There are a number of variables missing here which would help to decide if any system is over priced.
1) What panels? Are you looking at cheap as chips poly, middle of the road mono or Rolls Royce Hybrid? Since your quote is for 250w panels I'm assuming cheapish poly or old stock mono as most quality panels these days are 260w and above. Brand and model are very important factors, not all panels are equal.
2) I assume you mean a Solis inverter with duel tracker? A good inverter and not expensive with good support back-up. You can pay a lot more for a lot worse. How much extra are you paying for the 25yr warranty on the inverter and who offers this? I'm guessing it will be a few hundred pounds as the failure time of transformerless inverters is around 5 years. (If only we could have the old SMA SWR inverters back - sigh...)
3) How complicated is the scaffold? Conservatory? Are you remote? All add ££ to scaff
4) Any other factors on your house that make installation difficult? What did their structural report say? (Ask to see their assessment. If they cannot provide it walk away)
5) Voltage Optimiser - did they test the voltage on your house? Are you near Cambridge? Any transformers / railways / heavy industry near by? As standard I always take a voltage reading during a survey - anything above 250 volts I always recommend an SPD as a minimum but VO would be better. It gives the inverter an easier and longer life (see comments earlier - modern electronics are not as robust and are vulnerable to transient voltages. In Cambridge I have seen a steady 260v in some locations - imagine what the surges go up to). All this should be explained clearly in the quote if they are offering these as optional extras.
5) Yes - six years is way too short! Did you give them your yearly kWh usage for their calculations?
6) Any 4kw system that costs less than £5000 will have made compromises either with component choice or profit margin. If it is a one man band who works for wages maybe, but a professional company has overheads which you have to pay for somewhere along the line. That doesn't give them the right to rip you off, but you should allow them enough margin to stay in business! At the end of the day you are paying for a system that will take at least a decade to pay off - it would be nice if the installation company stayed around long enough to service it and keep it operational. A decent price for a basic well specced 4kw Mono installation would be £5000 so add all the bells and whistles and the fact that your quote uses questionable 250w panels it does look a little overpriced.
7) Are they including pigeon proofing?
I wouldn't say that installing PV is any riskier now than it has ever been, other than the risk of your installer going bust and leaving you in the lurch. So do your homework.
The returns are the returns; do your sums and you will see if it makes sense. Any chance you might move in the next 10 years? You can't take it with you.
How long has the installer been in business, can you meet past customers and ask questions? Do they employ their own installers or sub out (V.V.V.V. important!!!!). Do they know their stuff or is it all sales fluff?
Once you are signed up the FiT is guarunteed for 20yrs and we know electricity prices will only go up, so assuming the PV works as expected you are future proofing yourself for the next 20 years. If you bought a car for £5000 you would lose money as soon as you turned the key - can you think of a better investment at the moment?
And remember, decent solar panels should last at least 60 years. If you install PV on the house you plan to retire in, compromising on quality now is a false saving...
