A solar panels comparison enthusiast writes: Dear Solartwin

There is much too much unexplained PV panel jargon about!

What confuses me about your website and that of other companies in your field is the assumption that I understand kW and how they relate to the amount of energy I consume on a day to day basis.

What does it mean if I get 800kw per year from my system? May I suggest layman terms may help people understand better what you are trying to tell them and how this will benefit them. Treat me as if I am from Mars and know nothing about what you are trying to tell me but do it so I am not patronised. Hope my comments help!

Solartwin reply:

The multiplicity of energy units in use is really confusing. Please excuse me if you already know this but there are (a) quantities and (b) rates and (c) system peak ratings. To do an analogy, and to now refer to these in reverse order for a coal burner or furnace before them considering the same concepts in terms of PV energy:

System rating = peak power: (“this coal furnace can burn up to 100 bags of, say 95% pure coal per week”) Of course, normally you would not run it flat out.
Rate of use (eg sacks of this coal per week)
Quantity of energy (eg sacks of this coal)

Now to look at a modest domestic PV installation instead:

kW(p) = PV aystem rating = peak power: Say a domestic PV installation (usually costing about £6-7k) can deliver 1 Kilowatts-peak (ie 1000 Watts, in other words this is the power needed to run 50 x 20 W low energy light bulbs or a third of a normal 3 kW electric kettle) in full perpendicular sun when the sky is not cloudy.

kW = PV array output. But most of the time your 1 kW(p) PV installation is delivering much less than this. On a cloudy day your PV panels may deliver only 100-300W. And for at least half of the time, ie at night, it is delivering virtually nothing. So 1 kW(p) is a maximum, unless you shine extra sun light onto it using lenses reflectors, which is not recommended except for specially designed PV’s. So, as with the furnace, the PV’s are not normally operating flat out, but well below peak.

kWh = “Units” on an electricity bill. This quantity of energy units costs, in UK about 12 pence each, at present. Just a note on equivalnec: if a PV system delivers 1 kW for one hour, it delivers the same amount of energy (and therefore the same amount of money saving) as a the PV installation running for, say, 4 hours at a reduced power of only 250W. Either way they generate one Unit (1 kWh) of energy. Because meterologists can predict to within about 10% what the annual solar radiation levels are in every location across the world, it is possible to predict in UK, how many Units (kWh) a PV installation in any particular place will deliver per kWp of installed capacity. If the installation is on a typical south facing roof in UK, each kWp delivers about 850 Units (ie kWh) of energy (about 10% more in Cornwall and about 10% less in Scotland).

PV load factor. There is yet another energy measure (a ratio) which is sometimes worth considering although the renewables energy industry are not keen that you do, for reasons which will become apparent next. This is the average power output of a system over a year. There are 8766 hours (day and night) in an average year of 365.25 days. So 800/8766 = 0.09126. This means that your PV installation works not at it rated 1000 watts peak all year but at 91 watts average. In other words it has a load factor of 9.1%. Similar load factors are used by the anti-wind lobby to condemn wind farms! Back to the 800 kWh energy assumption (I assume that you meant 800 kWh rather than 800 kW)…

So if you were to get say, 800 kWh per year from your PV installation, money wise, you would expect to get:

12p x 800kWh = £96 worth of electricity generated. OR 3p x 800 = £24 if you were to export it all to the grid and use none of it. But not both. So say:
600 kWh used at home saving 12p per kWh = £72 plus
200 kWh sold back to the grid at 3p per kWh, unless your supplier pays you more = £6
Total benefit here = £78.

Then your feed in tariff is 41.3p per kWh
800 kWh x 41.3p = £330.40 tax free income.

So your total tax free financial benefit from this example installation is:

£78 = £330.40 = £408.4 more money in your pocket (but only if the £7000 which the system cost to buy was in a zero interest account – otherwise you need to deduct the net of tax interest it warned you).

But on this simple basis if you paid £7000 then your return on investment is 5.83%.

If you pay 20% tax on any investment income then 7.29% is the rate you would need to invest at to match the PV installation.

But these figures do not allow for the difference that with a bank you can usually get all you invested sum out again. Nor that PV installations tend to lose power slightly, perhaps 10%-20% over 25 years.

If you sold your home with a PV installation on it, it is still unclear what percentage of your £7000 original investment will come back to you as a increase in price (after stamp duty and estate agents fees).

Please let me know if this stuff is (a) interesting (b) patronising (c) worth taking further (d) incorrect!

Many thanks again for your infinitely expandible question. Do you mind if I put it, and its reply on our website? Sorry it is so anoracky!

Regards, Barry