Solar panels 
The following is for INSTALLED solar panels, NOT DIY...
You have £10,000 sitting in an investment account, which you don’t know what to do with.
I have spent some time explaining to people why it isn’t me who says that solar panels don’t work, it’s maths. I’m going to show a very simple calculation to explain that. So let’s dive in...
The problem is in using the energy your solar panels are generating. A typical 3kW (kilowatt) array will provide (on average) 7 kW a day. The trouble is that it may be providing that when you can’t use it! You could be at work, on holiday, down the pub...and no one is home to use what is being provided.
You could send it to your electricity provider for a tiny amount of payment (so that’s
no good). Or you could ensure (by planning) that you do use it. Or you could store
the energy in batteries (at considerable cost). Clearly, the middle way is better

Let’s say that your hot water demand is 7 kW a day 
No.
The problem is that some days your solar panels will be bathed in sunshine, and other days, they won’t get any sunshine at all, maybe even for days, or even a week or more. So some days your panels will generate enough energy to heat up your hot water cylinder, but on other days they won’t. Now, this is no good. If you send that energy back (on days when the sun is shining more than you need) you won’t get paid much. Or you might waste that energy.
A way around this is to store that energy in batteries...and that’s where costs really kick in. An array with batteries is going to cost just shy of £10,000. And the batteries will fail in time, requiring replacement. Your £10,000 is currently earning 4% interest, that’s £400 a year added. But, of course, inflation hits it, so we need to do the maths...
Purchase cost of a TYPICAL 3kW system with batteries 
Generates 2,500 kW a year
At 27p per kWh that is £675 'saved'
Minus the cost of maintenance & repairs which is £4,000 over the 25 years, so £160 a year
This means we are down to £515 a year 'saved'
We put that in a savings account (which with compounded interest comes to £10,724 after 15 years)...to pay for the new inverter and battery array which costs £10,724!
So the overall payback from a TYPICAL system is...£0...you get a big fat nothing
from your solar panel array after 15 years. After this time, they have also degraded
by 9%. The above data even IGNORES the loss of interest (at £400 a year) on the £10,000
spent!!! It assumes you have been gifted the panels and installation. If you purchase
them, then you will be out of pocket big time, obviously. Not only do you have that
initial capital expenditure of £10,000 you also lose interest on it at £400 a year

ALL solar panel data is supplied by Contact Solar, and is for a typical system with batteries.
When you see it as simple as that, you realise that it’s not worth bothering, and
that your solar array will NEVER pay its costs back. In fact, it will be a cost burden

I have assumed a battery pack change at £3,495 for a 3kW array and an inverter cost
at £3,714 
Now, I know some people struggle to comprehend this, and say I must be wrong. But
the problem is that this is mathematics 
Remember, this is supposed to be an exercise in saving money! Although it is an ideal to be generating your own electric, the fact is that it just doesn’t work out financially. It’s actually an exercise in how to lose money.
So, what if you had kept your £10,000 invested at 5% for 30 years? Your investment would have grown to £21,000 even accounting for inflation at 2.5% per annum. It would be worth, in real terms, double what it’s worth today. Remember, inflation hits BOTH investments at the same rate.
Solar panels are therefore mathematical madness at ‘saving’ you money! If the cost of electricity drops (from the quoted 34p per kWh) then your reason for solar panels is even less. If you can get better than 5% interest, then your reason for solar panels are even less.
Also, you could get better than 5% interest by investing it into a pension for 25
years. You receive tax relief at your highest marginal rate for personal pensions
contributions; however, access is restricted until you’re 55. Once funds are in a
pension, they grow tax
The bottom line is that you would do much better to keep your money, and not spend it on some glass panels on your roof. It really is as simple as that! You can play around with interest rates, inflation rates, and costs of energy, but the maths STILL comes out in favour of keeping your money.
There is no real
Update:
I have had a report of a man who installed a £14,000 system 5kW , and is getting £2,000 a year from it in energy savings. He says it is paying him back in seven years. He says it’s the bee’s knees and he’s chuffed he’s getting £2,000 a year.
Really?
Let’s look at the figures:
Cost of investment £14,000
Loss of interest £9,100 after inflation (completely ignored!)
Replacement parts £5,000 (5kW system over 25 years)
Replacement batteries £6,000 (remember just one, over 25 years 
Replacement inverter £6,000 (remember just one, over 25 years 
Cleaning £625
Total £40,725
Divided by his saving of £2,000 a year (but minus panel degradation) =
21 years...three times longer than he thinks it is.
I understand that people who have spent thousands of pounds don’t want to admit that they have bought a lemon, but we have to admit to mathematics.
The only way solar panels can work out economically is if you install them yourself.