Solar Panel installation basics

Heedtheadvice

How much you generate over the year will depend upon your roof slope too as well as the weather in your particular location. The weather, in particular cloud cover can vary over the S of Scotland so that will make a bit of difference too. That can rougly be derived as a guide from your rainfall figures but there is not a direct link.

Posters in the South report annual generation from 3200 to 3800kWhours per year for nominal 4kWp systems with inverter limiting for export to 3.68kW.  more than 4kW peak capacity on the roof and you will increase those figures by a ration depending upon what peak limit your DNO (Scottish Power distrubution division running the grid?) will agree to.

Over the whole year, similarly located, I get generated approx what I used to buy from the grid (3600kWh) but only saved buying 1000kWh (still buy 2600). This is mainly because my lifestyle has changed through retirement, greenhouse heating etc. and the difficulty using all the generated energy and having to buy at the times I need energy. The latter two do not match! Still, I save a good chunk and help the planet too.

I too wonder at some of your assumptions such as using all your generation. This is possible outside of peak generation (the early summer time) only unless you have some 'device' that 'uses' the generated power such as an EV car that you need to charge during the day or lots of storage.

Timing use with generation is not easy. Can you use 20+kWh every good day in May, June or times in July August? That is equivalent to a kettle or heater at 3kW on for about 7 hours mid day. If you have such power needs then you will be sadly dissapointed in winter!!

South facing always seems that it should be best but East West facing split is very useful with generation more evenly spaced throughout the day when required. It dies not make a huge difference in total generation over a whole year. I face South but would love some extra generation morning and evening relative to the curve of power generated from S facing. As an example heating the washing machine water at 10 in the morning and then drying in the afternoon I believe is more useful than starting at 1p.m. and trying to dry the following iffy day (given some long machine wash times)

As mentioned in one of the very good series of responses above battery storage seems the answer to time shift generation to demand times. Two main issues in that it is still costly to purchase enough storage to time shift significant amounts of energy (compared to buying from the grid) and it is only certain days outside poor winter generation that it is of benefit.

Your view of it all will be modified depending upon your aim being a financial or environmental benefit. For the former it is easy to get a system that has reducing benefits as size (and cost) increases but the more generation capacity you have, even if you cannot use it all could be more environmentally usefull as you will need to use less grid power and export (within peak limits) what you cannot use which benefits everyone.

(I'll also p.m. you.)

JKenH

My figures (kWh) taken from my Myenergi hub for last month (August) were 560 generated and 443 of that consumed
July 800/597
June 791/527
May 745/566
April 754/654
March 265/217 (part month only as statistics incomplete)

We are at home during the day, cook by electricity (no mains gas) heat our water by immersion heater (I have a Solar IBoost diverter), a couple of ASHPs which we use for heating and an electric car. 

What you need to bear in mind is for certain uses such as water heating and car charging you are only saving what you might be paying on a cheap overnight tariff. We are on Octopus GoFaster so pay 5.5p a unit between 01.30 and 06.30.

QrizB

mcn99 said:

Also looking at what is the realistic payback period, for say fitting a 6kw system,  15 years ? and what sort of installation cost should i be looking at.

For a very rough guide to payback, consider this.

• One watt of solar panel installed on a good south-facing roof might generate one kilowatt-hour of energy in a year.
• If you've got a straightforward roof and a basic system that watt of panel might cost you one pound, installed.
• If you export that kilowatt-hour of energy, you might be paid 5 pence. It will take 20 years to earn the pound you paid for the panel.
• If you consume the kilowatt-hour, you will instead avoid paying your supplier 16.7p for it. It will take six years to earn the pound it cost.
  

As has been mentioned by others, because generation doesn't easily line up with demand (generation in the summer and in daylight; demand in winter and in the dark) you'll do well to use more than 30-35% of the energy you generate. So that kilowatt-hour is more likely to be worth (0.35 x 16.7 + 0.65 x 5 = ) 9p and payback will be 11 years.

If your installation costs more or less than £1 per watt, you can pro-rata up and down accordingly. And if your predicted output is more or less than 1kWh/W you can pro-rata that, too.

I don't think anyone has yet pointed you at the EU's PVGIS website, where you can get a good indication of the output of a solar array anywhere in  the world. You should drop a pin at your own location and see what it predicts for your system.

Edited for the inevitable typos.

Hexane

Dolor said:

Your installer will submit the appropriate paperwork and you will pay c.£350 for the DNO to review your proposed installation.

I paid nothing for the DNO to review my installer's proposed installation, although I'm not in southern Scotland. Ultimately the DNO agreed a 6kW export limit at no extra cost.

arty688

I would also chuck in an inverter change every 10 years.

The panels you choose can make a big difference to the cost of the system as well, I did a spreadsheet and the cost per w for just the panel can varied from  26p to 78p . the Sizes of the panels are all different as well. My best bang for buck system was 13 490w JA panels giving me 6.4kw and my most powerful system 9kw was using 26  JA 345w panels. 

The datat is about 12 months old so is not up to date.

mcn99

Just starting to looking if i can get a government loan, dont really need it but it makes installation look much more appealing and may be the deciding reason to go ahead.

i have looked at different panel types, but no idea how they impact the cost per w, and so payback time. I have a very large south facing roof, so space not an issue. (usable roof all south, no west or east facing )

If I go for a 6kw system, is it possible to only feed say 4kw to grid and divert rest to always heat house or charge car?, is that a common install ?.
  

QrizB

mcn99 said:

If I go for a 6kw system, is it possible to only feed say 4kw to grid and divert rest to always heat house or charge car?, is that a common install ?.

Yes, those are common add-ons.

Do keep in mind that you'll rarely, if ever, see 6kW of output from a 6kW system. Typically your inverter will be rated 10-15% smaller than the panels, and you'll only see that rated output on cool bright days.

The long-term daylight-hours average of my system, for example, is around 20% of the nominal rating. A great day might see double that.

arty688

diverting excess to heat the water might not make economic sense if your current system is gas although that may change. Does make carbon sense though.

QrizB

Also, back in the first post you were asking about batteries. The economics for those are a little bit trickier but, broadly speaking, work like this:

• How many stored kWh per day will you reliably use? (My overnight usage, for example, is typically 2-3kWh a day).
• How much will your battery cost for each stored kWh? (Commercial systems are between £500-£1000/kWh).
• What is a kWh worth? (Let's assume 16.7p again)
• How much spare solar energy do you have? (In the summer you'll have loads, in the winter not a lot.)

Imagine you fit 4kWh of battery for £3000 (£750/kWh), have enough spare solar to charge it every day, and enough overnight usage to discharge it every night. Each year, each kWh of battery will deliver 365 kWh of electricity, worth (365 x 0.167 = ) £61. Payback will take (750 / 61 = ) 12.3 years, roughly 4500 cycles (4500 * 0.167 = 750).

Then compare that to the cycle life of the batteries to see whether they're likely to live that long!

arty688

And if you want the man maths to comedown in favour batteries , they will be cheaper to put in with the panels probably more like £500KWh and they could charge and discharge more than once throughout the day if its a sunny/cloudy day as well as the possibility to charge overnight at a cheap rate. This of course would increase the cycles though.

With solar and batteries you can generally mangle the maths to suit what you want. I personally think solar and batteries area good thing to spend your money on and i can see energy prices going up.