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Discussion Starter #1
My PV system gets commissioned on Wednesday, and I'm trying to work out my best strategy for prioritising either my EV charger or my hot water immersion heater via my zappi/eddi.

For the EV, I do practically all my home charging on Octopus's off-peak rate, so I know that every kW of solar I put into the car saves me 5p.

However, it's a bit more complicated for the hot water. My normal means of hot water heating is a gas-fired condensing system boiler (13 years old) I'm paying 3.17p/kW for gas, but clearly there's an issue of efficiency here. How much of a kW of gas fed into the boiler is transferred to the water in the tank, and likewise how efficient is the electric immersion heater - close to 100%?

If I assume 100% efficiency for the immersion heater, then by my man-maths I reckon the break-even point would be a gas boiler/system efficiency of 64%. If the efficiency is below that, then I should prioritise the water heating; above that and the EV gets it first.

Can anyone give me a rough idea of how efficient centrally-heated hot water cylinder systems are (I know there will be many variables, but a rough idea would still be useful)? Again, I'm not talking about the general efficiency of boilers, but the actual energy transfer into the water in the cylinder taking into account energy losses within the system.

Thanks,

Neil
 

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Surely you need only consider the best way to use your PV power, if you are not at home to charge, and not using it for other appliances, then use it to heat the water.

My car sits at home often, the Zappi then switches on and off when the washing machine, ASHP are working, off with black clouds but on with sunshine.

Will you have any power monitoring heating (immersun) or Zappi?

Generally gas is cheaper, but not if the PV is going to the grid.
 

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Again, I'm not talking about the general efficiency of boilers, but the actual energy transfer into the water in the cylinder taking into account energy losses within the system.
100%, but the cylinder then has heat loss, so how good is the insulation, some heat loss also on the pipe work, but once inside, none is lost.
 

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Even in non-condensing operation that boiler should be near 80% efficient. Maybe knock 10% off as it's getting on. In condensing mode it'll be better.
So if the car needs power use the solar for that and give the HW to the boiler.

Though I'd have thought some judicious jiggling of timers would avoid them being in conflict very often.
 

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Unless you use lots of hot water the hot water is easier to keep hot than it is to get charge into the car. If my car is at home I plug it in and the water still gets heated as the Eddi is working all the time the car isn't charging, and on cloudy days that's quite a bit of the time.
Also the water starts heating before there is enough surplus to start charging and carries on after the car has stopped so I find I can let the Zappi have priority and still have free hot water.
 

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Discussion Starter #6
Surely you need only consider the best way to use your PV power, if you are not at home to charge, and not using it for other appliances, then use it to heat the water.

My car sits at home often, the Zappi then switches on and off when the washing machine, ASHP are working, off with black clouds but on with sunshine.

Will you have any power monitoring heating (immersun) or Zappi?

Generally gas is cheaper, but not if the PV is going to the grid.
Sorry, I should have been clearer - this is surplus PV once the demands of the house have been satisfied

100%, but the cylinder then has heat loss, so how good is the insulation, some heat loss also on the pipe work, but once inside, none is lost.
Again, my wording wasn't very clear but what I'm looking for is the efficiency of the system from a kW of energy being fed into the boiler, through to the water in the cylinder. So straight away the boiler's probably around 80% efficient, then there's further energy/heat loss as the heated water is transferred to the cylinder. Insulation of the cylinder isn't really a factor, as the energy loss there will be the same irrespective of the heating system used (gas/immersion)

Even in non-condensing operation that boiler should be near 80% efficient. Maybe knock 10% off as it's getting on. In condensing mode it'll be better.
So if the car needs power use the solar for that and give the HW to the boiler.

Though I'd have thought some judicious jiggling of timers would avoid them being in conflict very often.
Unless you use lots of hot water the hot water is easier to keep hot than it is to get charge into the car. If my car is at home I plug it in and the water still gets heated as the Eddi is working all the time the car isn't charging, and on cloudy days that's quite a bit of the time.
Also the water starts heating before there is enough surplus to start charging and carries on after the car has stopped so I find I can let the Zappi have priority and still have free hot water.
OK, thanks all. It sounds like the most cost-efficient strategy for any surplus is to give the EV first refusal, and only if the EV won't/can't take any then use it for HW.
 

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I switch my boiler off when the heating goes off in May and haven't turned it on again yet, but I don't have gas. For me it is cheaper to pay 5p overnight than it is to us the boiler, wood pellet stove.
We have quite high losses between the boiler and the hot water tank but we have quite a run, boiler is in the garage. Once the heating is on I heat the hot water overnight using E7 rate with a boost set on the Eddi.
For me E7 is cheaper than the boiler, and I expect it is for you too. It would be cheaper to prioritise the hot water but it doesn't work out like that for us. I expect you will be able to keep the water hot even with the car plugged in, unless it is charging most of the time.
 

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Your EV will charge at anything from 6A up to 16/32 MAX depending on Ev, and the charger itself. But it cannot charge at anything under 6A. I have a 4kW panel, so when it's really sunny I set my granny EVSE to 6/8/10A depending on my need to charge faster, or not. So I might trickle at 6A, and any excess is used by my Solic 200 to heat the immersion tank. Can you set your Zappi to have a variable capped max rate which would allow excess to go to immersion?
 
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