Could anyone sanity-check my fag-packet solar ROI calculation?

itm2

I've been thinking about having solar panels on our roof (in South West London) and have been using this utility to attempt to calculate the energy which I might be able to produce per annum:
https://re.jrc.ec.europa.eu/pvg_tools/en/#PVP

I've selected the "Crystalline silicon" technology, and an "installed PV power" of 2.4 (based on 12 square metres available for panels on the south-west facing half of my 35-degree roof).

The yearly PV energy production figure is showing as 2,244kWh, which is about 20% of my current annual electricity consumption. At current rates that would equate to an annual cost saving of £460, so a 17-year payback period if the installation costs are c£8k. As we're both at home during the day, we're well positioned to consume any energy that we generate during the daytime.

Can anyone in a similar situation confirm whether that estimate of 2,244kWh is a reasonable ball-park estimate of what we might expect to generate from 12 square metres of solar panels in the Greater London area?

Is it also true that the life expectancy of today's panels are in the region of 25 years?

Martyn1981

Hiya, just some quick thoughts.

Well done on using PVGIS, that's what most of us use and find to be very accurate, and the first place anyone asking about PV will get directed for a good guestimate.
If you don't have any shading issues then the cals should be good. I stuck a random pin in London, with your specs and got 2,224kWh, so looks like you got it right.

Hope this doesn't sound pedantic, but it will depend on the layout of that 12m2, and whether it's all available, and can be maximised with panels, typically around 1m by 1.6m, let's say 1.1m by 1.7m to account for different sizes and gaps between. Also you'll need to leave 200-300mm around the edges for planning rules, wind lift, and rain (to fall into gutters).

£8k for 2.4kWp is extremely expensive, even (I assume for London). I don't know what a good price would be (given it's London), but I'd have thought closer to half that.

If you consume around 11,000kWh's of leccy pa, then you would expect to use a high proportion of that generation, but probably times in the summer when you'll still export some.

Life expectancy is hard to know, but PV seems to be long lived and exceeding expectations. I don't think it would be naive to expect today's PV to be good for 40yrs+. 

itm2

Many thanks for the feedback - good to know that the 2,224kWh figure is at least in the right ball park.

Interesting that you think £8k is expensive. I got an indicative quote last year from am organization called Solar Together, who were working in collaboration with the local council on a group discount scheme. That's where I got the £8k figure from - although it wasn't based on a survey - just a submission of high-level info about my location.

itm2

I've just managed to retrieve details of the indicative quote that I got last year. I think the figure of £8k would have included battery storage. Without battery storage, the quote was £3,754.

At the time the company was asking for a £150 deposit to proceed with a site survey. Is it normal for solar providers to charge for an on-site visit and estimate?

QrizB

itm2 said:

I've just managed to retrieve details of the indicative quote that I got last year. I think the figure of £8k would have included battery storage. Without battery storage, the quote was £3,754.

That's better. Still somewhat higher than you'd pay in the provinces, but better.

At the time the company was asking for a £150 deposit to proceed with a site survey. Is it normal for solar providers to charge for an on-site visit and estimate?

No, that isn't normal at all.

confusedpaul

I had one of those indicative quotes from Solar Together for Hampshire. I never pursued it any further as I wasn't comfortable paying for a survey and there was no detail on what equipment they would use.
 We ended up contacting 2 other companies. One national that have been name checked on Fully Charged a few times and one that I found out later get some of the contracts for these Solar together deals.  The national company have been great, we didn't do it last year but went back to them again this year, changed the specs and requirements and have gone with them.
The solar together contracters on the other hand, really tried pushing social energy storage where you give up control of the battery. The quote was garbage, I am no expert but knew they had underquoted badly,  £235 for a Zappi charger? The consumption and payback estimates were way off.
This year we also got another quote from a different company, their online quote based on our requirements ignored what we asked for and went for the cheapest most convenient install for them.
Get a number of quotes from different companies, we are possibly paying more than we could get it for elsewhere, but we know it's a fixed price for quality equipment and a company I am happy to trust with my money. Ours is not the simplest with flat and pitched roofs across 3 aspects with shading, but it really did show the difference between companies when asking them to quote.

Hexane

itm2 said:

The yearly PV energy production figure is showing as 2,244kWh, which is about 20% of my current annual electricity consumption. At current rates that would equate to an annual cost saving of £460, so a 17-year payback period

Careful here - that calculation appears to assume a 100% usage of the generated energy. It's easier to approach 100% with only a 2.4kWp system than with a larger one, but without a battery or an EV it's still near-impossible to reach 100% even if you're home all day. My quote (based on a much larger system) assumed 35% usage, and many assume 50% usage.

To give you an idea of how it works, if your system is generating nearly 2kW for several hours at a time for several months of the year, then to use 100% of the generated energy you would have to *guarantee* to consume at least 2kW constantly for all of that time. You could have a row of 2kW mini-kettles and turn them on as necessary and sell the hot water to someone? Not really. Turning the dishwasher or the washing machine or the oven on when you have spare power doesn't work, they have varying cycle times. (And sometimes their minimum power draw will be more than the spare generation that you have.)

If you have a battery or EV then it becomes a lot easier. I have a hot water diverter and that only soaks up a very small amount of the excess power. If your house is full of constantly-on computers or something else such that your baseload electricity consumption is more than 2kW constantly during daylight hours from spring to autumn then in that case 100% consumption is easy too. Otherwise it's really hard.

Martyn1981

itm2 said:

I've just managed to retrieve details of the indicative quote that I got last year. I think the figure of £8k would have included battery storage. Without battery storage, the quote was £3,754.

At the time the company was asking for a £150 deposit to proceed with a site survey. Is it normal for solar providers to charge for an on-site visit and estimate?

Now you're talking. I actually paid £4,150 for 2kWp (with Solaredge tech) in July 2012.

But to be fair, whilst PV panel prices have dropped massively since then, there is a lot of fixed costs, and again I assume higher prices for London, but somewhere in the low to mid £3k's would probably be 'reasonable' I assume for London.

That will improve your returns massively.

Going back to consumption, and I mentioned you would probably export in the summer, is your high consumption due to leccy heating? I appreciate you are home during the day, but your winter demand may be multiples greater than your summer demand when PV will be generating a lot, and almost every day.

Have a look at your bill and see what your monthly or quarterly consumption is, and how it varies.

Don't want to misdirect you, but I suspect a system of that size, at £3.5k or less, would work well for you, but with limited space you might want to consider higher priced panels with higher efficiency. This will need further discussion based on quotes you get, and plans to move forward, as panel price rise fast as you chase higher efficiency (just for example, say 5% more generation for 10-20% more panel cost), but with so much fixed cost (scaffolding, labour, inverter, vans, profit etc) it might work well for you.

All the best, happy hunting, and keep chatting, loads of people on here will give tons of good advice and support.

Spies

Without a battery, assume 60% self use in the winter months and 30% in the summer months.

itm2

We have conventional gas central heating, with a combi boiler. I do have 3 desktop computers which run 24/7 (long story), plus a couple of laptops which are used for a few hours each day.

As I sit here at my desk, at 10:30 on a "normal" weekday, my smart meter is showing 900W of electricity being consumed. That's with me using my main desktop computer, my wife watching TV/PVR next door, and no washing machine/dishwasher/etc running. The heating is also off (!!!). Yesterday's total was 22.3kWh

Looking back at usage from previous years, average daily electricity usage from April to September has been around 22kWh. From November to January it's around 30kWh.

My initial thought was that ROI would be harder to achieve with a battery, as the battery costs were high and presumably they degrade far more quickly than the solar cells.

It sounds like I need to do some work on a more elaborate spreadsheet to estimate usage and returns, with self-usage calculated based on my provider's current kWh rates (around 20p at the moment), and unused capacity based on a typical feed-in tariff (maybe 5p?)

2nd_time_buyer

itm2 said:

We have conventional gas central heating, with a combi boiler. I do have 3 desktop computers which run 24/7 (long story), plus a couple of laptops which are used for a few hours each day.

As I sit here at my desk, at 10:30 on a "normal" weekday, my smart meter is showing 900W of electricity being consumed. That's with me using my main desktop computer, my wife watching TV/PVR next door, and no washing machine/dishwasher/etc running. The heating is also off (!!!). Yesterday's total was 22.3kWh

Looking back at usage from previous years, average daily electricity usage from April to September has been around 22kWh. From November to January it's around 30kWh.

My initial thought was that ROI would be harder to achieve with a battery, as the battery costs were high and presumably they degrade far more quickly than the solar cells.

It sounds like I need to do some work on a more elaborate spreadsheet to estimate usage and returns, with self-usage calculated based on my provider's current kWh rates (around 20p at the moment), and unused capacity based on a typical feed-in tariff (maybe 5p?)

I think something is wrong there with your consumption. The computers should be around 100w. The TV less than 100w too. I would look at turning everything off in turn and try to see where the high drain is occurring - or there is a problem with you smart meter.

I notice that my smart meter was recording around twice the consumption that appear on my bill for the first 6 months of last year. However, it seems to be working now. It is a first generation meter and I suspect there may have been an issue when I moved to Octopus.