more than good enough for most people’s uses. Even 15% is good enough to make a real difference. And people are very good at pointing out that you shouldn’t be an efficiency snob.

I don’t think I’m an efficiency snob, but what does concern me is a slightly different issue. It seems like advances in PV technology are happening every week. And the price of solar panels is decreasing all the time. I worry that I’ll get some solar panels and then 6 months later there will be residential panels available for half the price that are an extra 10% more efficient.

Just in the last 2 weeks I’ve seen the following stories which always make me think in a few months to a year solar panel efficiency is just going to leap forward massively:

A quarter more efficient?

A potential breakthrough in efficiency? (and good news for LED lighting)

And a possible 80% efficiency?!

Of course this talk of efficiency may be irrelevant to me when the time comes and we've actually bought our house. You also need to pay attention to positioning. A south facing roof is ideal (south-west and south-east are fine too). You can put them on roofs facing other directions too, but you lose efficiency (about 15-20% for west or east. I think it’s far worse for north).

And then there’s the angle of your roof too. A 35 degree angle is ideal in London, but the optimum angle changes depending on where you are in the world (or even just the UK).

So I’m not an efficiency snob, I’m really not. But depending on what house we end up with, I might end up having quite an unsuitable roof to place my panels on, so every percentage of efficiency may count!