More PV for my pension?

Martyn1981

Ectophile wrote: »

In case anyone's wondering where the rather obscure figure of 3.68kW comes from, it's exactly 16 amps at 230V.

Yep. Here's some boring background:

In fact the earlier inverters were limited to 16A, but because UK voltage is only a 'pretend' 230V, and is allowed to vary from -6% to +10%, that meant export could hit 4,048W, so the rule became 3,680W, and folk may well see earlier 16A capped inverters exceeding that figure.

Folk may also see their inverters shutting down when grid voltage reaches (or exceeds) 253V, to prevent the PV systems pushing it even higher. Again, newer inverters should have this function. If this happens, you need to contact the DNO, as it's their responsibility to maintain an appropriate voltage.

Mart.

michaels

Martyn1981 wrote: »

Hi, I'm single phase. But 3 phase typically allows 3x the export without prior permission (11.04kW) but you'd need to check.

Obviously the supply cable can cope with more than 3.68kW, since most are rated at 80A or 100A, which is around 18kW to 23kW. However, what the DNO's are concerned about is the amount of leccy being 'pumped' backwards, and the ability of your street transformer, and the small local substation to cope with the total export of all SSEG's in the area (small scale electricity generators).

So all systems have to be registered with the DNO. In the case of 3.68kW (or less) inverters, they are notified after the install, but for anything that could (even if only theoretically) export more than 3.68kW you need to get prior permission.

For the 2kWp WNW extension, I asked nicely, they checked my house in summer 2012, noted that the street had been upgraded to individual supplies in Mch, and OK'd me for 5.9kW. The reason for 5.9kW is that I had to send them the certification documents for each of the three inverters, so they could check they were UK approved. They then added them together, and approved that as a theoretical max. I have asked, and they've answered that they simply don't care if I add more panels, only if I change the inverters for 1 (or more) that total more than 5.9kW.

Hope this makes sense!

Your best bet would be to ring them and ask, you may get a donut, or someone who knows exactly what they are talking about. Ask if it's worth 'asking' officially, which your installer will have to do. Get a feel for what might be allowed, eg only 3.68, or 5 or 6. From there you can estimate what could be installed split across the rooves. I'd link both systems through a single inverter, otherwise you'll be asking to add a huge amount of extra kW.

I genuinely doubt that a 6:4kWp split across E/W rooves would loose much to capping if you could get a 5kW inverter, but 6kW probably better.

SolarEdge also have inverters (I think) that can cap based on export (rather than generation), which might be accepted by the DNO, but you'll need to research. They are also launching the storage system in conjunction with Tesla, but this'll be pretty expensive at this point in time, and again, you'll need to see if this has some sort of export capping package, that might (just might) be accepted by the DNO in lieu of a generation cap.

Almost certain, that none of that will make any sense, I've even lost myself on that one.

Mart.

Mostly made sense but a little bit confusing is that you seem to talk about putting both arrays on a single inverter but previously you had said it might be important to keep the outputs on two different export meters so that I can make sure I get the current higher (up to 4kwh) tariff on the probably higher output W facing array?

Martyn1981

michaels wrote: »

Mostly made sense but a little bit confusing is that you seem to talk about putting both arrays on a single inverter but previously you had said it might be important to keep the outputs on two different export meters so that I can make sure I get the current higher (up to 4kwh) tariff on the probably higher output W facing array?

Don't worry about that, your FiT rates and generation will be so close, that you might as well share them. Apportioned, you might find some W units being classed as E units, but if you really want to go 'that' big, I think you'll have to share an inverter.

Asking for a single inverter of 5kW(ish) is one thing, but asking to add 4kW (or so) inverter on top of what you already have will be a push, and less likely to get approved.

Course, you could just wire up the new array straight to that monster FF of yours!

Mart.

michaels

So trying to size an invertor for my existing 4kwh W array plus a 4.5kwh E array. PV ouput has useful 'insolation' graph but will only show this for days I have output, which doesn't include June or July unfortunately.

So with a true E/W split the max is going to be at the solar midday but how many kwh? Would I need 7kwh to aviod being 'capped'?

Martyn1981

michaels wrote: »

So trying to size an invertor for my existing 4kwh W array plus a 4.5kwh E array. PV ouput has useful 'insolation' graph but will only show this for days I have output, which doesn't include June or July unfortunately.

So with a true E/W split the max is going to be at the solar midday but how many kwh? Would I need 7kwh to aviod being 'capped'?

Tricky question. Let me think!

My 5.58kWp can sustain 4.2kW if the weather is good, from around 11am to 3pm, but only when the sun is high, say peak 3 months straddling Summer solstice. Obviously this is only on the best days, when all the oranges line up.

4.2/5.58 * 8.5 = 6.4kW.

But that's an iffy guess. Also my rooves are 30d, so yours at 35d may need slightly less, so let's say 6kW.

But, remember, you don't hit peak that often, or for that long, so losses probably (sorry about the probably, but who the hell am I to guess) wouldn't be silly on a 5kW inverter.

I'm not trying to undersize you, just seeing how low you could go to give you a fighting chance of getting approval from the DNO. They may simply say that the property isn't suitable for anything over 3.68kW.

Mart.

cells

michaels wrote: »

My maths says a 6k install paying 572 for 20 years returns 7.1% - what am I doing wrong, surely it can't be that high?

it is probably more than that, as that writes down the value of the kit to zero at year 20 and its likely to be worth something at that stage rather than nothing.

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Maybe you could account for it as your 7.2% return in the first 20 years and then

£1k maintenance/repair to give it another 20 years life which is a 20% return for the capital employed at that stage (assuming no FITs just £200 annual value form the ~5-6MWh output)

michaels

Contacted my installer from June, they said they are already running down the PV side of their business in preparation for January no more FIT...but they said they should be able to do me a good price as they still have a fair bit of kit, I will of course keep the board posted. TO be comparable to my June install we would need 4kwh for 4.5k as no more iBoost needed. would be nice if it were even cheaper than this as this side of the roof would be a single aspect array with no shading or other problems so no need for solar edge.