How to calculate the generation in kWh of the PV Panels

[+Sid]

Thanks Bob & binky,

To know that the calculators are usually based on old specs is helpful

The problem for me is that in order to generate enough electricity daily to cover the worst of the winter I would probably have to cover both of my SE facing roofs with panels. I can get 16 x 395 = 6.32kWp on my main roof and probably another 6-8 on the smaller one. Whilst I would want a battery I can't see it helping much here if there isn't enough light to charge it during the short winter days.

I have since found what seems like a good and sensible local installer who has done a payback calculation for me based on 6.32kWp and come up with a figure of 5 years. Trouble is he has assumed that I will be utilising all of the energy produced all year around. Well, there is absolutely no way that I will be doing that and when I use the PVGIS calculator the proper way and look at the monthly figures and, knowing my usage each month of the year (I have kept records), I can see that I will exporting a great deal of it and my spreadsheet calculations lead to a payback time of 16.5 years.

It's interesting that you have produced a lot more than expected but everyone seems to quote annual figures rather than figures for the worst months which is really what matters to me.

I don't want to give up on this. The local installer has told me of an installation nearby so I will try to find out first hand what they are producing daily at the moment. Meanwhile I'll keep burning the log fire.

 

[binky]

AHH, bs for sellers v reality. Short answer is winter outputs are always going to be low, and size of array matters. I would always recommend a larger array, but with batteries, off peak deals can make sense over array size. Have a read of all the posts on here on this subject, and post questions on anything you don't understand after that. I think everything you need to know has already been covered, but I will say whoever told you 100% self use is deluded. That's me being polite for talking high quality rose fertiliser.

 

[+Sid]

A graphic. If there was a 50% grant from the Government I would probably do it.

 

[Bob Smith]

Not sure I understand your diagram Sid.
You cannot compare things like this from my point of view simply because at night your PV won't produce and you need electricity. Of course, you'll have batteries but will they be large enough to cover your needs at night and some days when there is no sun?.

Don't forget that you can sell your excess of electricity so if you sell enough of it it could offset what you end up buying in Winter. To be this is a better way to look at it.

I initially thought I could get big batteries (kind of 500kWh or more) so to store all my excess electricity generated in Summer and use it in Winter. Unfortunately, it does not work like this.


If you can afford it the more electricity you can produce the better. Then you have a nice problem to have which is: What to do with that electricity.
You can store it but you will soon reach the limit of battery storage (I mean now I know, it was kind of ridiculous to think I could get a stake of batteries to store 500kWh. I am not that rich), use it to heat your water, maybe also your EV cars and of course use of the many export tariff.

Another thing to consider is to be able to buy electricity at night at a lower price to store it and use it in the day when there is no sun


To me, it is important to understand the different variables and play with them to find the sweet spot (which depends on each individual's objective).
There is another post that talks about one of the parameters to consider
https://electricianforum.co.uk/threads/which-export-tariff-and-why.55047/

You will find that if you want to size your system to cover your winter needs (if you have the space), it will end up being oversized in summer.
No one has come up with a subscription model yet where you have more panels in Winter than in Summer


If you want diagrams


The one above is for a 3.6kW installation with 6 Panels facing East and 7 facing West. Panels efficiency 20%. It was installed in April this year and it is located in Preston. There is also below the agile tariff.

The diagram below is mine, in Cheshire 16 panels all facing South East. Installed in 2016. 4kWp and panels 15% efficiency




Feel free to ask around and talk to your neighbourghs who have PV. They should pretty much all tell you the same. The reason is quite simple. Statistics.

I personally does not know anyone who regret to have purchased PV systems. Of course bad installers or faulty products do happen but that is bad luck. If installed properly and the equipment does not fail then you will only regret not to have installed more lol.

This is my case, I started with 4kWp mainly because back then with the FiT, no one would discuss more than 4kWp, now there is no limitation. If DNO does not want to approve a big installation, you can always limit your export but make sure you have enough storage otherwise you will be wasting electricity.

Hope this helps and good luck with your installation.

 

[+Sid]

So are you paid a fixed price under the FIT scheme?

Under SEG a supplier could simply pull the rug from under at any time and pay you 0.1p/kWh if they liked?

 

[Bob Smith]

Between 2010 and 2019, there was what was called Fit Scheme
They paid you:
A generation tariff for every kWh generated (regardless if you used it or not)
An export tariff (As most of the time it was not possible to measure what was exported, There was a so called. "Assumed". ie it was assumed that you export 50% of your generation.)

Those tariff were set at the date of the commissioning for a 20 years period.
I have mine set up in 2016. The Generation Tariff is 12.3p/kWh and the Export a 4p/kWh
So for 1000kWh generated, I get paid 1000*12.3p + (1000/2)*4p

Such scheme does not exist anymore. You can still get paid for what you export and there are various tariffs. You can change pretty much as often as you want. It is not a commitment for 20 years.

 

[binky]

So are you paid a fixed price under the FIT scheme?

Under SEG a supplier could simply pull the rug from under at any time and pay you 0.1p/kWh if they liked?

that will always be a concern, unlike FiT nothing is guranteed or even index linked. However, making holes in your bills has brought solar back into being very worthwhile. When the government killed off the FiT 6 years ago, I basically shut my solar business down, the numbers were all looking at around 12+years payback to just break even. Of course the recent spike in energy prices has completely changed the economic picture, with leccy at 40-60p per kWh, I would expect a payback of between 5-8 years, perhpas less. You would have to look at your energy consumption to really work out numbers, and get clever about using what you generate during the day - you will become a weather watcher!

 

[Bob Smith]

If you want to crunch numbers, I came across a new youtuber yesterday and he made this spreadsheet.

https://www.garydoessolar.com/utilities/solarcalculator/
This video where he explains how to use it is here

 

[+Sid]

Interesting video Bob. I'm going to have hours of fun!

 

[Bruspark]

Generation = spec maximum x night x cloud x part shadow x weather x sun angle x temperature x dust and bird poop x losses x they lied anyway

 

[binky]

Generation = spec maximum x night x cloud x part shadow x weather x sun angle x temperature x dust and bird poop x losses x they lied anyway

The official figures which are averages for kw of panels at whatever angle to south and roof pitch are based on real world testing. I've found these to be quite reliable over the years, and improvements in efficiency of the panels and inverters means most systems beat the official figures, provided people do bother to keep the panels reasonably clean. Only real fly in the ointment the last few years has been longer periods of gloomy overcast weather. But even that has been offset by exceptional warm weather with blazing sunshine.

 

[bladerunnerpv]

@+Sid Probably a bit late to add this, but in case it helps...

If you plug your PV system design numbers into something like easy-pv you can get an indication of kWh for Import, Export, Self-consumption etc, over the year and daily too, also will show battery usage to fill the skirts of the PV generation. Will give you a feel for pros and cons if you play with different options.

Assuming the bulk of your appliance use is outside economy 7 for simplicity. Your biggest saving imho will be to maximise self-consumption. Look at the price per kWh from your electricity supplier and deduct price x self-consumption, that's your saving p.a. Adjust for always on like fridge, freezer which you can calculate separately. If your main appliance use is during E7 then payback will be longer. BUT you've always got the standing charge to pay.

Link: https://midsummerwholesale.co.uk/easy-pv

Oddly the generators have increased their standing charge prices way above even the high inflation. If it was just the consumption prices I could understand that to an extent in the current climate.

Calculating for time of use tariff is another matter! I don't have a PV system yet so system owner's info will assist that one.

 

[Bruspark]

Supposedly the standing charge increase was part my contribution for bailing out the bust providers they shouldn’t have allowed to trade.

More realistically the remaining suppliers are worried that as prices rise people won’t use as much energy as they have ordered /bought for them - so expect a fixed charge soon to assure their profits

 

[Bob Smith]

Here other useful links to size your system.

https://app.opensolar.com/
It is an app some installers use but there is a free version of it

or this one

https://easy-pv.co.uk/
but it only provides list of components sold my midsummerenergy

Both can help estimate how much energy in kWh will be generated

 

[binky]

MCS provide a document that gives expected outputs per kWh per year based on orientation and roof pitch, and based on location in the uk. I've tried posting it here a few times, but cant, so Google it

 

[bladerunnerpv]

Regarding time of use tariffs I recently saw a 'Gary does solar' video on utube where he had a usage/saving calculator which looked like it might be worth a second look.

Not tried it myself, no idea what assumptions are behind it.

 

[Bob Smith]

Correct here it is

https://garydoessolar.com/utilities/dailymodellingutility/
There is a free and a payable version. He keeps improving it. Not sure if he is doing the programming himself or give the requirements to someone.
I remember suggesting him sometime ago to add the possibility to have several arrays. Not sure he is listened or not but the pro version will have this soon.

 

[binky]

MCS provide a document that gives expected outputs per kWh per year based on orientation and roof pitch, and based on location in the uk. I've tried posting it here a few times, but cant, so Google it

Tried again , but apparently you need Microsoft office..legal copy