I’m currently spending the equivalent of about £3600 fueling my car annually
OK, but if you drive a lot more miles then you need a lot more solar panels 
Looking at this from the other end, we already know that someone doing serious commuting needs a decent charger, and that is charging from the mains which is available 24/7. If you are charging from solar panels you need a bigger charger, according to the ratio of sunlight_hours/total_hours (which is about 1/6.5). And the panel output to match. And assuming you don’t drive when the sun shines.
is that it would be parked at their work place for most of the day and only connected to the panels at the evening/night when they aren’t producing much power.
True.
Then you get into the “big battery” discussion.
I believe @stolman, if he is still out there, may have data on this.
Daytime workplace charging is being heavily encouraged and funded hereabouts.
As to your assumptions:
I think we’re above 20% efficiency for solar panels and you can get ones close to 25%
Our small EV uses 12.5 kWh/100 km, the big one 16. Admittedly, careful driving and hardly any heating needed, and heating does consume a lot.
Cloud cover brings the efficiency of PV panels down to 10-25%
The bulk of my installation is now 13 years old, no degradation in output, no failures of electrics or electronics, zero maintenance, other than a light intensity meter that failed but I didn’t replace anyway.
As to PV in general, whether it makes sense or not from an economical perspective depends mostly on variables like location, policy of your energy supplier regarding excess energy, the timing of energy usage in the house.
We use electricity to drive our 2 cars for about 30k km a year in total, cook, pump up our own water, heat and cool the house (although do use wood for heating so I am a bit of a cheater) and run the pool. We don’t use any other source of energy. Energy bill varies per month but should now be 60-90 euros a month (assuming .35 euro/kWh peak, .25 low and 18 feed-in reimbursement), a large portion of which is the fixed fee..
I haven’t made a calculation of ROI, and likely have not yet paid back my investment because 13 years ago the price of solar panels was a lot higher than now and in Spain one could not inject excess energy into the grid, this only happened recently thanks to the socialist gov’t. But I can assure you that nowadays the PV proposition is really a no-brainer here, given the prices of fuel, gas, electricity and cheap solar panels. Not to mention the ROI related to the name of this thread.
Some data:
Peak power: 10 kW
Annual yield: 16 mWh
Bought from grid: 6 mWh
Fed into grid: 7 mWh
So yes, impossible to match generation and usage without a battery, but using the grid as a battery is OK I guess, especially when I feed in at the time of day when it’s mostly needed.
What is your panel area and, if I may ask, how much would it cost today including the electronics?
Then double it for the UK 
About 70m2.
Cost estimate 12-15k€ including mounting structure and installation work.
That’s a huge array. We have what is called a “5 bedroom house” (it has only 3 now ) and no way could we get even half that on our roof.
The price seems to be similar to the UK rates.
That would cost $25-30K in my area, and would be a standard size array on a single story house that would have perhaps three times that floor area.
Orienting the array to face in the right direction for max energy production is another issue, as is the esthetics of the house. A coworker built a utility building (basically a hangar sized garage and workshop for his dune buggy projects etc) on his property with roof oriented as required, then installed his own solar system for about half the commercial installed rate. So far it has reduced his electricity bill to zero, which given AC use in the summer is probably about $200/month saved. He played games with the declared cost to inflate his state tax write-off and the government ended up paying his entire actual solar installation cost. Hopefully for him his tax returns won’t be audited too closely over the next few years.
How much did he pay for the array?
There is obviously zero doubt that a big enough array will power anything you need to power, including a Tesla doing 300 km a day
Even in the UK where the sun shines on average 1/6 of the 24hr period.
There are considerable variables:
IIRC his materials cost was $15K, or half the typical installed cost for an array of the same size… but I’m going only on word of mouth.
His peak electricity bill with AC on would be something like $400/month, the $200 month figure was intended by me as a rough average over a year and apparently his bill is zero year round. I think he’s pushing power to the utility much of year, and not being paid much for it. This is in a warm climatic zone (which is a way to buy less costly land locally) so is probably running house AC three months a year on a single story 3000 sq ft house. That also means he has 3200 hrs of sunshine per year, which would increase solar electrical output.
Given that legal tax write offs and some level of tax evasion combined to have the government pay his entire materials cost, his payback period was zero. The expense was sweat equity, plus in his case the cost of the utility building he wanted anyway – the solar panels were as much as anything as rationalization to build the building.
Electric cars BTW played no part in any of the above, so the power is used for the house or fed back to the utility. My coworker drives a large diesel truck to work every day, the kind that can pull a very large trailer on the weekends, and uses I’d guess 4 gallons a day for his 60 mile round trip commute. His diesel fuel cost is also a write off, on the $50K net annually fruit grove business that he runs on the home property as a side business. Sometimes he drives a new Jeep that probably uses a bit less fuel.
Meanwhile another coworker has no solar panels but charges his $80K Tesla at work every day for free. ‘Everybody’ has an angle on this stuff, particularly with 32 cent per kW-hr artificially inflated electricity cost. Set up a game and people will play it, particularly engineers.
I look at all these solar panel schemes and wonder "if they are such a great investment, how come every supermarket, office block and factory doesn’t have the roof covered with the things…
…but maybe I’m missing something.
