Free solar panel discussion

daytona600_2

if sky or the bbc wanted to stick a transmitter on your roof
you would charge them rent
then why would you rent your roof for free to fit solar panels
so someone could make a 25 year tax free & index linked profit
at your expense

free indeed

grahamc2003 wrote: »

Why on earth do you think it is 'free'?

You enter into a contract - your side is to sign away the use of a large part of your roof for 25 years. There is a cost in doing that - the opportunity cost of not using your roof for anything else for 25 years.

Next year you may have a spare 20 grand which you would like to buy your own panels with in order to get fit payments an order of magnitude greater than the value of the electricity you use. The loss of that possibility would be part of the opportunity cost.

You may also, in ten years time, decide you'd like an extra room in the loft, and the panels may rule out that possibility.

Or in 20 years time, you may decide to sell up and attract a buyer who doesn't want your aging solar panels. If your contract doesn't allow for early buyout, then your sale could fall through with your potential purchaser (with his solicitor'sadvice) unwilling to take on the lease. (And that could prove VERY expensive).

So please don't keep saying it's 'free'electricity - it isn't, it is the electricity you get for giving up the use of your roof. If you had a house you rented out for £1000per month, I doubt you would say that was 'free' money.

Nang

Cardew wrote: »

That might give you a rough idea of generated electricity used in the house. However if you have an export meter you know EXACTLY how much is consumed in the house; so you don't need a comparison of 'before and after' data.

If you generate, say, 2,500kWh and you export 1,437kWh(and get paid 3p for each of those kWh) you know you have used 1,063kWh in your house.

If you look at a couple of yakky58's posts he has detailed his consumption figures down to the last kWh - in a year he used 990kWh in his property.

Opps..

Sorry its me being slow, I see what you are saying. If he knows he used 990 of the power he generated, he saved 990 x whatever his elect rate is (say 12 pence) = 990 x 0.12 = £118.80.

Where it falls down, is the assumption you are making is that he hasn't changed his consumption habits since having it installed.

You do change them though its inevitable, for example if its calm cold and sunny I use the tumble drier instead of hanging the stuff out etc.

Andy...

Cardew

Nang wrote: »

Opps..

Sorry its me being slow, I see what you are saying. If he knows he used 990 of the power he generated, he saved 990 x whatever his elect rate is (say 12 pence) = 990 x 0.12 = £118.80.

Where it falls down, is the assumption you are making is that he hasn't changed his consumption habits since having it installed.

You do change them though its inevitable, for example if its calm cold and sunny I use the tumble drier instead of hanging the stuff out etc.

Andy...

Sorry but it is your method that assumes that he hasn't changed his consumption habits since having it installed.

If you have an export meter fitted and it shows that you have used 990kWh in the house in a year, then you have saved 990kWh from your annual consumption regardless of your new consumption habit.

Nang

Cardew wrote: »

Sorry but it is your method that assumes that he hasn't changed his consumption habits since having it installed.

If you have an export meter fitted and it shows that you have used 990kWh in the house in a year, then you have saved 990kWh from your annual consumption regardless of your new consumption habit.

Your not seeing the whole picture.

When you have solar panels you increase your consumption at sunny times, Refer to the example I already gave. You will in practice use more energy than you used prior to having them, because you know it is free. It skews the calculation.

You need to know how much elect you bought from your supplier last year when you didn't have the panels, and how much you bought this year when you had the panels. Only then can you can figure out the true cost reduction.

Andy..

Cardew

Nang wrote: »

Your not seeing the whole picture.

When you have solar panels you increase your consumption at sunny times, Refer to the example I already gave. You will in practice use more energy than you used prior to having them, because you know it is free. It skews the calculation.

You need to know how much elect you bought from your supplier last year when you didn't have the panels, and how much you bought this year when you had the panels. Only then can you can figure out the true cost reduction.

Andy..

I will try once more:

Obviously you will attempt to maximise your consumption, from the PV panels, in the house because each kWh utilised in the house is worth, say 12p.(to use your figure)

To achieve this you will change your pattern of use so appliances are used midday as much as possible.

So after this change of use, if you have used 990kWh in the year you have saved 990 x 12p.

If you have been less dilligent in adapting your pattern of use, you might have saved, say, 880kWh in the year; hence you save 880 x 12p.

So the only way of calculating EXACTLY how much you have saved is by use of an export meter.

Using your method; let us assume that:

1. you were billed for 4000kWh in the year before panels were installed and billed for 3,500kWh in the post installation year. So your consumption is 500kWh less.

However we know for certain that you used 990kWh from the panels because we have an export meter. What do you conclude?

2. you were billed for 4000kWh in the year before panels were installed and billed for 2,500kWh in the post installation year. So your consumption is 1,500kWh less.

However we know for certain that you used 990kWh from the panels because we have an export meter. What do you conclude?

Nang

Cardew wrote: »

I will try once more:

Obviously you will attempt to maximise your consumption, from the PV panels, in the house because each kWh utilised in the house is worth, say 12p.(to use your figure)

To achieve this you will change your pattern of use so appliances are used midday as much as possible.

So after this change of use, if you have used 990kWh in the year you have saved 990 x 12p.

If you have been less dilligent in adapting your pattern of use, you might have saved, say, 880kWh in the year; hence you save 880 x 12p.

So the only way of calculating EXACTLY how much you have saved is by use of an export meter.

Using your method; let us assume that:

1. you were billed for 4000kWh in the year before panels were installed and billed for 3,500kWh in the post installation year. So your consumption is 500kWh less.

However we know for certain that you used 990kWh from the panels because we have an export meter. What do you conclude?

2. you were billed for 4000kWh in the year before panels were installed and billed for 2,500kWh in the post installation year. So your consumption is 1,500kWh less.

However we know for certain that you used 990kWh from the panels because we have an export meter. What do you conclude?

For Point 1, I would conclude that I made a 500 wh saving and used an additional 400 watts that i problably wouldnt have if I hadnt had the solar panels installed that year.

You see what I am getting at?

Andy..

Jon_Tiffany

Cardew wrote: »

So the only way of calculating EXACTLY how much you have saved is by use of an export meter.

Not quite, there is another method, you can calculate it by using an energy monitor.

zeupater

Nang wrote: »

For Point 1, I would conclude that I made a 500 wh saving and used an additional 400 watts that i problably wouldnt have if I hadnt had the solar panels installed that year.

You see what I am getting at?

Andy..

Hi Andy

I sort of see where you're coming from .... but Cardew is right. The only way to know with a degree of accuracy how much energy you have saved is to have an export meter and a total generation meter as attempting to compare one year's usage against another's is open to being skewed by so many variables.

As an example, consider the temperature differences within the first three months for the last five years .....

.......Jan / Feb / Mar
2006 4.3 / 3.7 / 4.9
2007 7.0 / 5.8 / 7.2
2008 6.6 / 5.4 / 6.1
2009 3.0 / 4.1 / 7.0
2010 1.4 / 2.8 / 6.1

This introduces a massive heating demand variance if you are using electricity for heating, but even if your major heating source is not electricity there is the effect of running a 100W CH pump for more/less hours, temperature changes effecting the cycle of the fridge/freezer, what kind of food you have a preference for when the temperature differs, and on, and on ....... Also, what happens when you change other variables, such as buying a new plasma TV, or a LED TV .... or change your lightbulbs for low energy .... or buy an electric hob to replace a gas one (because you are generating electricity, not gas) .... or .... or .....

Now consider Cardew's solution ..... you know what you've generated & you know what you've exported, so you know what proportion of what you've generated has been used by yourself .... change as many variables from one year to another the answer is accurate.


## EDIT ..... Jon's post "Not quite, there is another method, you can calculate it by using an energy monitor." .... agreed, if you can get an energy monitor accurate enough to do so, mine has an unacceptably poor percentage inaccuracy in low current conditions .....

Jon_Tiffany

zeupater wrote: »

## EDIT ..... Jon's post "Not quite, there is another method, you can calculate it by using an energy monitor." .... agreed, if you can get an energy monitor accurate enough to do so, mine has an unacceptably poor percentage inaccuracy in low current conditions .....

The monitor will for the most part be measuring high current. A small margin of error is acceptable for the intended purpose. What level of accuraccy do you require?

Most people will not have an export meter, however, many people will have an energy monitor. Its a far better solution to find out how much of the generated leccy you have used based on real measured figures rather than guesswork.

grahamc2003

I think what Nang is saying is that the amount of solar energy used isn't necessarily the saving. For example, say he puts the dishwasher on at lunchtime when half full, just to use the solar power, then, although say 3kwh may be used, the saving over what would otherwise be the case without PV is only half that (1.5kwh), because the dishwasher would otherwise be run only when full.

Jon - do you mean a quite sophisticated energy monitor? I ask because my Owl would be pretty useless for monitoring PV usage - mind you, mine can't be connected to a PC. Are you assuming a pc connected energy monitor for real-time (and constant) monitoring of pv output and load?