500-750L HW Tank? Maximise solar PV diversion

[Andehh]

We are extending & renovating a bungalow with 6 bedrooms (4 children &  regular visitors). It will have 2 en suites and a family bathroom, bath and at least 2-3 showers every evening, but during the summer we go through a lot of paddling pools & hot water to wash the cars (hot water taps being installed all around the exterior).

 

We also have 4kw of Solar PV on the roof.

 

My idea is to go for the biggest tank I can find, 750L, so that during the summer when we generate 10x the electricity we need it can be diverted into the cylinder. If we have a couple of cloudy days, then the HW has had enough heat put into it, it will be able to 'coast' for a few days without boiler or solar PV input...until the next sunny day when it once again will be brought back up to max temp.

 

Does this logic work? We are on mains gas.

 

Many thanks,

 

 

[A_L]

18 minutes ago, Andehh said:

so that during the summer when we generate 10x the electricity we need

 

Unlikely, a well sited 4kW array will generate a max of around 30kWh on a 'perfect' day. All of this would increase 750l by about 34°C

Edited May 24, 2021 by A_L
minor typo

[SteamyTea]

17 minutes ago, Andehh said:

Does this logic work? We are on mains gas.

Not really.

 

A larger store will mean greater overall losses and 4 kW_p is pretty small.

To heat 500lt of water by 30°C will take about 17 kWh of electricity, add in some losses, about 25 kWh.

On E7 that will be about £2.50 a day, on gas about £1.

You could put in a dedicated ASHP and reduce electricity costs down to about the same as gas (and have a much lower  carbon footprint), some of that power can be supplied by the PV though.

 

Water saving showers and a timer may be better.

Edited May 24, 2021 by SteamyTea

[Carrerahill]

31 minutes ago, Andehh said:

We are extending & renovating a bungalow with 6 bedrooms (4 children &  regular visitors). It will have 2 en suites and a family bathroom, bath and at least 2-3 showers every evening, but during the summer we go through a lot of paddling pools & hot water to wash the cars (hot water taps being installed all around the exterior).

 

We also have 4kw of Solar PV on the roof.

 

My idea is to go for the biggest tank I can find, 750L, so that during the summer when we generate 10x the electricity we need it can be diverted into the cylinder. If we have a couple of cloudy days, then the HW has had enough heat put into it, it will be able to 'coast' for a few days without boiler or solar PV input...until the next sunny day when it once again will be brought back up to max temp.

 

Does this logic work? We are on mains gas.

 

Many thanks,

 

 

I doubt you are going to make all this extra energy however consider a Sunamp - they are like half the size of a conventional tank and thus reduce the need for a massive calorifier.

[Andehh]

Talk about asked & answered! ?

 

Thanks very much for the quick & thorough feedback, looks like ill reduce the tank size to 500L and dull down my ideas of mega free energy.

 

 

[Ferdinand]

Or get a bigger array.

 

Mine is 10 kWp.

 

You need a different sort of connection if that is still the case, and somewhat different paperwork..

 

Someone who has done it more recently needs to coment.

[Ferdinand]

I might also say - think about 2 boilers.

 

Or one boiler and one electric, design so as it can be changed later.

[ProDave]

Allowing for other electricity uses, it is a rare day when my 300L UVC does not have the spare capacity to absorb all that my 4kWp solar PV produces.

[Andehh]

Thanks all, good to keep seeing perspectives on this! Still settling on 500L and then PV diverter off the 4kw Solar system into the tank.