Hot water cylinder with solar PV

[Dave123]

Hi all,

I need your help! I have little to no working knowledge with solar PV, and am currently relying on my builder who is doing a lot of renovation work in my (fairly old and poorly insulated) house but also planning to put in a 15kW solar PV system with battery storage. This will be south facing in south west UK. This system will not be linked to the national grid, so it will be an offgrid solar PV system with a battery, but with the grid as a back up, I don't have the opportunity for any payments from exporting to the grid.

I'm about to get a new gas boiler put in, with a new hot water cylinder. I have a large 4 bed house.

I'd like to be able to use any excess solar to help heat the hot water cylinder in summer/when there is excess solar, and let that gas heat it the rest of the time.

What is the most cost effective/least likely to break down way of achieving this? I imagine its a case of getting the cylinder just to have an immersion heater in it, but... when I look at devices that claim to help divert energy to the cylinder they all seem to mention detecting what you're exporting to the grid as a trigger for instead exporting it to the cylinder... and since I won't export anything to the grid, I therefore don't know if they'll work.....

Thanks in advance,

Dave

 

[solarsimon]

It "won't be linked to the grid" but you'll have the grid as a backup?
If you have a grid connection (which you'll almost certainly need occasionally in winter) then these diverters simply adjust the power that they deliver to an immersion to ensure that no power is exported.
Do you have a grid connection or not?

 

[Dave123]

It "won't be linked to the grid" but you'll have the grid as a backup?
If you have a grid connection (which you'll almost certainly need occasionally in winter) then these diverters simply adjust the power that they deliver to an immersion to ensure that no power is exported.
Do you have a grid connection or not?

Hi Simon,

Thanks for reply. Yes we have a grid connection, sorry for clarity, we will not be able to export to the grid. The wording on the diverters I've looked at says "it detects when you are exporting to the grid" as the trigger for when it then decides to instead send the power to the immersion heater. Which I had (presumably wrongly) assumed meant that it basically clamped onto whatever wire that was exporting to the grid and when it detected electricity running through that wire, it kicked in.

So, from what you're saying, basically I don't need to worry about this? And instead the diverters don't (generally) rely on detecting electricity being exported out to the grid to decide when to send to the immersion?

Go easy, I'm a doctor not a solar power expert and I've posted this in 'beginners corner' for a reason!

Thanks,

Dave

 

[solarsimon]

If you are exporting 1 Watt it'll turn on 1 Watt of heating, bringing it down to zero. It'll keep adjusting the power sent to the immersion to hold it at zero. There's a chance you'll be exporting trivial amounts, but it should be tiny.... and then, at some point, your water gets up to temperature & the thermostat disconnects the immersion. The power would then be exported. What export constraints are you up against? Which country?

 

[Dave123]

If you are exporting 1 Watt it'll turn on 1 Watt of heating, bringing it down to zero. It'll keep adjusting the power sent to the immersion to hold it at zero. There's a chance you'll be exporting trivial amounts, but it should be tiny.... and then, at some point, your water gets up to temperature & the thermostat disconnects the immersion. The power would then be exported. What export constraints are you up against? Which country?

Thanks Simon,

I think maybe you're understanding what I'm asking - I'm in the UK. I'm not allowed to export to the grid unless the system is fitted by a "certified engineer", which this will not be.

And if it was fitted by a certified engineer, I'd only be usually allowed a 4kW system. So I won't be exporting. This is the crux of my question.

So if the diverter is comparing the amount generated by the panels to the amount being utilised by my system, then that is fine. But if it is any way looking at how much I'm exporting, then it won't be fine, as the answer to that will always be zero.

Dave

 

[solarsimon]

You need to be MCS registered to be eligible to receive SEG payments. If you're under 16A /3.68kW then anyone can simply notify their DNO that they have fitted a system.
Bizarrely, there's no formal certification in the UK for electricians, but there are constraints around building control.

 

[Dave123]

Thanks but this doesn't apply, as I said my system is 15kw, so I'm coming back to my original question?

 

[solarsimon]

Thanks but this doesn't apply, as I said my system is 15kw, so I'm coming back to my original question?

You can have 15kW of PV connected to batteries with any notification at all.
It's the size of the inverter that governs the WPD/National Grid submission/approval.
You can also heat immersion direct with DC from the PV, but it can be tricky to switch DC power.