PV on the roof - experience and advice

[Socks]

Well, me and Mrs Socks are thinking about solar panels.  We have a pitched roof, one side South South East facing and re-roofed a couple of years ago because of nail rot so it is in good condition.

Looking at a few estimates and our current electricity usage, best option appears to be 8 panels, 3.4 Kwh system with a 4.8kwh battery as most of our use is in the evening.  And also change energy supplier to Octopus because they offer a good payment rate for exporting surplus electrickery.  (As well as supplying 100% renewable energy.)

If I've done my sums right, this looks like a payback time of about 7 years and over a 25 year timescale something like a 10% return on the £7,000 installation cost.

Anyone on here with recent experience of doing this and / or any advice or  comments?

[Pingu]

Have you seen this thread?

https://yacf.co.uk/forum/index.php?topic=122749.0

[Socks]

Thanks, I hadn't seen that, will have a look

[Wombat]

My first thought is that you need more of both PV output, and battery capacity.

There are just the two of us at home, and we are fairly careful, but we do have oddities such as a borehole pump for our water as extra, and cooking has to be electric.  We have the usual 4kWp system, and only a few months back, had a 5.2 kWh battery installed.  this time of year the battery goes totally flat in the evening, and its just staggered up to 50% charge today (yesterday it barely exceeded 5%).  In the summer this heats all our water, and fully charges the battery, which will then only go down to maybe 15% by the morning.

I'm not sure if it has changed sine the demise of the Feed in tariff, but the effective maximum on the usual connection protocol was claimed to be 4kW, but it wasn't, that's a porkie, it was only 16 amps, so actually 3.68 kW.  if you can't fit more than 3.4kW worth up there, then there is no discussion, but I'd aim for as much as you can manage, especially as the efficiency of panels has gone up a bit since ours were installed 5 years ago, so you can squeeze a bit more in than previously.  There is logic in having panels capable of generating more at peak ties than the inverter can export, as it means that more of the time you are generating more, as most of the time you are at nowhere near peak output.

[Socks]

My first thought is that you need more of both PV output, and battery capacity.

There are just the two of us at home, and we are fairly careful, but we do have oddities such as a borehole pump for our water as extra, and cooking has to be electric.  We have the usual 4kWp system, and only a few months back, had a 5.2 kWh battery installed.  this time of year the battery goes totally flat in the evening, and its just staggered up to 50% charge today (yesterday it barely exceeded 5%).  In the summer this heats all our water, and fully charges the battery, which will then only go down to maybe 15% by the morning.

I'm not sure if it has changed sine the demise of the Feed in tariff, but the effective maximum on the usual connection protocol was claimed to be 4kW, but it wasn't, that's a porkie, it was only 16 amps, so actually 3.68 kW.  if you can't fit more than 3.4kW worth up there, then there is no discussion, but I'd aim for as much as you can manage, especially as the efficiency of panels has gone up a bit since ours were installed 5 years ago, so you can squeeze a bit more in than previously.  There is logic in having panels capable of generating more at peak ties than the inverter can export, as it means that more of the time you are generating more, as most of the time you are at nowhere near peak output.

Thanks Wombat - I don't think we have space for more panels, however there's a chap coming to have a look next week so I'll bear this in mind.

[SoreTween]

Check what panels they intend to use.  3400W / 8 = 425W peak per panel.  That's not bleeding edge but pretty darn close - make sure the panels listed in the quote match their claimed system output.  Not that I'm distrusting at all but would I be surprised if they installed older panels & blamed your orientation or azimuth for underperformance?  Inspite of South South East being pretty good?  No, I would not.  plug in solar is a useful reference for what is & isn't bleeding edge in panels (sorted by date added in that link).
Big name panels will likely last longer.  They should all meet the output/life curve claimed but do we trust cheap chinese brands?  Maybe not but the cheap chinese brands are so much cheaper than LG, Panasonic and so on that if they are limping along at 150W after 12 years you can probably replace them with another lot of cheap chinese panels and still be quids in.

Are you going for on top or inset?  What is your azimuth, angle & latitude?  There's online calculators that will estimate your output from these to help verify salesbastards claims.

[orraloon]

New fit, 2 weeks ago, 12 panels 3.6kW, 13kWh battery.  SW, 214 degrees roof orientation.  Moved to Octopus, export tariff active as of yesterday. 

Today generated 16.1kWh, big chunk exported.  Ongoing, awaiting the full techie sizing assessment, is the move off gas CH to ASHP.  Grant and lot of 0% loan funding in place for the full thing.  Still tuning the payback calculations.  Which will be assisted by the Octopus Flux off peak / peak import / export rates.  Ongoing...

[Canardly]

Had ours fitted last May. Certainly very happy with the installation so far. No battery as yet for us, as could not make the sums work. YMMV. Octopus are/is very good.

[quixoticgeek]

As a rule of thumb, in typical middle of the UK latitudes. In the depths of winter, you will get 0.6kwh of yield per day, for every 1kw of installed panels (assuming south facing, angle equal to latitude). So a 4kw name plate capacity install will get you 2.4Kwh per day in the depths of winter.

Some days more, some less, but it's a useful number for back of the envelope calculations.

In summer that same 4KW install is going to be an embarrassment of riches, way more than you can know what to do with. (Unless you got a big EV you empty every single day).

With a move away from burning bits of dinosaur in order to get heat for space and cooking means that our electron consumption is going to keep going up. 4KW of panels feels like a lot. But in the depths of winter, if you were hoping to not be pulling things from the grid, you're gonna be struggling.

A friend of mine is building an off grid install with electric heating. He's put in 15KW of panels in the last couple of weeks, with 30KWh of batteries. More is still to come. This is based on being able to heat the home with electricity in February. As well as cook.

Why am I throwing numbers at you? It's a bit of expectation management. If you are hoping to reduce your grid usage in winter, that's not gonna be very useful. Your battery will be empty at the start of every day, and you will need power from the grid. At the same time, in summer you're gonna be able to sell some surplus power during the day. But you may find that the battery isn't big enough still by the time you throw an electric oven into the mix.

Don't get me wrong, all solar (assumed installed where it will get sun) is good solar. Just don't be realistic in what to expect. Especially in winter.

And consider more batteries than you think you need.

Oh and don't forget to do a demand side assessment. A power measuring plug socket thingy is great. Find out how much your devices actually use. Work out what your daily demand really is. And when it is.

Good luck. Keep us updated.

J

[aidan.f]

Socks, I'm local 2U 3.4 kW peak solar and 9kWh battery - happy to show you the install. The problem: may turn you into a 'thriftatarian'

[Socks]

Socks, I'm local 2U 3.4 kW peak solar and 9kWh battery - happy to show you the install. The problem: may turn you into a 'thriftatarian'

Thanks Aidan - I'll send a pm

[DuncanM]

We have approx 4kW of solar and a 9kWh battery.  The solar has been on our roof for >6 years, and the battery was installed last year. Over the summer, we could fill the battery and still have extra power for the car or heating water or whatever.  In the winter we can charge the battery over the 12:30-4:30 cheap period of Octopus Go , and it's an unusual day when we actually use the whole capacity of the battery.
One thing to be aware of when choosing the battery is the power that the inverter can supply. Our battery is 9kWh, but the inverter can only supply 3kW, so our power shower (>8kW) will draw from the grid, no matter how much power is available from the battery or the panels. If you want to be fully off-grid, you would have to ensure your inverter can supply the most power hungry device in your house.

[bhoot]

We had 10 panels and a 3KWH battery installed in January, on a pretty much due south roof in London, although partially shaded by taller buildings at some times so not quite optimum.

One thing we have been really pleased with is the option to divert our excess (once the battery is charged) to our hot water cylinder. We have a very old fashioned cyclinder with an immersion heater in it - and mercifully it still worked. For about 6 months this year we have hardly used any gas at all as most days we got enough hot water from the immersion (although we are fairly frugal in both electricity and hot water usage). We had to get an extra unit for this, but it was all done as part of the installation.

We could have had extra battery capacity but the calculator didn't suggest it was really worthwhile - and ur experience would seem to support that. In the winter when you want all the battery power for the long evenings there wouldn't be enough sun to charge up more than we have, and in the summer when you could charge more battery we don't really need it as there is more daylight etc.  Of course having an electric car to charge would probably change that equation.

[Socks]

Good points Duncan and bhoot.  We no longer have a hot water tank and immersion heater, so probably not an option.  I'll have a look at inverter capacity and battery size.

[Nutbeem]

We had our system installed 16 months ago:

12 Panels = 4Kw system, plus a 5Kw battery & a Solar Buddy Diverter (for the immersion heater)

We’ve got a South facing roof, 30 degree slope, on the south coast and 2 of us in well insulated bungalow, so pretty much optimal for Solar Panels.

We’re all electric & my work shifts allow me to be around in the day and use the washing machine, dishwasher etc. at times when we have ample solar &/or battery power.

Yield so far this year has been 5017kWh
Best month was June, 835kWh total, daily average 27kWh
Worst was Dec, 137kWh, daily average of 4.4kWh, better than QC 0.6%, but I presume this is our location. It will be interesting to see how we do this year?

Working out payback time involves maths that are beyond me, but our average monthly payment has dropped from £195/month to £45/month. We paid for the system with savings that would otherwise have sat in the bank being slowly eroded by inflation, so I think we’re winning.

With regard to the battery various sources have said we should get 10kW but I couldn’t make the payback maths work for the extra cost & I think for our needs 5kW works fine:

Spring, summer and autumn the battery fills quickly and easily lasts through to sunrise with charge left over. We use very little from the grid in these months.

Nov – Feb we struggle to fill the 5 Kw battery so I can’t see how having larger would be of any benefit?
Also on the battery front:

1.   We’re on economy 7 so now winter is here I set the battery to partially fill from the grid for 40 minutes before we go onto day rate. That way we are still effectively using night rate electricity (15p versus 45p a unit) until the sun gets high enough for the panels to start working. I’ll turn this option off sometime in early March.

2.   Don’t just think of the battery as providing power from sunset to sunrise, it also plays a role in the day. To explain this; if I put the washing machine on when the sun is shining the panels should provide the electricity it needs. The reality is that part way through the machines cycle it may get cloudy or start to rain and panel output drops from 4kWh to 0.5kWh. When this happens the battery picks up the slack rather than the grid. This is why even a 5kW battery is beneficial, as through the day it can be charging or discharging as required, then slowly discharge through the evening to power lights, TV, etc.

I usually wait until the battery is charged to >40% and the panels are producing at least 1kWh before running heavy consuming stuff. I then try not to use anything heavy from mid to late afternoon so that we go into the evening with as much charge as possible.

Aiden is quite right when he says you’re likely to become a “Thriftatarian”

[Socks]

Thanks Nutbeem.  We've received two quotes so far, waiting for one more.  Cheapest option seems a bit underpowered - 8 x 395w panels, 4.8kwh battery.  Our average daily usage is about 6kwh so the other quote seems better, if a little more cost.  8 x 3.4kw panels, 5.12kwh battery.

Using off peak rates to charge the battery and in effect, time shift our use to lower rate seems like a neat trick!

[Mrs Pingu]

Are you sure that's 8x 3.4kw panels? Seems huge to me.

[Nutbeem]

Are you sure that's 8x 3.4kw panels? Seems huge to me.

8 panels producing 3.4Kw in total I would expect

[Nutbeem]

Thanks Nutbeem.  We've received two quotes so far, waiting for one more.  Cheapest option seems a bit underpowered - 8 x 395w panels, 4.8kwh battery.  Our average daily usage is about 6kwh so the other quote seems better, if a little more cost.  8 x 3.4kw panels, 5.12kwh battery.

Using off peak rates to charge the battery and in effect, time shift our use to lower rate seems like a neat trick!

If you can afford the outlay I doubt you'll regret the decision to install Solar. Besides the long term cost saving there's something very satisfying about generating your own electricity 

We went with the company we were happiest with rather than the cheapest. Fitting and installation is quite involved so having the right equiptment and having it properly fitted for us was more important than saving a few £'s.

We also did the switch to Octopus when they started taking on new customers (our exisitng supplier wanted to increase our payments even though our useage was going down and kept ignoring our export payment request).

We've been very impress with Octopus; prompt communication, bills that are readable, nice website. Their 15p export tarrif is the icing on the cake. On some days in the summer what we earned for export was more than our useage and standing charge.

[aidan.f]

Back to batteries...

One problem is management  - almost always relies on battery manufacturer providing webservers and support for their product.

If they:  /lose interest/are incompotent/go bust, then unless you are a bit of a geek and kno how to apply MODBUS* you have a dumb product. 

This the problem with the moderm world but /Hive/Sonos/Alexa/Ring etc.. are a much smaller pieces of e-waste.

*MODBUS - is a standard open control protocol. I would advise before you buy finding out how well documented your product is, e.g. is there a LAN not WAN integration for LAN based home automation e.g. Home Assistant. Happy thrifitarian geeking

[Canardly]

If cost is an issue atm then you could fit a hybrid inverter and have a battery installed at a later date which would be a very straight forward process. You should  really be trying to max out the installation size for your property if poss whilst you have scaffold up and so on. Your installer ought to be informing you of possible options.

[Socks]

Are you sure that's 8x 3.4kw panels? Seems huge to me.

8 panels producing 3.4Kw in total I would expect

Yes