4k,6K or 8K???

[gc100]

Hi all,

 

Having discovered we can afford to add the PV now as part of the build, I'm having trouble deciding what size array to put in. I may be limited to 4K as from my previous readings the DNO's only allow above 4K if installed via a MCS supplier??? Is that true? It my local electrician who'll be doing the job, so not registered.

 

Putting aside that for a moment, given both my wife and I work from home, kids, and future EV car purchase in the next 5 years probably how do I go about sizing this correctly? I'm aiming for an airtight house with good insulation, but due to the fact its a barn conversion, then end result is a bit of an unknown as to what the actual energy demands will be (ASHP heating BTW). Is it just finger in the air stuff or can I somehow work this out say from the SAP calcs? I'd rather do it now as adding additional panels now, will be much cheaper than paying for scaffolding again later down the road. The house is about 160m2 single story with vaulted ceilings, and we're going for a 11KW ASHP. 

 

Thanks

 

Edited February 18, 2020 by gc100

[Jeremy Harris]

First off, you can fit any size PV array you wish, there is no restriction from any DNO on the number of panels or the panel total power output.  There is an arbitrary distinction between a maximum AC export limited to 16 A per phase system and one that can export more than 16 A per phase.  This is a left over artefact from the old FiT system, that used 16 A per phase as a notional break point between only having to notify a DNO of any PV installation and having to seek approval from the DNO.

 

If you choose to fit more PV capacity than the notification limit (which is a nominal 3.68 kWp), but don't want the hassle of getting DNO approval for such a large export, then you can just install an inverter that is limited to 16 A per phase maximum export.  This doesn't have a massive impact, as most of the time any PV system won't be generating anywhere near its peak output.

 

The issue of MCS approval applies to any system, irrespective of size, if you wish to claim the SEG payment, currently about 5.5p/kWh for exported power to the grid.  If you don't wish to make use of the SEG scheme, then there is no requirement for MCS approval, the system just needs to comply with the regs in the same way as any other domestic electrical installation.  Whether the SEG is worth having depends very much on personal circumstance.  We have a 6.25 kWp PV system, that is limited to ~6 kWp by the inverter.  We're on the FiT, but our export payments are similar to the SEG rate, we get 5.38p/kWh at the moment.  Last year we received a bit under £150 in export payments.  You need to work out whether such an income is worth the additional cost that might be associated with having an MCS installer do the work.

[gc100]

Thanks Jeremy. TBH Im not really interested at all on the export side, its really about consuming what we generate, but there is obviously a balance. This is a dumb question, but what happens to the excess generated electricity if not exporting? 

 

I'm kind of presuming battery tech will get better and cheaper in the coming years, do may look to leverage that at some point I suppose.

[Jeremy Harris]

1 minute ago, gc100 said:

Thanks Jeremy. TBH Im not really interested at all on the export side, its really about consuming what we generate, but there is obviously a balance. This is a dumb question, but what happens to the excess generated electricity if not exporting? 

 

I'm kind of presuming battery tech will get better and cheaper in the coming years, do may look to leverage that at some point I suppose.

 

 

Nothing at all happens when an inverter limits export.  The panel voltage rises slightly, but that has no effect on anything.  The noticeable effect is that the output from the inverter may stay closer to its maximum rating for longer, rather than going up and down a fair bit.  Our system very rarely actually generates the 6 kW that the inverter is rated at, as most of the time the 6.25 kWp array can't generate anywhere near maximum.

 

We self-consume as much as we can, but to some extent having lowered our overall energy use works against this.  Hot water is the main consumer of excess PV generation, plus we try and run things like the dishwasher and washing machine when the system's generating, but most of the other loads don't easily lend themselves to only being on when we're exporting.

[Ferdinand]

Remember that you will be able to use it to power the ASHP in cooling mode should you choose that type of ASHP in summer.

 

But given your suggested sizes, that will apply to all of them.

 

F

[gc100]

Thanks both. So any idea how I could size this? How did you size your own system?

 

[Jeremy Harris]

20 minutes ago, gc100 said:

Thanks both. So any idea how I could size this? How did you size your own system?

 

 

 

I sized our system by fitting the maximum number of panels that I could physically fit on the roof!  Not very scientific, but it seems to have worked out OK.

[AliG]

The sizing calculations are quite complicated and also depend on the price of the system.

 

The more PV you install the less likely you can use incremental capacity to offset paying for electricity at 12p/kWh so the lower the incremental returns become. Offsetting  this is that incremental capacity also gets cheaper.

 

One thing you can consider is what will your peak electricity usage be and how often will you hit that peak. A 11kW ASHP has input of 3.5-4kW for a start, however your PV system will likely generate very little electricity on days when you run your heating. 11kW is a big ASHP for 160m2.

 

Before you do the calculation though, the first thing is to figure out how much roof space you have for the PV panels. They work best when close to facing due south. On our house we have 5kW as it was quite awkward to fit more than that on the roof which has multiple corners, your barn may just have a large straight roof.

 

You will get around 170-200kW per square metere of roof, so an 8kW array would require over 40M2.

 

My guess on the size of house that you have is that a larger system will be oversized.

[joth]

1 hour ago, gc100 said:

I'm aiming for an airtight house with good insulation, but due to the fact its a barn conversion, then end result is a bit of an unknown as to what the actual energy demands will be (ASHP heating BTW). Is it just finger in the air stuff or can I somehow work this out say from the SAP calcs?

Remember the PV won't generate much during the winter so you mac heating load calcs won't be much use for guiding how much solar to install.

Presumably you don't have 3 phase mains supply? That gives you the option ofup to 12kW before needing the G99 application.

If you are single phase and do make the application, put the max amount you might want to install on it (so 8kW), they are may come back saying only 6kW export is allowed or something in which case I'd probably aim for that. (while you can have more with an export limiting inverter, they cost more and will be another point of contention to resolve in the G99 application)

If they grant 8kW you can always decide to install less than that. 

 

1 hour ago, gc100 said:

I'd rather do it now as adding additional panels now, will be much cheaper than paying for scaffolding again later down the road.

 

Not to mention  the VAT savings?

 

 

[gc100]

1 hour ago, AliG said:

The sizing calculations are quite complicated and also depend on the price of the system.

 

The more PV you install the less likely you can use incremental capacity to offset paying for electricity at 12p/kWh so the lower the incremental returns become. Offsetting  this is that incremental capacity also gets cheaper.

 

One thing you can consider is what will your peak electricity usage be and how often will you hit that peak. A 11kW ASHP has input of 3.5-4kW for a start, however your PV system will likely generate very little electricity on days when you run your heating. 11kW is a big ASHP for 160m2.

 

Before you do the calculation though, the first thing is to figure out how much roof space you have for the PV panels. They work best when close to facing due south. On our house we have 5kW as it was quite awkward to fit more than that on the roof which has multiple corners, your barn may just have a large straight roof.

 

You will get around 170-200kW per square metere of roof, so an 8kW array would require over 40M2.

 

My guess on the size of house that you have is that a larger system will be oversized.

 

We had originally sized a 8.5 ASHP, but we have large vaulted ceilings in nearly all rooms hence we're going on the safe side and increasing the capacity.

 

I have about 115m2 of south facing roof (20% pitch), so if I mount the panels sideways I could fit 56 panels which is about 15KW system. So roof space isn't really the limiter . Using the EU calculator with my pitch and azimuth it says a 4K system would in the summer months generate 480KwH per month. How do I translate that into peak kw generated in the day for a given hour ? 480/30days/12hrs = 1.3 kW ? That seems a bit low.

 

Edited to add: I'm actually considering just doing the whole roof as the cost will be slightly cheaper than the zinc that is currently specified. Even we we don't get anywhere near using that capacity currently its going to be cheaper than just a straight zinc roof. Or am I being silly?

 

Edited February 18, 2020 by gc100