Solar Panels UK - is it worth it?

[barry.perfect]

I also built my own spreadsheet, built it up of a fair bit of time starting off workinging out where they hell our 17000kwh/year consumption goes (turns out the always on network and server equipment adds up a fair bit, on top of the car and a HP). Mine is really messy tho as it continuously compares 4 different panel & battery combos.

Use PVwatts to get your monthly and daily generation vs usage I then matched what I can move to either high gen or cheap rate electricity and from there worked out savings per day/month/year. Then take that rate and project into the future. I've not accounted for energy price increased other than the obvious near horizon because the more it goes up, the quicker the system pays for its self.

This is my graph at 28p/7.5p, assuming putting what solar you can into the battery, then filling the battery over night if needed (over 25 years):
View attachment 781555

Lines are P (panel count) and b (battery count in KW).
If the day rate goes up to 50p/unit and night to 12p, payback is between 3-4 years:
View attachment 781557

The big array with the 10k battery in this condition puts you £80k up in 25 years .

Thanks for the info on PVWatts, hadn't seen that before sounds like a good resource for doing the ROI maths.

Are you just using the 17000KWh figure and dividing it by 12 for the monthly usage or are you factoring in the higher usage in the winter when you have the least Solar?

 

[HenryT]

you can’t just tell the powerwall to charge up during ‘go’ hours and then top up with solar during the day and discharge to avoid grid import during the day?

That's exactly how I have my Solis inverter and Pylontech battery set up organised and it seems to work perfectly. Typically we don't draw anything substantially from the grid until 3pm on a bad day and as late as 8/9pm when there has been plenty of sun. Last bill for Jan/Feb showed an average cost of 9p/kWh. (We were fortunate enough to renew our Go tariff on the same rates of 5p/13p).

The inability to directly set what the Powerwall does was one aspect which put me off that option, though I do appreciate it has other benefits.

 

[Avendit]

Thanks for the info on PVWatts, hadn't seen that before sounds like a good resource for doing the ROI maths.

Are you just using the 17000KWh figure and dividing it by 12 for the monthly usage or are you factoring in the higher usage in the winter when you have the least Solar?

Different answers at different layers of the calculations tbh. Due to solar almost never going to be enough to completely power the house, I've focused on a subset of 'background' load that I want to cover as much as possible - and that is very consistent over the year. I'll also experiment with as much of the movable load as possible - eg moving hotwater (via the heat pump) to mid day and low rate running only. We can't escape the ongoing heating requirements, but I did find that using an eco 7 or octopus rate that covers the 6am heating demand peak to bring the house up to day time temp could be more advantageous than shopping for the deepest plunge pricing if it doesn't cover this time.

Check back the thread for some tips on manipulating the PVwatts data: Solar Panels UK - is it worth it?. I may have taken this too far, but it was interesting.

 

[Mrklaw]

Different answers at different layers of the calculations tbh. Due to solar almost never going to be enough to completely power the house, I've focused on a subset of 'background' load that I want to cover as much as possible - and that is very consistent over the year. I'll also experiment with as much of the movable load as possible - eg moving hotwater (via the heat pump) to mid day and low rate running only. We can't escape the ongoing heating requirements, but I did find that using an eco 7 or octopus rate that covers the 6am heating demand peak to bring the house up to day time temp could be more advantageous than shopping for the deepest plunge pricing if it doesn't cover this time.

Check back the thread for some tips on manipulating the PVwatts data: Solar Panels UK - is it worth it?. I may have taken this too far, but it was interesting.

thats cool. Are you able to do any estimation (even by eye) of what your target PV generation is that you're using for battery calcs? Eg I'm trying to take an average day in eg March, accepting that Nov-Feb are probably too extreme to try and cover with battery unless I literally size a battery to my peak consumption period and charge up off peak, which is unlikely to be cost effective

So if I ignore winter, and size for spring, I likewise accept summer probably doesn't need that size battery and I may export or have excess but I do want to try and cover 8 months of the year

 

[Dilly]

Actually, it did not work as the Powerwall only charged at 1.7kW; I was hoping for a 5kW charge rate. I only topped the battery up from 6% to 42%. I'm now hoping the Time Based charging is activated soon.

I could be wrong now but it used to only store about 48 hours worth of data. That was when there three settings. The third one was a sort of halfway house and pretty useless anyway. Perhaps it uses more data now.
mine often charge at about 3.5 even though they can charge at 7. Might be worth checking that yours hasn’t been capped at 3.6. They used to be. My first one was and got uncapped when the second was installed.

 

[Avendit]

thats cool. Are you able to do any estimation (even by eye) of what your target PV generation is that you're using for battery calcs? Eg I'm trying to take an average day in eg March, accepting that Nov-Feb are probably too extreme to try and cover with battery unless I literally size a battery to my peak consumption period and charge up off peak, which is unlikely to be cost effective

So if I ignore winter, and size for spring, I likewise accept summer probably doesn't need that size battery and I may export or have excess but I do want to try and cover 8 months of the year

Cant remember if I said in those posts, I looked at a few scenarios - a terrible day in Jan, a random day that was closest to 'average' for the year, which was about 13kwh generation and a good day in June. I also ran the Best day, but tried not to pay too much attention to that. I then do a time series simulation of the battery level, charging to different sizes at different times and see how that pans out for the day.

If I can cover off that early morning heat demand, either with a bigger battery, a later cheap period or as someone suggested, over heating our underfloor heating earlier in the night then from an average day onwards we can pretty much miss peak rate power completely, or just tap into it in the evening a bit.

I think one weakness of this approach however is that the average I'm basing my thinking off can actually be generated in really different ways. We can hit that 13kwh generation level once or twice on really sunny days in Feb, but it might also just be a nasty day in June. Presumably the curves interplay quite differently in these cases, and there would be much more heating demand on a sunny Feb day than in June!

It would be good to methodically work through the whole year doing this rather than taking 3-4 samples, but other than saying the data is there in the PVWatt data, I'm not sure of next steps. You need to add in the battery fill up over night, simulate the different modes the battery might be in (store & drain, solar only) and allow the battery sizes to be changed, and rates and times to be changed. Add in the heat pump data (which I have to some extent). Probably a whole new approach and data structure needed TBH. And even after that its stunningly personal and offers no value to anyone else, unless we do -another- layer of abstraction. Gah.

 

[RedMod3]

1.7 kW is normal for charging to the standby reserve (and for Stormwatch). It'll charge from the grid at the full rate if the AI deems it necessary.

Thanks, so I assume when I get my second PW, the combined charge rate will be 3.4kW.

 

[Avendit]

thats cool. Are you able to do any estimation (even by eye) of what your target PV generation is that you're using for battery calcs? Eg I'm trying to take an average day in eg March, accepting that Nov-Feb are probably too extreme to try and cover with battery unless I literally size a battery to my peak consumption period and charge up off peak, which is unlikely to be cost effective

So if I ignore winter, and size for spring, I likewise accept summer probably doesn't need that size battery and I may export or have excess but I do want to try and cover 8 months of the year

Fairly early in my explorations I arrived at this:

This is monthly total production across the year for the planned array, colour coded for green: covers all internal house usage (406kwh), including cooking, washing & dishwasher. Amber is internal house usage without those specific things, red is, well, failure and see what you can do with the battery. This is from an array that is being sold as generation 5000ish kwh/year. Assumption is that the battery can ensure we use as much of the generation as we need.

 

[RedMod3]

Staying with Octopus Go would be easier as it is my current tariff.

Did you compare the payback between a powerwall v other batteries (e.g. GivEnergy) when making your choice. If the Tesla Energy Plan isn't viable then the attraction of a powerwall goes down due to it's high cost?

I contacted PureDrive, but they never replied, so I decided to exclude them. I read GivEnergy Ts&Cs, and didn't like an aspect of them (don't remember exact details now), so they were excluded. So many people here and someone I know have strongly recommended Tesla, and I like the company, so I chose Tesla because of the size of the batteries and the 13.5kWh capacity. I was originally going for one PW, but I got a good price on the second at 5% VAT, so I took it.

I didn't go into over complex spreadsheets, as they imply a degree of accuracy that is just not achievable. Instead, I took my monthly solar generation added on the extra I'd get from my extra panels and then calculated the effect of one and two PWs with different GO rates.

 

[scdoubleu]

Thanks, so I assume when I get my second PW, the combined charge rate will be 3.4kW.

As I only have the one PW, I can't say. Western Power refusing permission for a 2nd.

 

[WannabeOwner]

ofgem says the average house uses 12000 kWh a year of gas. Some of that is hot water , so maybe roughly 10000 kWh of gas used on heating. I don't know what the average house size is, but that's probably at least 3-4x as much as that passive house requirement.

First entry in Google suggested 700 sq.ft. but I have no idea if that is the same average sized house as ofgem live in (nor whether that is footprint rather than floor area and average is 2-storey). I also don't know if 15kWh /sq.m. p.a. for Passive House is "heating" kWs (and thus equivalent to Gas) or "electricity converted to heat". So I may be comparing Apples with Oranges and it needs more research ...

65 sq.m. * 15kWh = 975 (kWh per year, per house)

So if that is directly comparable with your Ofgem gas usage ... then Average Passive House seems to need 10x less energy for space heating.

 

[RedMod3]

As I only have the one PW, I can't say. Western Power refusing permission for a 2nd.

My PWs are/will be set for zero export

 

[squarepeg77]

I have some high resolution CSV files tracking our household usage and solar PV generation, captured via @VanillaAir_UK’s pvs2pvo script that gets data from an EcoEye monitor.

Recently I’ve been putting together a simple Python model for battery storage. I could share it when I’ve polished it a bit, if it’s of interest?

 

[WannabeOwner]

Western Power refusing permission for a 2nd

Can you (offer to) limit export to get around that? Especially if your aim for having batteries is so as NOT to export anything ... notwithstanding that Export at Peak may enable a decent price, more especially once we get to time-of-use metering.

 

[squarepeg77]

In the meantime here’s what our 4kW PV system has generated for the past 4 and a bit years

 

[phil4]

One thing I've never understood is why there aren't some simple modelling tools out there.

What I mean by that is the sort of thing where you tap in how much juice the house uses, perhaps grouped into periods, morning, day, evening, night. Pop in what Solar you have, and what batteries and it churns through and tells you how self powered you are.

Even something that works out the best battery capacity for a given solar array seems to be missing (ie. so you don't reach 100% charged on sunny day, but not so big that you don't even get close).

I totally appreciate that these sort of things can be very specific to each house/person... but I'm surprised there aren't even high level tools to try and get an idea.

 

[Mrklaw]

One thing I've never understood is why there aren't some simple modelling tools out there.

What I mean by that is the sort of thing where you tap in how much juice the house uses, perhaps grouped into periods, morning, day, evening, night. Pop in what Solar you have, and what batteries and it churns through and tells you how self powered you are.

Even something that works out the best battery capacity for a given solar array seems to be missing (ie. so you don't reach 100% charged on sunny day, but not so big that you don't even get close).

I totally appreciate that these sort of things can be very specific to each house/person... but I'm surprised there aren't even high level tools to try and get an idea.

spirit energy have something a bit like this. you plug in electric rates including time of use, size of PV, house consumption and you can select up to three different batteries for comparison. I used that as a start poitn for building my own mini spreadsheet

 

[phil4]

spirit energy have something a bit like this. you plug in electric rates including time of use, size of PV, house consumption and you can select up to three different batteries for comparison. I used that as a start poitn for building my own mini spreadsheet

When I go there now, it doesn't offer any battery stuff and wants your details so they can call you.

It's not a bad start though if as you describe. Thanks for mentioning.

 

[Mrklaw]

Some useful links that are still open:
rule of thumb for PV and battery sizing https://www.spiritenergy.co.uk/kb-batteries-70-percent-self-supply
Calculator - it asks for email address at the end but even if you do multiple passes it only sends you one email. calculator

 

[phil4]

Some useful links that are still open:
rule of thumb for PV and battery sizing https://www.spiritenergy.co.uk/kb-batteries-70-percent-self-supply
Calculator - it asks for email address at the end but even if you do multiple passes it only sends you one email. calculator

Thanks for those.. it wasn't so much that I wanted to avoid giving them my details as I'm already in the process of getting a quote and install from them, and don't want to go confusing things