Solar Panel actual output over day/year versus theoretical output

I think Harry gets about 15%, and that's pretty good. He's about 10 miles SW of Worcester. The average for commercial solar farms in the UK is about 9%, 20-25% in Summer, down to about 4% in Winter, IIRC. South of England better than the north, obviously, but east versus west probably makes little difference. Optimum orientation and lack of shadowing more important. It's questionable whether solar panels in Northern Europe actually produce more energy in their lifetimes as is used in their manufacture. South of France, North Africa, California, yes, fine there, but not in Northern Europe. Without subsidies, or misleading claims by mfrs and installers, it's doubtful whether anyone in the UK would install them.

Some links to individual records/comments/experience:

What I *actually* want to know is the 'average over the year' *watts* from a panel. I could work it out backwards from the above but a simple "you'll get about 20% of the panel rating averaged over the year" would be more useful. I want to know how much panel I need to install to run a water pump, or how big a water pump I can reasonably power from a 260 watt panel.

Thanks, *that's* the sort of stuff I wanted to know. So my 20-25% figure is a summer figure. The winter figure means I can only run a

One book which stems from the OPEC induced oil crisis of the 1970's is called Sun power, by J.C. Mc Veigh, Pergamon Press 1977

I'd recommend avoiding the Kindle edition which is overpriced by about an order of magnitude. I have a copy that was withdrawn from the library system and sold off as scrap. You want table 2.4 which has experimental monthly figures based on Kew data 1958-1968. It is in MJ/m^2.

There have been a few improvements since it was written but it isn't a bad guide to DIY solar collectors 4 decades on from when it was written.

Better than that in summer and much much worse in winter. Peak summer to worst winter ratio is about a factor of 9. Week by week is more extreme.

Solar powered devices where I live in North Yorkshire wreck their batteries every winter. This is a shame since they work perfectly in midsummer but fail to remind people that on frosty mornings entering a tight bend too fast results in hitting a tree.

When I used to try to charge a leisure battery off a small panel to run an intermittent stable light in the winter, the actual performance was in the low single figures percentage, not actually worth doing. (Now I just use a couple of camping lights with lithium batteries).

We currently have a 5W panel trying to maintain charge on a small sealed battery (~16Ah) to run an electric fence (~50mA) round daughters rabbit hutch from dusk to dawn (managed by the PWM charge controller).

TBF, the panel isn't optimally placed ATM (temporary / test setup) and so only get's any real / direct sunlight in the mornings, the end result is even with all the sun we have been getting recently, supply can't keep up with demand.

As the days get shorter and less sunny this will only get worse and so I've either got to reposition the panel, add another panel or run some light SWA out to the rabbits store shed.

I *think* it's reducing the rate at which the batteries get discharged but when adding the panel I also swapped batteries (from 22Ah to the smaller one).

Cheers, T i m

well it's not difficult e.g assume you live in cambridge, a 1kW panel would give you 863kWh/year, so a 250W panel a quarter of that is

I think averages are a bit misleading in this context. On a sunny day in mid-winter, the output might be quite high, but only for a few hours, after which time you get next to nothing, and you get next to nothing for the full 24 hours on dull cloudy winter days.

That's the problem with any renewable power supply; they're intermittent, and unless you have battery back-up of sufficient capacity to tide you over the dull days and darkness, and sufficient renewable power not only to supply what you're trying to run but also to recharge those batteries, you're stuffed!

I note that most (all?) of those solar powered garden lights you see in garden centres etc these days, are useless in winter. The dullness/darkness is too long and the capacity of the photocell is insufficient to recharge the battery in the few hours of sunshine you might get once or twice per week, so the device never works.

But what matters to him is what happens on the worst days in winter and how many of those you get in a row so that he doesn?t get enough water moved.

That may well be what matters, but what he actually asked for was

"the average output of (say) a 250 watt panel over the year"

That is true - on a clear winters day the cold conditions mean that the PV array runs at higher efficiency, but not by enough to make up for the very low sun. To first order you can predict output in clear sun to vary as cos of the angle between normal to the panel and the sun and derate output by 0.3% for each degree C the unit is above 20C.

The other rough heuristic is that the flux arriving directly from the sun and the diffuse scattered component averaged by month are roughly equal provided you have the collector at a sensible azimuth and angle.

There is a slight advantage to pointing ESE or SE rather than due south since fair weather clouds in summer afternoons can rob you of power.

It is even more of a problem for such things like the radar activated "please go round the dangerous bend" on remote hillsides. They work brilliantly in midsummer but are stone dead two hours after sunset in winter and *NEVER* working on those cold frosty mornings. The batteries in them are ruined completely every winter without fail.

Again you are better off switching them off and putting them in the garage. The combination of hard frosts and the batteries being brutally discharged for a long period does them no favours at all.

Incidentally most of them are not that hot at our high latitude either since they come on too early in our long twilight. They work much better at lower latitudes when sunset to darkness is much faster.

Yes (OP here), I can see very little use for small panels except maybe for maintaining very low consumption devices like simple phones.

We have three 'full sized' (i.e. 260 watt) panels on our boat which is only a small (10 metre) boat and they are OK in the summer, in France, to run a domestic fridge plus lights etc. Through the winter they maintain the batteries (which is handy since I can remove the shore power connection and thus reduce the corrosion risk).

I'm coming to the conclusion that even a similar sized panel (i.e. 250 watt or so) panel isn't going to run much of a pump all the year round. Since just the panel will cost about £100 that's not a sensible economic proposition.

Average insolation is indeed about 10% of peak.

I get emails enthusing about how Gridwatch shows how great solar power is. And why is coal still on the BIG dial

They are never posted at midnight Or in te winter.

With or without something to store energy? The output from a panel will vary from zero to 260W so if you attach a pump that uses a couple of hundred watts it isn't going to start on a cloudy day.

So you really need a battery of some sort and then use the average to size the pump.

Does it need to run on demand or will it be OK running one day in three in summer and once a fortnight in winter?

It was handy to take one when cycle camping. We left it strapped to the top of the trailer and it would charge batteries for the GPS or torches whist we were on the road and again with the panel outside the tent when pitched up. 'Luckily' we had a reasonably sunny week and enough batteries to ensure we had some stock should it become overcast too long. Not having power hookup for the week paid for the panel and would have been overkill in a small tent in any case. ;-)

We see plenty of big panels on the narrow boats near here but they are often in the shade for at least one half of the day and dirty and not optimally aligned etc. I question what use they are above maybe keeping an alarm online?

When I took battery 'A' off the rabbit hutch electric fence solar panel and put it on a smart charger indoors you could actually hear it sigh from the pleasure of a real charge. ;-)

I think you can get these things doing what you want but you really do need to go OTT on both the panels and batteries to *guarantee* enough power when you want / need it.

It's like when you see places in the back of beyond living off grid when there may be several pretty large solar arrays, wind turbines, water wheel *and* a 'battery room' full of pretty big batteries (sometimes Milk Float / Submarines sized setups).

Again, it's one of those things that if money was no object (and assuming you had the space etc) I would probably have a large solar setup, just because. What I wouldn't do though is expect other people to pay for it ... ;-(

Cheers, T i m

Rule of thumb in Eastern England is average of 10% rated power over 1 year. For example a 250W panel will yield an average output of 25W over the year, giving a total yield of 220kWh.

peak to trough is a factor of about 9. And thats just the annual ...

Gridwatch is not definitive, because it never ever reaches the maximum rated capacity on all te solar panels in the country at te same time but this is what I get

mysql> select avg(solar), max(solar) from day where timestamp like '2018%' ;

So in 2018 the average solar was 1.2GW and the max was 9GW The min of course was no solar at all.

I believe greencunts reckon there was well over 12GW of installed panels during 2018

In winter - espoeciallcy December, its dire

select avg(solar), max(solar) from day where timestamp like '2018-12%';

So the average is a fifth of the annual average, and the peak is well down too.

It's more comlex than you seem to appreciate. Where abouts do you live I'm guessing a panel is scotland might be differnt from that in cornwall, then there's the angle, is it fixed or what direction does it face, or perhaps it'll be directional and controlled. Then there's the panel itself, there's differtn types and they age too and depending on how hot they get that will also change it's effcicincy and useful life.

I've seen this with students they calcluate what they'll get from a 12V panel, which is normally spec'd for far higher light levels than we get in the UK, they bring it into the lab expecting 12V at say 3 amps and they end up with 6V and milliamps (because of indoor lighting too).

I could work it out backwards from the above but a

Do you expect the water pump to run at night ?