So now I have a solar PV system how do I make the most of it???

Exiled_Tyke

zeupater wrote: »

Hi

Looks like you've fallen into a logic trap here ....

Basically, leaving solar aside, it's not power you need to consider - it's energy, so if you heat a given volume of water from a given start temperature to boiling point with either a 3kW or 1kW kettle then (within reason - kettle mass, in cycle heatloss etc) the same amount of energy will be needed, it's just the timescale which varies ..... now, upon bringing the effect of solar into the picture it's an assessment of the average ratio of self generation vs imported power which becomes important. Realistically, the proportion of annual generation hours above 3kW is pretty low, much lower than that where above 1kW is available and significantly nowhere near the hours those where only 500W is spare, which is probably where the majority of generating hours are over a year, therefore represents what is likely available when you want a cuppa (morning/evening/winter/dull/raining etc).

Of course, everyone will be different, but as a pure sanity exercise - if we leave high generation and low generation aside and assume that there is a constant ~500W of generation available, then a 3kW kettle would be importing 83%((3-0.5)/3) of it's power with a 1kW kettle importing 50% - of course with time being a relative variable, the percentages also apply to the total energy .... a saving of 33% (83-50) of the energy therefore exists in this scenario .... so that leaves the payback, which is pretty simple a £10 kettle costs the equivalent of 83kWh of electricity at 12p/kWh(10/0.12), which would be the equivalent of boiling around 800lites of water (rule of thumb ~1kW for 1Hr raises 1Tonne by 1C), which would be (say) 1600 kettles part filled with 500cc of water ... what's that ? ... just over 4 boils per day for a year (1600/365) to justify the kettle ? .....

So, a basic sanity check works out at ~4x500cc boils/day to breakeven over 12months, that gets me to around lunchtime most days, but remember, this is just a basic exercise ... all I know is that our 1kW kettle gets used quite a lot and, using our own variables, definitely paid for itself years ago .....

HTH
Z

Sorry but there are too many generalisations in this to make the figures useful. If there are many people who like me start the day with a cuppa before generation starts and then again after work (when there's only adequate generation in the peak of summer) then averages won't work. Similarly smoothing figures out all year round falls foul of the same problem. I think that if the calculations take both these factors into account then it's a relatively low proportion of homes that will benefit. And then there's the assumption that the kettle only cost £10 which is possible but again probably not the price many will have paid for their 1kW kettle.

Joe's original calculation in terms of cost per boil I think is the right starting point but then the remaining uncertainty is the occasions that the saving can be achieved. He assumed 6 litres a day, 365 days a year which I know is certainly not achievable for me.

zeupater

Exiled_Tyke wrote: »

Sorry but there are too many generalisations in this to make the figures useful. If there are many people who like me start the day with a cuppa before generation starts and then again after work (when there's only adequate generation in the peak of summer) then averages won't work. Similarly smoothing figures out all year round falls foul of the same problem. I think that if the calculations take both these factors into account then it's a relatively low proportion of homes that will benefit. And then there's the assumption that the kettle only cost £10 which is possible but again probably not the price many will have paid for their 1kW kettle.

... as mentioned, it doesn't apply for everybody because of their own usage patterns, but the logic that 1600x500cc average daytime boils should be in the right ball park for most with solar pv to break even on £10kettle is likely pretty sound ... as for the kettle price, well seeing that the current discussion revolves around the £9.99 Aldi one referenced earlier on the thread it's a totally relevant start point - ours is a travel kettle which was probably around the same price - I also saw a Swan travel kettle at the weekend for the same price in 'The Range' and Argos have a selection ranging between £10 & £20 (Not sale prices), so there are plenty available ...

Anyway, it's dark now and I've just used the 3kW glass jug-kettle which cost around 5x the price - functionality is no different as it heats water to the same temperature, it just aesthetics - but would price and payback be considered an issue on that one? .... two kettles!, well isn't that the height of extravagance - but at least it provides a little individualism into our existence ....

HTH
Z

EricMears

Of course, there are many people who already have a 1kW kettle so that they can take it away in their caravan. Arguably they would be paying nothing for the kettle when using it for cheap PV-operated boiling water because they've already accounted for its cost on their caravan budget.

Cardew

ajbell wrote: »

You boil water when you have the power not the other way round.
I will boil water just to stop exporting.

Just doing your bit to save the environment?

Doc_N

Best steer clear of smart meters if all this is to continue......:)

Exiled_Tyke

Doc_N wrote: »

Best steer clear of smart meters if all this is to continue......:)

Given that most of us struggle to use our 50% and that one report on here suggested that even using devices to divert energy to an immersion heater only just gets to the 50% (and doesn't have a reasonable payback period, nor is very green), then I would welcome an export meter. It would make me a lot more conscious about how I use energy and would really get me focused on exporting as much as possible as well as making me better off. At the moment it is far too tempting to put the tumble drier on knowing that I'm not paying for its electricity (and I confess it's been on more since I got my panels than previously)

orrery

Exiled_Tyke wrote: »

Given that most of us struggle to use our 50% and that one report on here suggested that even using devices to divert energy to an immersion heater only just gets to the 50%...

I'm using about 2/3 of my generation (measured using the smart meter and generation meter).

...(and doesn't have a reasonable payback period, nor is very green)....

I don't understand that statement - can you expand?

ASavvyBuyer

Interesting and amusing to see the calculations and comments about using a low wattage travel kettle. We already had one for when we went away, so we just started using that, as it boils the right amount of water for two large mugs. Therefore, a bit greener too, as it helps avoid boiling more water than needed.
If buying a new one, a payback time of a year or two seems quite reaonable.

ASavvyBuyer

“ ...(and doesn't have a reasonable payback period, nor is very green)....
”
I don't understand that statement - can you expand?

Yes, found that an interesting comment. Can't quite see why using a fossil fuel (gas) to heat water is greener than using the power from the solar panels.

For us the payback time worked out at about 2 years.

Conservative estimate of about 4kw a day for about 200 days a year to provide all the hot water we needed from March till September. We already had a less than 1.5kw element in the base of the tank. Switching device cost £30 from REUK, plus added a 2kw dimmer in the circuit to adjust power for poor days and was easy to fit into existing immersion heater circuit.
Therefore, with gas at about 2p/kw and at best 80% efficient, 10p/day for 200 days = £20. So diverting pays for itself in a couple of years.
Realise that not everyone would already have the lower wattage element or same setup, but what we have works for us.

tunnel

Got my new Aldi kettle today.
For the record, 8 out of 10 cuppas today were completely covered by my own generation and yet had it been the "big 3kW" kettle ALL of those cuppas would have left me needing to BUY electric.