4-5kw solar PV: advice please!

ahw23

I suspect nobody's going to be around right now, but if anybody is, some input would be really helpful. The scaffolding is all up, the installers are coming tomorrow. The DNO, despite hassling, have still not made their mind up and have until mid January to do so. There seem to be 3 options: 6.5kw + limiter, 5kw + limiter (to minimise risk of capping if DNO say no) or 4kw. I need to phone and confirm in the morning. OH has a cold, so can't think straight, and I'm feeling rather under pressure as I'd hoped it wouldn't come to this. Apparently, we are highly likely to get permission, as it's most often refused in rural locations where extensive extra infrastructure would be needed, whereas we are in the middle of a large village (albeit one with a solar farm and a fair few rooftop panels). Does anyone have any idea how much capping would occur if we went ahead with 6.5 but were refused permission? OH has suggested that we could fit the full 6.5 and only get the export limiter if we got a negative answer in the new year, but I suspect that might not be quite allowed. All thoughts welcome! Thanks.

Martyn1981

Hiya, don't know how much capping you'll get, but can make some guesstimations based on:

1. As per my 23/11/16 post, I wouldn't expect much capping at all from 5kWp. [Edit: the losses from a 5kWp E/W system will be negligible, and more than made up for with a smaller inverter. 5kWp is not a concern, don't go for 4kWp. M.]

2. My 5.58kWp E/W system tops out at 4.2kW (some cloud spikes to 4.9kW, but don't worry about these, they don't last long enough). This occurs from 11am to 3pm (at most) and only on very nice days, when the sun is high enough to hit all the panels at the same time, so perhaps June and July. I'd guess, on average at 20 days, for 2 hrs, with 0.5kW capping (4.2 - 3.68kW) = 20kWh pa.

3. 6.5kWp might proportionately peak at 4.89kW (compared to mine). So a greater capping loss, and logically more days per year when capping would occur as the bigger system is more likely to exceed 3.68kW more often.

4. However, the smaller inverter will probably be a little more efficient at lower generation times, which should make up for some of the capped losses.

5. The capped losses on a larger system, will of course mean a loss of FiT + export, however, the leccy itself would probably be exported (summer), so of no value in savings against import, however, the bigger system during lower generation periods, such as the winter, will mean extra generation that probably offsets import (10p .... 12p/kWh?).

6. Capping may seem annoying, but consider the cost savings of going bigger, which almost certainly mean a lower cost per kWp installed.

7. In the near future DNO's may well approve for domestic installs, a system approved for commercial installs, which caps export at 3.68kW, rather than generation. This is quite important, as it allows more generation, which can be consumed in the property, diverted to water heating, diverted to batts, and it's only when the actual export hits 3.68kW that the inverter reduces output. Could be an expensive retro-fit though.

8. Bigger systems, even with capping, would still allow for the future with more potential for charging an EV or batteries. So some future proofing built in.

ahw23

Well, it's now up- all 6.5kw, 22 panels! For all the planning and theory, it's still quite thrilling to see them up there, working, even in this grey murky light. Martyn, with regard to your point 7, I think it's actually an export limiter that we have, rather than a generation limiter. That's how the company explained it, anyway. It also records data of usage within the house and export. DNO have until mid-January to get back to us, so we still have a good shot of being able to disable that function and just use monitoring. Now to complete the paperwork and get it off to SSE...

Martyn1981

Congrats, and not a bad day to install either, as, on average, generation should get a little better now each day for the next 6 months.

Can I ask what make and model your inverter is, just being nosey and wanting to learn how the set up works.

Keep an eye on the battery thread (or perhaps pop back in 2 years, and then keep an eye on it) as with such a large system, you'd be able to make lots of use of a battery when prices become more reasonable, or possibly even a heat pump (small domestic A/C unit).

Have fun. :T

ahw23

It's the SE6000 from Solar Edge. I can't find the info. about the export limitation right now, otherwise I'd tell you what that was too.
We will be hanging around this forum for a while, so we'll keep an eye open about batteries or ASHP. The priorities for us over the next two years will be reducing energy consumption (adding extra insulation on external walls and other places, thermal blinds and maybe switching two old aluminium-framed windows), some form of ventilation with heat recovery (maybe just through the wall, but we have a humidity issue), and cutting reliance on gas for winter heat (probably a silicon carbide stove). Once we've done all that, we'll have a better idea of where best to use surplus and how much is actually needed. Thanks for all the help!

Martyn1981

SE6000, that'll do the job then! Nice.

There's a thread on batts, but nothing to ponder yet, still a few years to go me thinks:

On-grid domestic battery storage

but, you may find the ASHP thread interesting. Zeup has posted tons of info and facts, and also mentions the ability to reduce humidity, which might be interesting to you, as you'll probably have a lot of spare generation that could be used to displace GCH:

Discussion ... ASHP(Air/Air) with Solar pv ....

Have fun and post some generation numbers on this thread, if you feel like it:

Renewables: "talking 'bout my generation"