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

KevinG

My Economy 7 period is interesting - 2230-0030 and 0230-0730 GMT (so an hour later in Summer, cheap rate until 0830). This allows quite a bit of usage to be at the cheap rate and it is certainly cost-effective for me.

zeupater

Cardew wrote: »

I have an Economy 7 meter, although for years the readings were added together and treated as a single total.

I found that without any attempt to maximise use during the 7 hours cheap rate period, I used about 20% in that period. Don't forget that, depending where you live and when you get up in the morning, activity can be using cheap rate electricity - showers, cooker, etc.

With water heating and timed use of washing machine/dryer/dishwasher it would be easy for me to raise that percentage to 30% - 35%.

Hi Cardew

I'd be interested if you could rework the 20% E7 usage above with the assumption that the overall daytime electricity usage was reduced by a nominal 1000kWh/year to reflect daytime solar ... also (you've probably mentioned it before but I can't remember) do you heat your HW with E7 or gas ? ... if gas can you add ~6kWh/day to the overall annual E7 usage and rework the percentage ...

Given the twin-meter tariff structure for overnight electricity, I'd maintain that, given a suitably sized HW cylinder, it would almost certainly be cheaper to heat 100% of the demand from E7 than 50% from diverted solar and 50% from daytime electricity ....

Thanks
Z

zeupater

KevinG wrote: »

My Economy 7 period is interesting - 2230-0030 and 0230-0730 GMT (so an hour later in Summer, cheap rate until 0830). This allows quite a bit of usage to be at the cheap rate and it is certainly cost-effective for me.

Hi

With that pattern if you're 100% electric then you would likely get an hour or so of lighting/TV in the evening, a cuppa to take to bed, overnight immersion heating as well as running the fridge & freezer, then some TV news to help with morning depression, an electric shower, lighting, a bacon butty and a cup of coffee before a hard day at work ...... all while the pv is snoozing ...

No-wonder it's cost effective for some ...

Z

grahamc2003

The Immersun thing looks quite interesting - it is certainly much more sophisticated than the home-brew immersion 'controllers' discussed on here, mainly due to the variable power supplied to the immersion heater.

As usual though, when you click on 'technical details' in the blurb, you don't get much technical detail!.

The main point about not getting virtually all the exported energy to heat your water is imo, as Z said, the size of the tank (and the position of the elements, and the solar system size etc). For example, my tank only stores about 1.2kWh from the immersion, which would equate to about typically 30 mins of excess power from a 3.8kw system flat out) - this is enough for one hot bath (the other water heating is from the AGA, when the tank can store lots of heat).

If I'm reading correctly, there's a 500W threshold of generated power before any power is sent to the element. (i.e. users will still be exporting
up to 500w before water heating starts). The heating will then be on for a minimum of 1.5 hours - so if the sun goes in, and you import grid power, then the water heating element is still activated - exactly how much power the controller gives to the immersion isn't stated - it could be zero possibly, but it may also give it a minimum value, which may be 500W, which of course would be imported at day rates in that circumstance.

Without knowing more technical details which don't seem to be available on their website, it really is impossible to predict the level of net savings (that is, savings less any extra costs incurred, like day rate electricity). It is certainly not as simple as saying ' I export 1000kwhpa, and this device will use all that to heat water (in fact, that is impossible).

sly_dog_jonah

grahamc2003 wrote: »

The Immersun thing looks quite interesting - it is certainly much more sophisticated than the home-brew immersion 'controllers' discussed on here, mainly due to the variable power supplied to the immersion heater.

As usual though, when you click on 'technical details' in the blurb, you don't get much technical detail!.

The main point about not getting virtually all the exported energy to heat your water is imo, as Z said, the size of the tank (and the position of the elements, and the solar system size etc). For example, my tank only stores about 1.2kWh from the immersion, which would equate to about typically 30 mins of excess power from a 3.8kw system flat out) - this is enough for one hot bath (the other water heating is from the AGA, when the tank can store lots of heat).

If I'm reading correctly, there's a 500W threshold of generated power before any power is sent to the element. (i.e. users will still be exporting
up to 500w before water heating starts). The heating will then be on for a minimum of 1.5 hours - so if the sun goes in, and you import grid power, then the water heating element is still activated - exactly how much power the controller gives to the immersion isn't stated - it could be zero possibly, but it may also give it a minimum value, which may be 500W, which of course would be imported at day rates in that circumstance.

Without knowing more technical details which don't seem to be available on their website, it really is impossible to predict the level of net savings (that is, savings less any extra costs incurred, like day rate electricity). It is certainly not as simple as saying ' I export 1000kwhpa, and this device will use all that to heat water (in fact, that is impossible).

Hi Graham,

I suggest you read the Installation Manual which goes into much more detail. You've confused the '500W export' with the optional relay connection to a secondary load, which is separate to the immersion system (see page 14). Also the 90mins applies to the relay switched load, not the immersion load. The device optimises the export to be much less than 500W at any given time, so I understand (assuming the water isn't already up to temperature). I'll find out in a couple of weeks when mine is installed.

How big is your tank if it only takes 1.2kWh to heat a bath's worth of water? I've calculated that our 200 litre tank would require 16kWh to raise the temperature from 21C (ie cold) to 85C, assuming all the hot water has been used. OK so a bath isn't 200litres, but still the difference seems significant. Is your immersion thermostat set quite low?

At the moment we only use Gas to heat the water, the immersion is just there as a backup. Our gas consumption for heating averages 14-15kWh/day, which we're looking to reduce as far as possible through this device.

grahamc2003

sly_dog_jonah wrote: »

Hi Graham,

I suggest you read the Installation Manual which goes into much more detail. You've confused the '500W export' with the optional relay connection to a secondary load, which is separate to the immersion system (see page 14). Also the 90mins applies to the relay switched load, not the immersion load. The device optimises the export to be much less than 500W at any given time, so I understand (assuming the water isn't already up to temperature). I'll find out in a couple of weeks when mine is installed.

How big is your tank if it only takes 1.2kWh to heat a bath's worth of water? I've calculated that our 200 litre tank would require 16kWh to raise the temperature from 21C (ie cold) to 85C, assuming all the hot water has been used. OK so a bath isn't 200litres, but still the difference seems significant. Is your immersion thermostat set quite low?

Yeah, I think you're right, thanks. Having read the manual, I still think it's not explicit that the export threshold before water heating starts is very low (it may well be, it just doesn't say afaics). It does state that the WAIT state is Waiting for sufficient exported power to start heating water.
which obviously means there is some sort of threshold. Similarly, the minimum element heating time once switched on isn't necessarily zero (although it may be) - it just isn't stated, and without seeing the ciruit design, it's probably impossible to tell.

It certainly does look a clever and sophisticated device, with plenty of future options and versatility built in. I'd say it's possibly viable for someone with a large solar system and a large heat store - would be pointless (or rather not cost effective at all) for my 1.75kW system for instance. Not sure even a building site 120v transformer to drive the immersion is worth investing in for me!

My (old) tank is large capacity, but long and small diameter, with the element at the top. That's how I want it because it isn't very well lagged and is pretty inaccessible. So I spend quite a bit on day rates (usually before the sun hits my sw panels of course) when the kids all want a bath in sequence, and any suggestion for them to wait till half one when my system peaks goes down like a lead balloon. The temp is set low (55iirc) which is still hot enough to add cold water to for a bath. Takes about 25 minutes to heat up for a bath.

The tank is very old - I'm thinking of getting a solar thermal system on my SE roof with a high capacity well lagged thermal store before the tank starts leaking (again).

sly_dog_jonah

grahamc2003 wrote: »

Yeah, I think you're right, thanks. Having read the manual, I still think it's not explicit that the export threshold before water heating starts is very low (it may well be, it just doesn't say afaics). It does state that the WAIT state is Waiting for sufficient exported power to start heating water.
which obviously means there is some sort of threshold. Similarly, the minimum element heating time once switched on isn't necessarily zero (although it may be) - it just isn't stated, and without seeing the ciruit design, it's probably impossible to tell.

It certainly does look a clever and sophisticated device, with plenty of future options and versatility built in. I'd say it's possibly viable for someone with a large solar system and a large heat store - would be pointless (or rather not cost effective at all) for my 1.75kW system for instance. Not sure even a building site 120v transformer to drive the immersion is worth investing in for me!

My (old) tank is large capacity, but long and small diameter, with the element at the top. That's how I want it because it isn't very well lagged and is pretty inaccessible. So I spend quite a bit on day rates (usually before the sun hits my sw panels of course) when the kids all want a bath in sequence, and any suggestion for them to wait till half one when my system peaks goes down like a lead balloon. The temp is set low (55iirc) which is still hot enough to add cold water to for a bath. Takes about 25 minutes to heat up for a bath.

The tank is very old - I'm thinking of getting a solar thermal system on my SE roof with a high capacity well lagged thermal store before the tank starts leaking (again).

WAIT simply means that the household is consuming more than it's generating, so the Immersun is WAITing for either the generation to increase or for consumption to decrease (so that power would be exported), then it delivers just enough power to the immersion so that that export is minimised (but I'm guessing not nil). As generation or consumption in the house changes, so the power delivered to the immersion is adjusted to ensure no electricity is imported while the immersion is on.

Our situation sounds quite different to yours. We've got a 3.7kWp system which on the best days generates well over 20kWh, but even before we had PV the most we'd use per day was 11-12kWh. Therefore it makes sense to try to reduce our gas (water heating) usage from both an environmental and economic point of view. We don't have room to fit solar thermal now.

Our 200litre tank is so well insulated it only drops 1C per day (according to the specs) and the immersion element is towards the bottom of the tank. If the tank was completely cold (21C) it would take over 5hrs at 3kW to get up to 80-odd C. Since we're going to fit a tempering valve though the chances of the tank ever reaching cold is unlikely, as we'd be down-mixing the very hot water to 40-odd C so only using a bit at a time.

zeupater

sly_dog_jonah wrote: »

...
Our 200litre tank is so well insulated it only drops 1C per day (according to the specs) and the immersion element is towards the bottom of the tank. If the tank was completely cold (21C) it would take over 5hrs at 3kW to get up to 80-odd C. Since we're going to fit a tempering valve though the chances of the tank ever reaching cold is unlikely, as we'd be down-mixing the very hot water to 40-odd C so only using a bit at a time.

Hi

You'll find that the hotter the tank is maintained the faster the heat will leak away, so diverting too much energy could just mean that the losses will be higher ...

Mixing down to 40C was advised to be too low by our ST installers ... they advised to set the TMV in the mid 50sC to ensure that the water was still hot on the longest runs (kitchen/utility) so that it's still warm enough for use in the kitchen environment ... realistically, the TMV should simply be seen as adding an anti-scald device for the higher tank temperatures, mixing down to a more normal level.

A note for care, if you haven't already considered it, is to take care of where the cylinder is relative to where anyone sleeps ... if the tank fails you don't want 70/80C water flooding over someone ....

HTH
Z

Cardew

zeupater wrote: »

Hi Cardew

I'd be interested if you could rework the 20% E7 usage above with the assumption that the overall daytime electricity usage was reduced by a nominal 1000kWh/year to reflect daytime solar ... also (you've probably mentioned it before but I can't remember) do you heat your HW with E7 or gas ? ... if gas can you add ~6kWh/day to the overall annual E7 usage and rework the percentage ...

Given the twin-meter tariff structure for overnight electricity, I'd maintain that, given a suitably sized HW cylinder, it would almost certainly be cheaper to heat 100% of the demand from E7 than 50% from diverted solar and 50% from daytime electricity ....

Thanks
Z

I have gas CH, and have heavy electricity consumption.

We use approx 7,000kWh a year of which approx 1,350kWh is recorded on the E7 meter which is 19.3%.

If 1,000 kWh daytime solar electricity reduced that overall figure to 6,000kWh then the 1,350kWh would represent 22.5%.

If 6kWh a day was used at cheap rate to heat water my total consumption(with 1,000kWh solar) would be approx 8,190kWh of which approx 3,540kWh would be cheap rate, thus 43.2%

Taking my area(Midlands) the 'break even' point taking the cheapest tariffs is approx 20%.

I would most certainly agree with your conclusion that heating water by E7 would be the preferred option. The only exception would be if your heating was non-storage and used daytime electricity.

Cardew

sly_dog_jonah wrote: »

Since 7hrs/day is 29% of the day, that implies that your nightime electricity consumption rate is only 31% less than the daily average. Sounds like you may have some inefficient fridge/freezers running overnight?

Unless people are early risers, it's unlikely they will be able use the cheap rate when cooking etc, although as you say it varies by region and time of year.

I have a Neff American fridge-freezer and Liebherr fridge that are reputedly the most efficient available(A++??) and an older Bosch chest freezer in garage that uses approx 1kWh a day.

The mechanical Timer for changeover from cheap rate, to day rate, was set for 00:30 to 7:30 winter and 01:30 to 0830 summer, the clock has slipped slightly and it is closer to 8am/9am.

We do tend to get up early(the dog demands it!) and both wife and I use an electric shower, albeit only for a few minutes(can't understand how people claim to have 20 to 30 minute showers)

However the point zeupater and myself were making, is that using cheap rate E7 to heat water must be a cheaper alternative than using a single rate tariff, with solar when available. I suspect that even with oil CH there might be a case for E7 for some people.