MSE guide: plug-in solar panels

prowla

https://forums.moneysavingexpert.com/discussion/comment/82120626#Comment_82120626

Nope - I'm still not getting what you are saying.

An 800W panel-only plugin system will produce a maximum of 800W, regardless of the rating of the panels which provide the power to it.

If the panels produce more, where does the extra go?

Netexporter

https://forums.moneysavingexpert.com/discussion/comment/82121315#Comment_82121315

Panels produce their rated output at an irradiance of 1000W/m2 perpendicular to the panel face. Those conditions are rarely met in a real life installation, so panels also rarely reach full output. It is for this reason that arrays are "over-panelled" with respect to the inverter ouput, to compensate. Most enverters will take 150% overload, some as much as 200%.

You are correct you won't get more instataneous power that the inverter rating, but by over-panelling you will get more energy over a period of time.

jimi_man

https://forums.moneysavingexpert.com/discussion/comment/82121315#Comment_82121315

If you have an 800w system then the maximum it can produce is 800w, at best conditions, so sunny and unobstructed etc. However when it's winter or cloudy or slightly obstructed then it will produce less - a percentage of that - say 50% - so maybe 400w perhaps.

However if you have double that, so 1600w, then the panels will produce - at best conditions - 1600w, however only 800w will get through due to the 'throttling' by the microinverter. However on cloudy days, or slightly obstructed or winter then using the same ratio it will produce 50% of that which is 800w.

The idea being that as there is a limit of 800w then it's best to maximise that by pushing through 800w for as much time as possible. So the larger the panels then the more time the 'system' can spend pushing out the maximum 800w.

Obviously the figures have been slightly exaggerated to help you understand and I've ignored any slight output loss by the panels. HTH.

Heedtheadvice

https://forums.moneysavingexpert.com/discussion/comment/82121315#Comment_82121315

Is your concern maybe the advertising?

Correct, the panels ( in a non hybrid system) will not produce more than 800W at best so it could be considered a bit misleading using the headline figure of the panels capability, where it is higher than the 800.

In the thread it is also a bit misleading ( such as immediately above, amongst others) to say that the panels power ( when higher than 800Wp) produces a higher power. It is the inverter load that does not allow that potential higher power to be produced. No power is lost. Those types of comments are no doubt posters just trying to simplify what non technical readers struggle to understand.

However Prowla, you do not seem to accept the perfectly valid point being made by several posters, that with higher peak power panels there is a higher useable power generation capability under less than perfect conditions and those conditions will apply to many installations owing to physical limitations and imperfect solar radiation for much of the year. So peak power inverted is not increased but the total energy produced over time certainly is. Extra kWh for use.

Martyn1981

https://forums.moneysavingexpert.com/discussion/comment/82121315#Comment_82121315

I think others have tried to explain in great detail.

So I'll just cover some made up, but hopefully realistic examples.

On a nice sunny day in the ~4 better months, the 800Wp system will probably generate at ~90%, dropping down to perhaps 75% if the day is very hot and little wind. [This assumes pitch and orientation is at optimal levels. In reality, it almost certainly won't, and balcony installs may be steep pitched, reducing summer peak, and boosting winter gen, in that example, you'd benefit from even more than 890Wp of panels.]

So the 800Wp system will sustain a peak of around 720W at best, whereas the 890Wp system will generate ~801W (capped to 800W). I'm ignoring any inverter losses, but that will only benefit the larger system(s).

Briefly in Apr or Dec, if orientation is right, and sky is clear and cold, you may briefly see panels hitting 100%+, but of course, that will drop off, if sustained as the panels heat up.

All other times of the year, when generation isn't perfect, and also (bear in mind) when the sun isn't perfectly orientated to the panels, and their pitch, generation will be less.

So, a larger system will generate more per year than a 800Wp system. And in the case of 800Wp v's 890Wp, this is almost certainly the best example, as above 890Wp you might start to see some clipping, but I'd suggest (again) that 890Wp through an 800W inverter will see no (or negligible) clipping pa. Hence why I suggested that annual generation of an 800Wp v's 890Wp system will be in the ratio of 800:890.

In reality, I suspect a 1,200Wp system would be a better match, as generation will be less than peak before the sun lines up with the panel, and afterwards. Plus pitch issues as I mentioned. Just my opinion, but I would suggest that a 1,200Wp system, as I describe, would still see low clipping over a full year.

If the panels produce more, where does the extra go?

Into the home circuit, reducing import, or exported to the local grid.

jimi_man

In the thread it is also a bit misleading ( such as immediately above, amongst others) to say that the panels power ( when higher than 800Wp) produces a higher power. It is the inverter load that does not allow that potential higher power to be produced. No power is lost. Those types of comments are no doubt posters just trying to simplify what non technical readers struggle to understand.

Indeed. The term 'clipping' is used which is effectively what it is but the electricity is just not allowed to generated by the panels. However it is not quite correct to say that 'no power is lost' since it is limiting (clipping) the output which is effectively 'losing' some of the power. Probably a slightly pedantic point though!

The legislation allows for up to 2000w of panel power (still with the same 800VA inverter limit) though I think it said that electrician guidance should be sought above 960w. (I don't think it's mandatory).

The point being that the rules were obviously designed that people could, if they want, maximise their output in very poor conditions. The 800VA limit was never about the panel sizing, it was about the maximum amount of electricity that could be fed into the household system and that's the bit that seems to be causing confusion.

NedS

@jimi_man wrote:

The legislation allows for up to 2000w of panel power (still with the same 800VA inverter limit) though I think it said that electrician guidance should be sought above 960w. (I don't think it's mandatory).

Indeed, and will depend on the combination of inverter (MPPT input ratings) and panels (max current and voltage ratings) used, which is most likely why the legislation only allows for kits, where the two have been appropriately matched, rather than someone buying a micro-inverter and 4 x 500W panels on ebay and overloading the inverter inputs ending up with a very hot inverter.

It's just safer to sell 2 panels and an inverter as a kit where the constituent parts are well matched and there is less likelihood of wiring it up wrong (two panels overloading an MPPT input when connected in series).

prowla

https://forums.moneysavingexpert.com/discussion/comment/82121336#Comment_82121336

Yes, I absolutely agree with that and it makes good sense to do so.

However, you will never get more than the specified 800W out of the plug-in system.

However, if the inverter takes a 200% overload (ie. double), where does the energy go - does the inverter just disspipate it as heat?

Ectophile

https://forums.moneysavingexpert.com/discussion/comment/82122174#Comment_82122174

The excess power is lost within the panels. They produce electrons when the sunlight hits them. What you want is for those electrons to go around the circuit through the inverter. But if the inverter reaches clipping point at 800W, then the excess electrons re-combine inside the solar panels without going anywhere.

prowla

https://forums.moneysavingexpert.com/discussion/comment/82121787#Comment_82121787

Into the home circuit, reducing import, or exported to the local grid.

That is absolutely incorrect: the spec limits the output of a plug-in system to 800W.

It is not 800W but you can do more if you like; it is 800W maximum.

That's 800W output from the inverter, not more which the home can use, not more which goes onto the grid, not more onto the grid but you'll only get paid for the first 800W, not a net 800W after you factor in your house usage; it is a maximum of 800W output of the plug-in system.

@Netexporter reiterates an earlier comment that you can use overpanelling to extend the time the system can produce up to the maximum to include less sunny days or longer periods of the day, but it must never output more than 800W.