Solar Panel system - realistic output I should expect

[Oldskoolrools]

Hello all.
I’m new to solar power having recently moved into a home with an existing system. I have the following installed

• 23 x JASolar JAM60S01-295/PR panel, with a stated maximum power output of 295W per panel. Installed on pitched roof in 2018, which is a fairly shallow angle (estimate 30 degs to horizotal). South facing (full sun) with no shade. Ie could not be better placed as far as I can tell. Cleaned them recently.
• 1 x Solarriver 2300TL inverter
• 1 x Solis mini-3000-4G inverter

By my reckoning, the system should be able to generate 23 x 295W = ~6.8KW of power. The previous owner left notes, which said “in real terms” it generate 4.8KW. So far on the sunniest day of the year in the UK, the system has peaked at around 4.3KW (63% of theoretical maximum)
Appreciate there are many variables at play with solar systems, and I would not expect to hit maximum, but I can’t help feeling this is a little low (or the manufacturer’s maximum outputs simply aren’t realistic). My questions are

• Is the output I am getting realistic for the panels / system, or does it suggest a problem?
• Any examples of theoretical maximum vs actual maximum output anybody can share?
• Starting to wonder if I have a panel or two that have failed. Any recommended guides on how to test a PV system anybody can point me to?

Thanks in advance

[J.P.M.]

You reckon wrong. You're inverter limited (or over-paneled). The most you'll get out of your 6.8 STC kW array is about (5.5 kW combined max output *0.85) ~= 4.68 kW. The inverters are sort of acting as governors to limit system output.
Read on.
Download a free copy of "Solar Power Your Home for Dummies" from the net. It's a bit outdated but the basics contained in it are still valid.
After that, download a copy of something called "PVWatts" from NREL. Read ALL the help screens a couple of times and make a few runs. It's a pretty user friendly model and with inputs close to your array's conditions, results can be quite reliable. From those two sources you'll be able to make your own better, more realistic and more informed estimates of max. system output keeping in mind that the name of the game for most folks is to get an idea of likely annual output in kWh of system production, not necessarily peak instantaneous output.
Other things:
1.) Residential PV systems, even with adequately sized inverters, will most likely never produce their rated STC output (in your system's case 295 W/panel) under normal operating conditions.
2.) As a SWAG, your ideal system tilt is probably closer to your latitude, maybe a few degrees less. a 30 degree tilt isn't bad, but probably not ideal.
3.) Chances are, between 1st year burn-in losses of between 3-5 % and about 0.5 % additional annual deterioration after that, you've lost ~ 7 - 8% of your original performance which under the best of atmospheric conditions was probably no better than ~ 85% of STC output - see the dummies book for why.
4.) Then there is array fouling to contend with. Depending on the weather - particularly rain - a dirty array can lose an additional 0 to maybe up to 10-12 % or more of its clean performance.

Read the dummies book and a lot will be made clear(er).
After the read, come back and ask questions. Doing so will make the answers you get here a lot more understandable.
As far as testing for a failed panel, there are ways to test, but unless you have individual panel monitoring or have two strings per inverter, testing will probably take a trip to the roof.

FWIW, and with the little info available, I'd guess your system is performing about as it should.

[Oldskoolrools]

Hi JPM
First thanks for a very fast and comprehensive answer.
Downloaded the dummies guide and had a look at the PVwatts calculator. Initial calcs ignoring the inverter limit almost match my annual output, but that’s likely coincidence as I have made some assumptions until I can make some precise measurements.
The lack of inverter capacity appears a little bizarre, as the guy that installed the system actually ran his own renewables business installing solar panels and heat pumps, so I would have expected him to know what he was doing. That said, I already had questions about the air source heat pump set up that make me believe he might have been a little clueless, (it was configured so that the circulating pump runs 24 hrs, and not when the heating controller calls for heat. I estimate it was costing approximately £3/day in lost heat and electricity).
I am also confused as to why he installed 2 x inverters and not a single inverter, because as far as I can tell, all the panels were installed at the same time as they are built into the roof and not fixed above the tiles.
Anyway, I will have a good read and get myself up to speed. For now, great advice that points me in the right direction. Very much appreciated

[J.P.M.]

Hi JPM
First thanks for a very fast and comprehensive answer.
Downloaded the dummies guide and had a look at the PVwatts calculator. Initial calcs ignoring the inverter limit almost match my annual output, but that’s likely coincidence as I have made some assumptions until I can make some precise measurements.
The lack of inverter capacity appears a little bizarre, as the guy that installed the system actually ran his own renewables business installing solar panels and heat pumps, so I would have expected him to know what he was doing. That said, I already had questions about the air source heat pump set up that make me believe he might have been a little clueless, (it was configured so that the circulating pump runs 24 hrs, and not when the heating controller calls for heat. I estimate it was costing approximately £3/day in lost heat and electricity).
I am also confused as to why he installed 2 x inverters and not a single inverter, because as far as I can tell, all the panels were installed at the same time as they are built into the roof and not fixed above the tiles.
Anyway, I will have a good read and get myself up to speed. For now, great advice that points me in the right direction. Very much appreciated

Old Skool:

You are most welcome.
If I'd had more time, I'd have written less.
Still, not as comprehensive as epistles I've done in the past.
I've been accused of telling folks how to build a watch when they ask me what time it is.

The inverter size/capacity you have may make some sense.
As I'm pretty confident you know, England/Great Britian is a less sunny climate than places where PV is more popular.
As such, it MAY be more cost effective to undersize inverters.
Reason/Logic: For most homeowners of residential PV systems in less sunny climates, a PV system will be producing less power more of the time than in sunnier climes.
That can mean that an inverter with a capacity close(r) to the STC rating of the system will have less of its capacity used as a function of the available irradiance. So, it may make sense to not buy inverter capacity that will under-utilized for a large(r) portion of the 4,380 or so hours in a year that the sun is above the horizon.

Once you become (more ?) familiar with PVWatts, try this:
Do a run with your system as it is. Then, repeat the run but change the inverter size to match the array STC size (6.79 kW) and note the difference in annual output. The annual output may be greater with the larger inverter, but my guess is that the increase will be less than the difference in inverter sizes would suggest.

In general, it makes more sense to get inverter capacity close(r) to array STC capacity only as something called the "clearness index" goes up (that is, in sunnier climates).
In less sunny climates, an array spends more of its operational time running at a lower duty capacity. So, in such climates, more array STC capacity is necessary to meet a design duty (that is, the desired percentage of annual household electrical demand offset by PV).
However, in such less sunny climates, there is not as much of a need to have inverters that would have more of their capacity not used, so a lower capacity (relative to array size) inverter may be justified from a capacity and economic standpoints.

A trick with PVWatts that's not in the manuals (but is sort of a no brainer anyway) is to start with an inverter capacity that matches the STC array capacity and then lower the inverter size incrementally (while keeping the array size constant) until something called the "Life Cycle Cost") of the lost production equals the "Life Cycle Cost" differential between the two inverter capacities. There's more to it than that, but it'll get you started.

Some judiciousness for size increment selection is necessary as inverter sizes are all over the place.
In any case, my guess is you'll find that sizing inverters is not rocket science and anyway, about the most instantaneous power that's possible to get out of a stationary array is about 0.85 +/- maybe .02 or so of an array's STC rating. Anything much beyond that is a waste of inverter capacity and it's differential cost.

If you have a technical background, the bible of solar energy applications is by Duffie and Beckman: "Solar Engineering of Thermal Processes". contrary to the title, the tome contains what I believe is the most concise and short, but still complete introduction to PV I've yet seen. There is also a lot of info about the solar resource and a lot of reference sources that are applicable to either solar thermal or solar photovoltaic, and also a good chapter on solar process economics.
That volume too is available as a free PDF download.

Cheers,

J.P.M.