Solar Panel Guide Discussion

Martyn1981

Eric, just twigged that your set-up, 4kWp, south and shallow roof technically means you're in with a shot of a record, if (big if) we get a really nice day in mid or late June!

I wonder if anyone on here has managed a 30 from a 4kWp system yet, or more fairly, has anyone got 7.5kWh/kWp in one day yet?

Only asking about a '30', as I've heard it touted as a 'nearly got' target last year, but not a great June or July.

I got a new record today of 18.5, beating 18 I got late last August after install. Hoping for a 20 before April is out, then on to a 24 eventually.

Mart.

cheap-information

EricMears wrote: »

Assume the 26 kw was actually a daily output of 26kwh !!! A 26kw peak would be truly remarkable.

Peak output would presumably be whatever the inverter will handle - probably about 10% more than rated output before it's shut down as 'overloaded'.

Daily kwh would be several hours at that plus a few more at lower outputs. Every chance that would get well above your current max.

Not unusual to have an under-rated inverter though I've never been very impressed by the logic.

Yes, just checked and it was 25.76kwh on the 5th of May. I would have thought we would easily be hitting 30kwh in a few weeks time. Reason I asked about the inverter is that it is rated at 3500, however we peaked on that day, a smidge over 4kw/hour in power output (via the SolarEdge monitoring tool).

EricMears

Martyn1981 wrote: »

Eric, just twigged that your set-up, 4kWp, south and shallow roof technically means you're in with a shot of a record, if (big if) we get a really nice day in mid or late June!
Mart.

17.5deg is actually just a bit too shallow for optimum perormance on summer solstice day. That would be more like 30 deg (i.e. 53 deg latitude less 23deg for sun's distance N of equator)

But even my 17.5 deg ought to be better than the 45deg 'norm'.

If I'd designed the house for maximum PV yield , something like 35 deg would be ideal - i.e. fairly near to optimum @SS but still within same margin between May & August. However, it was actually designed for lowest possible ridge height but sensible headroom inside a (sloping) dormer . At the time PV was definitely a 'pipe dream' - payback on investmnt would have been around 400 years (yes, I really do mean four hundred !)

Martyn1981

EricMears wrote: »

17.5deg is actually just a bit too shallow for optimum perormance on summer solstice day. That would be more like 30 deg (i.e. 53 deg latitude less 23deg for sun's distance N of equator)

But even my 17.5 deg ought to be better than the 45deg 'norm'.

If I'd designed the house for maximum PV yield , something like 35 deg would be ideal - i.e. fairly near to optimum @SS but still within same margin between May & August.

Hiya Eric, similar here with 20 & 30 deg roofs, a summer optimised system. I've often wondered what roof orientation and pitch 'should' be deliberately built for PV. Steep South for winter, shallow east and west for lower but longer generation and twice the kWp, but no DNO issues, or shallow south, allowing north roof too for more generation, but that means even more summer excess, and DNO approval.

I'd guess that a large hip roof with steep south, boosted by shallow East and West if possible - interesting build!

You're probably aware already, but at under 20deg, you'd probably get around 70%+ performance from your north roof (compared to the south). After you spend a fortune getting 3 phase and DNO compliance!

Mart.

EricMears

Martyn1981 wrote: »

You're probably aware already, but at under 20deg, you'd probably get around 70%+ performance from your north roof (compared to the south). After you spend a fortune getting 3 phase and DNO compliance!

Mart.

Getting a bit OT here but my N facing roof has slope of around 75 deg.

May sound a bit odd but it's all part of a scheme to maximise solar gain and minimise thermal losses. Large flatish area facing S absorbs daytime heat. Small steep area facing S loses less. E&W are gable ends. Similarly, we have a lot of S facing windows, just a few facing E or W and two very small ones facing N. B&B construction plus concrete floors (with UFH) give maximum thermal mass.

Martyn1981

cheap-information wrote: »

I have a 4kw system in the north of England.

Also, we have a 4kw system but only with a 3.5kw inverter (SE3500), is this normal?

Hiya, inverter sizing seems to come under a swings and roundabouts heading. I just looked up the Solar Edge specs of the SE3500:

http://www.solaredge.com/files/pdfs/products/inverters/se-3500-4000-16a-inverter-datasheet.pdf

These give a recommended maximum DC of 3900, but note the ** that states “Higher input DC power may be installed; analyze yearly AC performance.”

Your installer may have gone for an undersized inverter to maximise efficiency (and hence generation) due to the UK not getting max sun for most of the year (that’s a polite way of saying our weather is ****). Many installers will go for a smaller inverter to improve low level efficiency, which could (how will we know?) actually give slightly higher total annual generation. The trade-off being an inverter running nearer to its max for longer, which potentially could shorten life expectancy.

May also have chosen that inverter because of your location ‘North England’ so less time at max again than S. England. Most likely went for that option as it saved both of you a few hundred pounds.

To make things even more complicated (sorry), we have a DNO approval limit in the UK of 16A or 3,680W (16A @ 230V) export, and for MCS approval, all inverters must now be built to meet this rule. Older inverters tended to be limited to 16A, so if grid voltage was at say 245V(or more) they’d output 3,920W (as an example). Now, more and more inverters are limited to 16A and 3,680W, so any extra generation may simply be wasted as the inverter caps output.

Just to complicate this issue even further, panel efficiency above 25C tends to drop off at about 1% for every additional 2.5C, so just when the sun ‘switches full on’, the panels get hot, and within a couple of minutes your system will probably only hold at about 90% max (3.6kW). Fun isn’t it, has your brain exploded yet?

As this question comes up a lot(ish), some time ago I found a great explanation on a technical forum, so I copied and pasted it in the following post (click on the arrow in the blue box). Have fun, Mart.

Martyn1981 wrote: »

This concept of "overloading the inverter" crops up a lot. It shouldn't be an issue. A simple example will suffice.

orrery

EricMears wrote: »

If your TV aerial is mounted significantly higher than ridge, it's probably been done for a reason...

True, but I put it there. If I stand on my roof (not something I try very often, you understand) I can see Sandy Heath so I should be OK with it in the loft. Anyway, it feeds the wife's box - mine uses the dish, so no real loss

EricMears

Martyn1981 wrote: »

May also have chosen that inverter because of your location ‘North England’ so less time at max again than S. England.

To make things even more complicated (sorry), we have a DNO approval limit in the UK of 16A or 3,680W (16A @ 230V) export, and for MCS approval, all inverters must now be built to meet this rule. Older inverters tended to be limited to 16A, so if grid voltage was at say 245V(or more) they’d output 3,920W (as an example). Now, more and more inverters are limited to 16A and 3,680W, so any extra generation may simply be wasted as the inverter caps output.

Whoops ! The further N you are, the more summer daylight hours you get (think 'midnight Sun' in Norway). The slightly longer absorbtion path through atmosphere would give a less intense noon peak in the North but I suspect that's rather less significant than the extra day length.

DNO approval wasn't hard to get. It may have helped our case that we have exclusive use of a nearby poletop transformer on an 11KV line but afaik it was just a case of installer having an extra form to fill in and waiting a few days extra for approval (but that was well within panel order lead time).

EricMears

orrery wrote: »

True, but I put it there. If I stand on my roof (not something I try very often, you understand) I can see Sandy Heath so I should be OK with it in the loft. Anyway, it feeds the wife's box - mine uses the dish, so no real loss

Relieved to hear that. You (or OH anyway ) would have been gutted if reception unexpectedly worsened !

Barter

orrery wrote: »

True, but I put it there. If I stand on my roof (not something I try very often, you understand) I can see Sandy Heath so I should be OK with it in the loft. Anyway, it feeds the wife's box - mine uses the dish, so no real loss

Who is Sandy Heath, and doesn't it seem a bit voyeuristic to go on your roof to look at her?

...and it's a bit non-PC to call someone a 'dish' these days; and don't get me started on "feeding the wife's box"...