Solar install, part 4 (export)

Read part 1 for more background.

I'm really pleased with the install, and we are making 40 to 50kWh a day at the moment. So now for some paperwork...

One bit is a micro generation certificate, this confirms it is all installed by approved installers, etc, and has the details. I need that for and export tariff.

I have been discussing with the installers - we have a 12.09kW system (i.e. 31 x 390W panels), but they are east / west split so never all get direct sunlight. With the split and roof angle they have determined that maximum is going to be around 9kW, so a 10kW inverter is more than adequate. It will be interesting to see in the height of summer - but interestingly, even though only April, the generation in June won't necessarily be as good as you expect just by sun angle, because it is hotter. Being hotter makes the panels less efficient, so may cancel out the extra sun angle.

I heard there is some trick with some panels with water cooling, i.e. using the waste heat as a heat pump and keeping the panels cool for maximum PV. But we don't have that :-)

So, I may be making 9kW some days, yay. But what do I do with that?

As you will have read, the primary target for this my own usage, and then the excess power goes to a battery, so I can then use the power later when the sun is not shining as much - maybe covering my whole 24 hour day's usage for most if the summer, which is mental.

The bad news!

The battery is likely to be August. So 4 months of no battery. So, I'll export to the power grid, simples!

Well, yes, but that needs paperwork to. The Distribution Network Operator have to give permission to connect, which they have. The DNO decide what they consider the limit for export, if everyone was exporting, before it would cause damage to transformers, etc.

It varies, it can be as low as 3kW. Thankfully, in our cause though, it is 6kW. So we cannot export more than 6kW, legitimately.

Well 9-6 is still 3kW, and whilst I have the hot tub heating during the day, normally, a daytime idle power (when hot tub not heating), is a bit more than 1kW.

The result is, on a really sunny day, we could be exporting 2kW too much. Ooops.

Well, not quite, the inverter is set up to limit to the agreed amount, 6kW. It adjusts the panel optimisers to ensure that overall they are not quite as efficient, if we go over 6kW. So on a sunny day I should see a flat top on my export stats at 6kW. It means I will be literally throwing away some sunshine, up until I get my battery.


However, there is another snag - the final handover paperwork is normally done when the install is complete. The permission covers the solar and the battery - but we have no battery for 4 months - so I have no paperwork. So I cannot get on an export tariff.

The installers are trying to work out with Western Power Distribution how the handle an interim paperwork handover for the solar only without losing the permission to install the battery later. Hopefully something we have sorted shortly.


So the next step is a tariff for the next 4 months. It looks like I can get 5p/kWh from so.energy, less from Octopus unless I move to them for import as well. Octopus also do an agile tariff, but not being able to control when I export (yet, no battery), just relying on sunshine, I am not convinced that makes sense (a variable tariff for every half hour noticed a day in advance).

So my plan for now is probably the 5p/kWh from so.energy for 4 months, if I can confirm how the 12 month minimum works penalty wise. Once I have a battery I will need to revisit tariffs as there are interesting options.


  1. If you know normal idle use is somewhat over 1 kW, can't you get the solar system set to produce a maximum of 7 kW so you should not export more than 6 kW, or is that a case of "that's not how the rules work" ?
    And now I understand why our SolarEdge solar inverter can have a set maximum (which we don't use). Selling them in the UK wouldn't be possible otherwise.

    1. The SE can be told to limit export, so that is way better than a system that can only make 7kW in the first place. It is actually only a 10kW inverter, so limited to that anyway, which we have seen on sunny cool days. We could go through hoops to operate more than 10kW, but we can't export it more than 6kW, and it would mean more batteries if we want to store it (one is on order). So plan for now is try with one battery, and see how we go.

  2. Very interested in this article, I too have a base line of about 1kW with daily usages in the order of 60kWh.
    The last installer I spoke to ran into issues with the DNO saying I could not have a large generation system as they combined the capacity of the solar array AND the battery export capability which exceeded their limits.
    With such high usage it is mostly pointless putting a token gesture small system in place... I feel it has to make a good dent or what is the point?
    With batteries I do not even want them to have the spare power produced, but they play the game "what if it is the sunniest day on record, the hosue uses zero kW and I suddently decide to discharge the entire battery to the grid also!
    Maybe a full island system is the way to go... but UK winters... Ugh!

    1. What is odd is that there is an agreed limit on export, and the inverter is configured to limit to that. So if it is the sunniest day, it makes no difference, the system will not export more than the agreed limit. The panels just get hot instead. Having a battery means I can use some of that excess in the night and so export even less. At the end of the day they have to expect you to stick to the "agreement" (and you probably legally have to) as otherwise you could just install panels and export anyway, so an agreement to limit export (in my case to 6kW) has to be a valid thing, surely, regardless of size of installation?

    2. Would this be allowable across multiple inverters? Can inverters aggregate their export output?
      I think a Tesla powerwall is AC coupled so can output up to about 7kW on its own. So you have a couple of Powerwalls and a 9kW system, I guess you can see why the operators get nervious.
      Plus I understand that in the event the power goes dark due to any planned maintenence they may request your systems produce zero, save some poor electrical worker on a pole somewhere being zapped whildt doing repairs... How does your system isolate in the event you have power and the gris does not?
      I think it may be easier to just have a large a system as is possible and agree to never export...

    3. Ah, you cannot export when there is a power cut. The tesla battery can have a "gateway" to island my house. I'll blog on how that works when I get it. I may have to work on some changes to consumer unit so some things (like hot tub) are on the grid side of that gateway so they don't run at all when running only off battery/solar.

    4. I look forward to future posts and tips...
      Did you look at other battery techs alongside Tesla?

      I will share how I get on with the next installer... it will be interesting one way or the other!

  3. You could monitor if your export is going to reach the limit and if you are not consuming a lot or are not able to store it (no battery right now of battery is full) then you could turn things on like the hot tub or the air-con

  4. Watching with interest. Just ordered 10x390w and 6.4 battery. Use about 10KWh a day. Pulled the fuse on the kitchen underfloor heating after first bill years ago. Not convinced it will be a great investment but money in the bank is losing value fast.

    Electric supplier will not fit smart meter as presumably poor signal but I am advised my existing meter is so old it will spin backwards so expecting that to force smart meter.

  5. Could you put that excess energy into an immersion heater?

    1. Yes, but I have a battery on order, so the hassle and cost of suitable equipment to do that for a couple of months was not worth it.


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