PV Self-Comsumption Model for sizing system + battery.

[Dan F]

Hi,

 

I'm starting to look at PV options, given this will effect the weight of the roof and need to be taken into account for our frame design.

 

I've spoken to a couple of suppliers and heard two opposing schools of though:

1) Plaster your roof with as much PV as possible, if you don't use it all you'll use more when you get an EV and even then what you don't use you can export at around 5.5p/KWh

2) Put a 4kWH system and be done, anything else is a pain with DNO and probably not justified.

 

I know the export rates aren't great so oversizing a system too much probably isn't a great idea, but the larger the system the more it's going to cover spring/autum months, when you save you 13/15p/KWh.  I can also clearly see how a battery would help even out the supply/demand and reduce the need to import from the grid for evening usage in the months where there is sun.

 

Given the above, it looks to me like some modelling is called for to find the optimum system and battery size.  I'm unsure though if any PV suplliers carry out this modelling, or if it's just all guesswork and trying to sell you as many expensive batteries and you'll buy from them.

 

I came across this study, which is exactly the sort of thing that would be helpful: https://www.bere.co.uk/assets/NEW-r-and-d-attachments/Lark-Rise-Self-consumption-study-by-Energelio-160429.pdf

 

It references a  "methods developed in the Darke and Taylor’s excel worksheet.", but I have been able to find anything further on this online.

 

Has anyone else used any modelling to decide on PV/battery sizes to optimize self-consumption and reduce annual costs, or have suppliers/consultants done it for you?

 

Daniel

 

Edited July 25, 2019 by Dan Feist

[PeterW]

23 minutes ago, Dan Feist said:

given this will effect the weight of the roof and need to be taken into account for our frame design.

 

Not necessarily so, an in roof system can replace the tiles/slates and be lighter than an existing roof finish. 

[Jeremy Harris]

Not at all sure that PV makes any significant difference to the loading on a new build roof, does it?  The structural loading from our 25 PV panels is less than it would have been had the roof been slate or tiles, by a fair margin, so the roof was just designed to take normal tile/slate loads and has plenty of margin given that about 3/4 of one pitch is PV.

 

With an EV and no battery storage (yet) we export around 60% of the energy we generate over the course of a year.  That's for a 130m² house that is all-electric, fitted with 6.25 kWp of PV.  In summer we export a lot more than we use most days.  A typical summer day generation will be around 25 to 30 kWh, of which we can use maybe 5 or 6 kWh on heating hot water and perhaps another 5 to 10 kWh on running the house cooling system and background loads.  Most days we won't need much cooling, though, so the daily average house load is probably around 10 to 12 kWh or thereabouts.  Car charging averages around 4 to 5 kWh/day, but only about half of this can be drawn from excess PV generation so as far as PV utilisation goes I might average around 2.5 kWh/day for car charging over the whole year.

 

However, we quite often have to manage household/car consumption in order to match demand to the power being generated from the PV system.  PV output is patchy on an hour by hour basis most days, due to variations in cloud cover.  The hot water system can do this load matching automatically, as it changes the hot water system charging power on the fly, depending on the amount of excess PV generation available.  It's not easy to do this automatically for other loads, and even car charging is tricky to match, due to the minimum acceptable charge power in the standard being 1.44 kW.

 

Using a battery storage system might allow a small chunk of excess PV generation to be used, but not a massive amount.  The greatest advantage that battery storage seems to give is the ability to load shift from peak rate times to off-peak rate times.  I'm sizing our battery system on this basis, as that seems to be both useful in terms of cost saving, and useful in terms of reducing peak demand on the grid.

 

It's not hard to model likely loads over time, and PV generation over time is easy to model using PVGIS, so with the two data sets it's reasonably easy to do some what-ifs and see how things might look in reality.

 

 

 

 

[Dan F]

23 minutes ago, PeterW said:

Not necessarily so, an in roof system can replace the tiles/slates and be lighter than an existing roof finish. 

 

That's what I thought, but out TF sales person, suggested looking into this and confirming.

 

22 minutes ago, JSHarris said:

It's not hard to model likely loads over time, and PV generation over time is easy to model using PVGIS, so with the two data sets it's reasonably easy to do some what-ifs and see how things might look in reality.

 

I have the PHPP model which has the montly PV output, so I can probably extend this and play with different system sizes and compare the amount exported/imported per month. BUT, this would be very approimate as it would be monthly and not take into account any peaks/troughs in demand at all.  Battery sizing is a bit more complex though, as you need to consider hourly demand also and potentially variance in PV output based on cloud cover also if you want to smooth out over days and not just over 24hrs.

Edited July 25, 2019 by Dan Feist

[Jeremy Harris]

The reality is that there is so much short-term variation in both demand and PV generation that detailed modelling to try and optimise battery size (or more probably inverter capacity) can be a bit counter productive.  I started off with the premise that I would model everything in as much detail as possible during the design stage.  Must have wasted hundreds of hours on it before we started the build.  99.99% of that effort was pointless, as experience has shown that all that's needed is a "big handful" estimate. 

 

Things that have thrown a spanner in my modelling have varied from the predicted thermal time constant for both the house, and the floor slab, being significantly longer in practice than theory (and estimates of material properties) would suggest.  The local microclimate threw both the PHPP heating requirement predictions and my simple spreadsheet model estimates out of the window, they ended up being significantly in error.  Overheating risk was massively underestimated by both PHPP and SAP, and we quickly realised that we needed active cooling and modifications to the glazing to make the house bearable in hot, sunny, weather.  Overheating also caused me to rip out our hot water system, before we moved in, and replace it, as the heat loss from the thermal store was unacceptably high and led to the services room getting to around 40°C, enough to damage the oak veneer on the door.

 

The PHPP PV prediction seems a bit less accurate than the PVGIS one, I've found.  PVGIS seems to pretty closely (within a couple of %) match our actual generation, so I tend to use that as my preferred output estimating tool.  I've also found that our export power is often limited on very sunny days by the local grid voltage.  The inverter soft-limits to ensure that the output voltage doesn't exceed 253 VAC, and this happens surprisingly often in summer.  It's a consequence of there being quite a lot of small scale PV generation in the village, together with a relatively low local demand (no industry anywhere near by).

[Nick1c]

I am (hopefully) going with an in roof system in the knowledge that it currently is not justifiable on a purely financial return basis. 

The rationale is that our ability to self use the power generated will increase over time with electric vehicles & time-shifting via batteries, making the numbers more attractive , a feed in rate may become available & it is a 'good thing'. If we need active cooling they will make the running of it free as I assume it will only be needed when the sun is out!

Our roof pitch is pretty much due N/S so we have no ability to change the time of peak generation throughout the day, if we oversize it to 'widen the shoulders' of generation we will use an export limiter if needed by the DNO. I have no plans to install batteries in the short term, but will make provision for it so we can use them as & when they become economic. 

[Ed Davies]

1 hour ago, Dan Feist said:

2) Put a 4kWH system and be done, anything else is a pain with DNO and probably not justified.

 

4kW (no H or h).

 

The actual limit is 16 amps which equals 3.68 kW. But that's the output limit on the inverter, you can have more PV than that as the inverter will just throw away the excess for the short times when the panels are generating the full nominal amount.

 

Also, might be worth asking the DNO what the limit for you would be. It depends very much on the local network and they might be happy for you to feed in more than 16 amps. If they need to increase the local capacity to allow you to feed in they have to pay, beyond 16 amps they can (and would) charge you.

[PeterW]

6 minutes ago, Nick1c said:

I am (hopefully) going with an in roof system in the knowledge that it currently is not justifiable on a purely financial return basis

 

Does that include the cost savings on the roofing materials, installation and additional benefits of using an in roof system..? 

[ProDave]

Re sizing,  First ask your DNO what they will allow you to connect.  Up to what is generally referred to as a "4KW" system (actually the inverter output is limited to 16A or about 3.68KW) you can connect without prior notification but you must notify the DNO once you have connected it.  This is what we have with 4KW of panels.

 

Above 3.68Kw you need prior approval BEFORE you connect it. Depending on the infrastructure around you they may just say yes or they may hit you with a cost to upgrade the network first.  This I am sure is what would happen here. When I submitted the form to notify mine, they initially read it as "4KW" and wrote to me saying as it was over 3.68Kw  they would provide me with a quotation to upgrade the network and I must not connect it until the upgrade work has been done.  That went away when I provided them with the manufacturers declaration that the inverter does limit the output to 3.68KW

 

Once you know what size the DNO will allow you then look at the roof space and see how much will fit in nicely, and how much you are prepared to pay for.

 

We self use at least 90% of what we generate.  In fact the exported amount is so low that I estimate even if we were able to sign up for the 5.5p per KWh export payment we would get less than £10 per year, so totally not worth it.  In order to sign up for that you have to use an MCS installer which is generally recognised to push the price up somewhat.  If you are not bothered about claiming the paltry export payment then you or any competent electrician can connect it, probably for a lot less than an MCS installer.

 

The key to maximum self usage is to use the big appliances (washing machine, dishwasher and tumble dryer) in the daytime, one at a time. If you are out all day set them on a timer.  The other key thing is choose a hot water system with a hot water tank and buy a device that sends excess PV generation to the immersion heater.  In the first half of this year my unit has sent 324KWh to the immersion heater.

 

The last thing I do to maximise self usage, is at this time of year I have the ASHP set to heat the hot water after 11AM when there should be enough PV for that to power it on a good day.

[SteamyTea]

2 hours ago, Dan Feist said:

I've spoken to a couple of suppliers and heard two opposing schools of though:

1) Plaster your roof with as much PV as possible, if you don't use it all you'll use more when you get an EV and even then what you don't use you can export at around 5.5p/KWh

2) Put a 4kWH system and be done, anything else is a pain with DNO and probably not justified.

Not exactly opposing, they can both apply.

9 minutes ago, Ed Davies said:

you can have more PV than that as the inverter will just throw away the excess for the short times when the panels are generating the full nominal amount.

Don't think you can under MCS rules, they closed that loophole a few years back I think.  It is the installed capacity of the modules that the DNO is concerned about.  Worth checking  though as things change.

 

@Dan Feistdo you know your current usage pattern, that can help.

As an example, I use about 1 kWh/day for my general use.  All the rest, somewhere between 3 and 20 kWh/day is on E7.  Those loads I could shift to PV/Storage if I wanted to.

 

Failing that, use the Golden Ratio, 1.62 kW installed capacity to kWh storage.

It is probably as valid as any.

 

@JSHarris have you got, or can you dig out, daily data on how much you use and how much you export. The exports could be considered the optimal battery size needed.  Then you can take away daily imports and see what is 'wasted'.

[Jeremy Harris]

4 minutes ago, SteamyTea said:

@JSHarris have you got, or can you dig out, daily data on how much you use and how much you export. The exports could be considered the optimal battery size needed.  Then you can take away daily imports and see what is 'wasted'.

 

Sadly not.  For some reason I didn't opt to log energy use on the house logging system, only environmental stuff.  Not sure why, as it would have been easy to do (the energy monitoring system broadcasts instantaneous power use, import or export, every 10 seconds).  I do have energy metering on things like the ASHP, hot water system and car charge points, but unless I regularly read the meters and not the readings I can't give a day my day data set.  TBH, energy costs are so low that it's sort of slipped off the radar as being something to spend a lot of time on.

[SteamyTea]

8 minutes ago, JSHarris said:

Sadly not. 

Shame

Would take 90% of the guesswork out of it.

 

[ProDave]

14 minutes ago, SteamyTea said:

 

Don't think you can under MCS rules, they closed that loophole a few years back I think.  It is the installed capacity of the modules that the DNO is concerned about.  Worth checking  though as things change.

 

The DNO are concerned about export current. Up to 16A you don't have to notify and they cannot refuse connection.  If you are not claiming the small export payment there is no need for MCS involvement either.  So subject to the voltage and current input limits on your inverter I cannot see an issue with more panels.

[SteamyTea]

2 minutes ago, ProDave said:

So subject to the voltage and current input limits on your inverter I cannot see an issue with more panels.

Needs to be checked out or we risk giving wrong answers. 

[jack]

21 minutes ago, SteamyTea said:

Needs to be checked out or we risk giving wrong answers. 

 

Agreed. I seem to recall that the limit was measured based on the panel capacity rather than the inverter output, but I might be misremembering, or the rules might have changed.

[ProDave]

25 minutes ago, SteamyTea said:

Needs to be checked out or we risk giving wrong answers. 

A quick search finds this http://www.ena-eng.org/ENA-Docs/D0C3XTRACT/ENA_EREC_G98_Extract_180902050358.pdf

 

The only talk is of the maximum current being less than or equal to 16A.  It does not seem to care how many panels are connected or indeed anything about the technology used, just the maximum current.

 

Unless anyone else finds anything to contradict that I still believe the maximum number of panels is a technical limit of the inverters max voltage and current inputs. It will still limit the output to 3.68KW / 16A

[scottishjohn]

 

unless you are already on FIT system started before end of march  ,then any new installations cannot  get payment for energy that is returned back to grid . the new system ,as I understand it is  called  SEG and its not finalised yet and only a few test sites are being run and will need a smart meter so they can pay you at different rates dependant on what the spot price for electricity  is at that time-it  could change tariff every 30mins 

so trying to make some sense of it for a new installation is hard at this time 

Edited July 25, 2019 by scottishjohn

[jack]

3 minutes ago, ProDave said:

Unless anyone else finds anything to contradict that I still believe the maximum number of panels is a technical limit of the inverters max voltage and current inputs. It will still limit the output to 3.68KW / 16A

 

You're probably right. I wonder whether that's changed at some point or it's just my memory failing!

[ProDave]

I think the issue is the previous FIT system may have imposed limits on actual panel capacity when claiming the FIT, but that is gone and any such limits gone with it.

 

We now only have to satisfy G98 (or G99 for larger systems) and unless someone can find to the contrary, I believe that is only concerned with maximum current.

[joe90]

I know I am a simpleton but why can they not have meters that run backward and forward (at a deemed rate) simples!

[jack]

5 minutes ago, ProDave said:

I think the issue is the previous FIT system may have imposed limits on actual panel capacity when claiming the FIT, but that is gone and any such limits gone with it.

 

The relationship with the FiT rings a bell.

[ProDave]

5 minutes ago, joe90 said:

I know I am a simpleton but why can they not have meters that run backward and forward (at a deemed rate) simples!

That would be good, but it would mean you getting paid at the full retail price for what you export.  The export scheme will only pay you at the wholesale rate.

[joe90]

7 minutes ago, ProDave said:

That would be good, but it would mean you getting paid at the full retail price for what you export.  The export scheme will only pay you at the wholesale rate.

 

Ah I said (at a deemed rate), I am sure a “smart” meter could go backwards at “payback rate” but go forward at “consumption rate”.

[ProDave]

4 minutes ago, joe90 said:

 

Ah I said (at a deemed rate), I am sure a “smart” meter could go backwards at “payback rate” but go forward at “consumption rate”.

That is how the new export payment scheme will work.  you first have to get a smart meter then the smart meter will record import and charge it at your normal rate and separately meter export and pay it at the much lower export rate.

 

Currently as far as I know, Octopus are the only ones to introduce this on a trial basis and are paying 5.5p per KWh exported.

 

[joe90]

44 minutes ago, ProDave said:

you first have to get a smart meter

 

Will they work without a mobile signal? (I think I read that on here somewhere).