Choice of intelligent switches ?

Martyn1981

sly_dog_jonah wrote: »

Hi Mart,

I suspect Hobbo is guestimating the savings based on Electricity 'cost' of zero. Due to deemed export the actual amount of electricity export is irrelevant unless you either have an export meter (very uncommon for domestic installs) or have a Smart Meter which can record export (I doubt these actually exist yet other than in prototype form). Therefore for now at least it is a safe assumption to assume that the 'cost' of the electricity is indeed zero.

Hiya SDJ, best of luck with it.

As I said, not stirring, just interested in the economics, particularly long-term as PV prices fall and gas/oil prices rise. As for deemed export, you and Hobbo are welcome to offset against some of my spare, I estimate export now at about 70%.

Mart.

legoman62

sly_dog_jonah wrote: »

Hi Mart,

I suspect Hobbo is guestimating the savings based on Electricity 'cost' of zero. Due to deemed export the actual amount of electricity export is irrelevant unless you either have an export meter (very uncommon for domestic installs) or have a Smart Meter which can record export (I doubt these actually exist yet other than in prototype form). Therefore for now at least it is a safe assumption to assume that the 'cost' of the electricity is indeed zero.

Our Immersun installation is done but hit a snag in that the immersion element is a dead duck, so needs replacing (along with the faff of draining the tank). As that'll need a plumber we'll probably get the tempering valve fitted at the same time to mix down the hot water to a safe temperature as it leaves the tank.

Unless Hobbo's gas is very expensive (ours is 3.7p/kWh inc VAT), his gas boiler very inefficient or he has a very large array I doubt he'd achieve savings of £120/year. Using our gas price and assuming the immersion is no more efficient than the boiler (for simplicity of calculation) he would have to dump around 3240kWh of spare electricity into his hot water tank, or just under 9kWh/day. Our 3.7kWh system has averaged 9.5kWh/day of generation over the past ~10months, so we couldn't have exported 9kWh/day unless we were only using 0.5kWh.

s_d_j
Do you need to drain your tank to fit a new immersion?
Is it a top fitting one?
I fitted my new immersion a while ago.
All I did was isolate water(and power)
Drain off a few inches from the top of the cylinder and remove.
Replace with new one and it's job done!

Will be interested to hear how the Immersun performs too.

sly_dog_jonah

legoman62 wrote: »

s_d_j
Do you need to drain your tank to fit a new immersion?
Is it a top fitting one?
I fitted my new immersion a while ago.
All I did was isolate water(and power)
Drain off a few inches from the top of the cylinder and remove.
Replace with new one and it's job done!

Will be interested to hear how the Immersun performs too.

Thanks for the tip legoman62. Our immersion is 40cm from the bottom of a 145cm tall tank, so it needs to be drained. Fortunately we're still within our 2 year guarantee period where the housebuilder is responsible for fixing free of charge (or their subbies). Beyond 2 years it's an NHBC-backed policy with an excess. Incidentally we also spotted that the immersion was lacking a safety cut-out (in case the thermostat fails) to prevent over-heating the tank. I think that has been a building reg requirement for at least 2 years, I'll have to check. Indeed it has, but our house commenced construction prior to April 2010 when it took effect (section G3, p.5):

http://www.stockport.gov.uk/2013/2994/developmentcontrol/14406/45614/019sanitation

The installer wired up an iron (resistive load) temporarily instead of the immersion to confirm the Immersun was working as expected, so once the immersion element is replaced we'll be all sorted.

Incidentally it's been a glorious day here, 18.8kWh in the bag already and still generating 0.25kW right now. Oops wrong thread. :beer:

SDJ

Cardew

sly_dog_jonah wrote: »

Unless Hobbo's gas is very expensive (ours is 3.7p/kWh inc VAT), his gas boiler very inefficient or he has a very large array I doubt he'd achieve savings of £120/year. Using our gas price and assuming the immersion is no more efficient than the boiler (for simplicity of calculation) he would have to dump around 3240kWh of spare electricity into his hot water tank, or just under 9kWh/day. Our 3.7kWh system has averaged 9.5kWh/day of generation over the past ~10months, so we couldn't have exported 9kWh/day unless we were only using 0.5kWh.

Presumably you missed Hobbo's earlier posts about his set-up?

He has warm air gas CH but gas doesn't heat his water; that is done by an immersion heater on a 'normal'(i.e. not Economy 7) tariff. So I assume he is estimating his savings at around 12p/kWh - thus approx 1,000kWh pa. He doesn't think that heating water with Economy 7 would be appropriate.

Hobbo's situation is atypical, your figures would apply to most properties with gas CH.

Hobbo2006

Martyn1981 wrote: »

Hiya Hobbo, can I ask a genuine question, and I promise I'm not stirring nor criticising. Is that £120 valuing the leccy at zero?

If you were on an export meter, how would the numbers work out at 3.2p or 4.5p? Anything that helps make PV more financially rewarding, will of course improve it's viability (to those who can fit such systems) and therefore help to speed up it's roll out and the leccy and gas/oil savings that it brings.

So I'd be very interested in your thoughts, just so I can squirrel a few more numbers away for consideration.

Thanks again.

Mart.

Hi Mart,

As Cardew said my hot water is heated via electricity so I've just looked at the reduction in the bills on my current tariff.

I'm with Southern Electric so from the 15th of Oct the units will be priced at 12.52p. I think the Immersun has the potential to save at least 1000 kWh's.

At our current rate of use our yearly bill for elec will come to £685 when the new pricing kicks in.

Heating the hot water on an economy 7 tariff would bring the bill down to £623 but requires a change of meter and a change in usage habits to be effective. With a family at home during the day, there is potential for some big bills.

With the Immersun I have assumed that it will contribute an average of 3 kWh's per day. With a 4kW PV system I'm seeing plenty of excess generation so across the year I think that 1095kWh's is realistic.

With the Immersun assumption the bill is £547.

so payback is 3.4 years at current tariff and 3.04 years at the new tariff from 15 Oct.

From looking at the graphs my wattson solar produces with generation and usage on one graph. I think that it should average more than 3kWh's per day and therefore payback in less than 3 years, which would be nice as there is a 3 year warranty on the device.

My main aim since getting the energy monitors is to use less electricity and I see the Immersun helping to do that.

The majority of the 21x 50w halogens we had have been swapped for 6w LED GU10 bulbs (still a couple of bedrooms to go) so I'll be interested to see how that effects our usage with winter coming. I did have to add more bulbs to retain the same level of light and had to source most of the parts from ebay (bulbs, bulb holders & addtional fittings) to keep the cost down. Overall the total lighting wattage has been reduced from 1302w to 343w.

The only gas appliances we have are the hob on the cooker and the warm air heating for the house..... I'm currently looking into whether getting rid of the gas usage makes sense.

sly_dog_jonah

Thanks Hobbo & Cardew for the replies. I hadn't read (or didn't recall reading at least) about Hobbo's set-up until now. Will be interesting to see how you get on. My gut feeling is that our savings will be of the order of £80-100/year.

Here's our Immersun installed next to our consumer unit. The installer managed to route the cabling without any trunking which I'm very pleased with. As mentioned before, we're just waiting for a new immersion element as ours had packed up (under warranty fortunately). Now I can start hunting round for a good solution to box in both the CU (which has always been on my list of things to do) and Immersun, although I need to allow airflow in/out of the bottom/top.

Cardew

sly_dog_jonah wrote: »

Thanks Hobbo & Cardew for the replies. I hadn't read (or didn't recall reading at least) about Hobbo's set-up until now. Will be interesting to see how you get on. My gut feeling is that our savings will be of the order of £80-100/year.

That still means the immersion using 2,000kWh to 2,500kWh pa.(that is valuing gas at 4p/kWh - not the 3.7p/kWh you used in above post)

Time will tell, but I would be surprised if you can achieve that figure, I would have thought Hobbo's figure of 1,000kWh more realistic

What is the maximum you think the immersion will be able to use in a single day, even if there is no residual heat in the tank?

Hobbo2006

sly_dog_jonah wrote: »

Thanks Hobbo & Cardew for the replies. I hadn't read (or didn't recall reading at least) about Hobbo's set-up until now. Will be interesting to see how you get on. My gut feeling is that our savings will be of the order of £80-100/year.

Here's our Immersun installed next to our consumer unit. The installer managed to route the cabling without any trunking which I'm very pleased with. As mentioned before, we're just waiting for a new immersion element as ours had packed up (under warranty fortunately). Now I can start hunting round for a good solution to box in both the CU (which has always been on my list of things to do) and Immersun, although I need to allow airflow in/out of the bottom/top.

looks really neat... Can't wait for mine now.

sly_dog_jonah

Cardew wrote: »

That still means the immersion using 2,000kWh to 2,500kWh pa.(that is valuing gas at 4p/kWh - not the 3.7p/kWh you used in above post)

Time will tell, but I would be surprised if you can achieve that figure, I would have thought Hobbo's figure of 1,000kWh more realistic

What is the maximum you think the immersion will be able to use in a single day, even if there is no residual heat in the tank?

OK you hooked me Cardew, I couldn't resist taking a look through my figures again to come up with a more considered figure.

PVGIS generation estimate for our system is 3060 kWh/year. In fact we should surpass that figure tomorrow with over 7 weeks left until our anniversary. That's an average of 8.4kWh/day generation according to PVGIS, but actually we're running at 9.5kWh/day since installation (+13%).

Assuming our electric consumption hasn't changed since installation, then the reduction in our import is solely due to our generation that has NOT been exported. I don't have any Q4 2010 import figures to compare with as we only moved into the house late 2010, but I've done a comparison of the figures for period Jan-Aug for this year and last year (the only complete months I have full comparable figures for):

Avg Import pre-solar: 10.77kWh/day
Avg Import post-solar: 7.13kWh/day (34% down)
Avg Generation: 11.01kWh/day
Avg Export: 7.37kWh (assuming consumption is unchanged. If consumption has dropped then the export figure would be higher)
Avg Export: 67%

The export average peaked in May at 12kWh/day but in January was <1kWh/day (and was probably worse in Dec). As I've omitted Sept-Dec, when Export will be less than average I've estimated that the annual export average would be about 6.5kWh.

So that's what is theoretically available to dump into the tank each day, maybe slightly more as we've probably cut our usage down since getting the solar installed. Also this is probably an underestimate of our export as in 2011 year my wife and baby were at home whereas in 2012 they are at work/nursery. So electricity consumption was almost certainly higher in 2011 which perturbs the above assumption about export somewhat.

Our hot water is currently heated by gas twice a day to 45C during 2x 1hr programs in the morning and evening. The thermostat is positioned in the lower 1/4 of the tank, so most of the water will be hotter than that (because it is hottest at the top), probably 48C is the average in the tank when hot.

Assuming the cold water is coming in at 19C our gas consumption of ~14kWh (when central heating is off) for a 90% efficient boiler would heat around 375 litres of hot water to 48C. That ignores the losses between the boiler & tank (although the distance between is quite short) and any losses from the tank itself (rated ~1C loss per day). Perhaps the 90% figure is based on heating hot water and the central heating at the same time?

I'm sure our actual usage is much less than that though, and here's why:
Our total water usage per day (hot & cold) is only 360litres (metered). The only use of hot water is in baths/showers and from sinks, as all the applicances use cold water. I find it hard to believe then that we would use more than 200litres of hot water per day, let alone 375litres. I can only attribute the above figures to losses between our boiler and tank.

So, lets go back to the export electricity. Assuming the immersion is 95% efficient (which is conservative), then heating 200litres of water per day to 48C would require 7.1kWh. That's less than our average export, so we shouldn't have a problem fulfilling that on the average day. On very good days of course there will still be some export once the tank is up to temperature.

Of course some days the spare power will be only be enough to part-heat the tank and we'll need to top up with gas heating, even if only a little. The question is how many such days will there be, and how much gas would that require? Only time will tell, but I think the Immersun will probably save us 20% of our gas hot water usage in Nov-Jan, 50% in Oct & Feb and 80% for the rest of the year (even that is conservative I think), or an average of 60% per month. That would be equate to 3060kWh of gas each year, which will save us £113 a year (based on EDF Blue Price promise prices inc VAT).

I realise of course that the gas kWh savings is much greater than the electricity we currently export, but I think that is down to the inefficiencies of the boiler rather than a flaw in my calculation. I welcome any pulling apart of the above logic though!

Hobbo2006

wow.... was easier for me to calculate how many units we were using.... I just slapped an energy monitor round the cable!

wouldn't your cold water be a lot colder than 19C though?