Solar newbie...

[-rick-]

A single inverter with two MPPTs will handle two strings with different condition (and importantly different voltages/number of panels).

[Dillsue]

9 hours ago, Crofter said:

I thought I'd need a second inverter because the extra three panels will see quite different conditions. Maybe I'm overthinking it. Tbh they don't add a huge amount, it would be easier just to stick to the main array of six.

If you have 5.68kw of POTENTIAL export then you'll need DNO permission.

 

As rik says a single 3.68kw inverter with dual mppts is the way to go. Each mppt operates independently of the other so will manage the differing output profile from the 2 sets of panels

[Crofter]

This might not be possible, but another thought:

In the same way that the PV diverter heats the water automatically, could a system be put together which would switch on the heat pump if certain conditions were met? (Sufficient solar input, tank topped off, temperature of house below X°).

It's a Panasonic A2A unit, installed last year.

I'd imagine that such a task would, if even possible, require a high degree of geekiness involving things like Pi or Arduino etc, not that I really know anything about those.

Maybe it would be better to consider adding a battery instead?

[-rick-]

Lots of inverters (not all) come with a controllable relay output that can be used to signal things like excess energy available. Your Panasonic likely has an input that could be programmed to set a higher temperature when there is a signal present. So combining these two could likely do what you suggest without any extra geekiness. But still requires some set up and knowledge. Whether it’s worth doing vs doing the work to export and sell excess to the grid idk.

 

A 3.6kw system with 5.6kw of panels should generate a decent amount of power that likely justifies the £250 charge to get export approval with Octopus (for a non MCS system).

[JohnMo]

1 hour ago, Crofter said:

This might not be possible, but another thought:

In the same way that the PV diverter heats the water automatically, could a system be put together which would switch on the heat pump if certain conditions were met? (Sufficient solar input, tank topped off, temperature of house below X°).

It's a Panasonic A2A unit, installed last year.

I'd imagine that such a task would, if even possible, require a high degree of geekiness involving things like Pi or Arduino etc, not that I really know anything about those.

Maybe it would be better to consider adding a battery instead?

I did this last March on mine, basically batch charging floor with excess PV. Most heat pump are able to do two different flow temperatures, normally for an UFH and radiator zone, so radiator zone calling for heat resulting in higher flow temp. Anyway use the zero volt contacts for second flow temperature to force ASHP into action. I did it via home assistant, there are other ways.

 

Currently doing something similar based on outside temperature, if average temp is below 5 outside, I boost flow temp in the middle of the day when CoP is best for a few hours. Then the ASHP site idle for a few hours while floor cools. But this is driven from a signal from a UFH controller so nothing smart going on.

 

If you have a diverter that can do two immersions, use a timer relay to give a decent cycle time for heat pump to one of the immersion outputs. But I think Panasonic can integrate quite well itself given the correct feedback.

[TedM]

I'm doing something very similar with my [electrically heated] holiday place in Wales that is occupied 90% of the time during May to Mid September.  We have a south facing 3kWp array on the roof and a 210L Megaflo tank with hot water usage basically being showers - no baths.

I've set up an [old] Immersun diverter to heat the tank and when that is satisfied any spare generation going into a single electric wall heater in the hall which is only connected to one of the Immersuns' outputs - it's an upside down house so it heats open-plan upstairs as well as the hall.  I like using the Immersun because, not only does it have two outputs, it can also be remotely boosted by applying any voltage to a specific connection.

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So I'm looking to design a system that can deliver close to 8kwh/day over that period.

Unfortunately there will be periods, even in the summer' when that's not going to happen.
I also run a Pi that monitors various house temperatures including the hot water.  This checks the water temperature very early in the morning (say 5am) and, if it's below a threshold that I've set, it switches on a wireless plug that sends power to the Immersuns' remote boost port so there's enough hot water ready for morning showers.
This is pretty important at a holiday let where guests expect hot water all the time!  You don't have to use a Pi to achieve this - I'm moving quite a lot of stuff over to Shelly relays which are simple to set up.
Our array and this setup supplies all our heating needs for 6 months of the year.

The one change I do want to make is to add a further 4 ground mounted panels that face west which would give us some very handy generation past 4pm during the summer.  I'll be adding another inverter to handle this power as the original array is on a very attractive FIT so I'm not going to mess with it!

Edited 15 hours ago by TedM

[Crofter]

49 minutes ago, TedM said:

I'm doing something very similar with my [electrically heated] holiday place in Wales that is occupied 90% of the time during May to Mid September.  We have a south facing 3kWp array on the roof and a 210L Megaflo tank with hot water usage basically being showers - no baths.

I've set up an [old] Immersun diverter to heat the tank and when that is satisfied any spare generation going into a single electric wall heater in the hall which is only connected to one of the Immersuns' outputs - it's an upside down house so it heats open-plan upstairs as well as the hall.  I like using the Immersun because, not only does it have two outputs, it can also be remotely boosted by applying any voltage to a specific connection.

Unfortunately there will be periods, even in the summer' when that's not going to happen.
I also run a Pi that monitors various house temperatures including the hot water.  This checks the water temperature very early in the morning (say 5am) and, if it's below a threshold that I've set, it switches on a wireless plug that sends power to the Immersuns' remote boost port so there's enough hot water ready for morning showers.
This is pretty important at a holiday let where guests expect hot water all the time!  You don't have to use a Pi to achieve this - I'm moving quite a lot of stuff over to Shelly relays which are simple to set up.
Our array and this setup supplies all our heating needs for 6 months of the year.

The one change I do want to make is to add a further 4 ground mounted panels that face west which would give us some very handy generation past 4pm during the summer.  I'll be adding another inverter to handle this power as the original array is on a very attractive FIT so I'm not going to mess with it!

That's a very similar application to mine, great to hear that it's working out so well for you.

Using a panel heater as a dump load is an excellent idea- and I presume that its own thermostat is enough to prevent overheating the property?

 

My understanding (and I could be wrong) is that a grid tied inverter feeds in a higher voltage than the grid, so that the house draws preferentially from it- and any surplus just gets exported, whether you are getting paid for that or not. So e.g. I could leave a panel heater switched on and it would take from either the grid or the panels depending on how sunny it is. Obviously not a very clever way to do things, but I'm just trying to understand how the system will work.

[JohnMo]

1 hour ago, Crofter said:

and any surplus just gets exported, whether you are getting paid for that or not

Only up to either a CT clamp on export says no or the inverters limits (e.g. G98) say no

 

1 hour ago, Crofter said:

So e.g. I could leave a panel heater switched on and it would take from either the grid or the panels depending on how sunny it is.

Not sure that would work, as the heater would be hot all the time, so hot 24/7 - is that want you want? You have a heat pump, assume that is set up to do the heating/DHW.  I would heat DHW to the highest setting the heat pump allows at lunch time each day and also boost central heating around the DHW heating to coincide with peak generation - around 11am to 2pm for both jobs each day. Anything else dump to immersion.  Don't get carried away with array size, otherwise you just have a job managing it and it will be wasted.  Look at it as a back filler for your normal loads only. Anything above G98 limits can be full time job in summer.

[Crofter]

6 minutes ago, JohnMo said:

Not sure that would work, as the heater would be hot all the time, so hot 24/7 - is that want you want? You have a heat pump, assume that is set up to do the heating/DHW.  I would heat DHW to the highest setting the heat pump allows at lunch time each day and also boost central heating around the DHW heating to coincide with peak generation - around 11am to 2pm for both jobs each day. Anything else dump to immersion.  Don't get carried away with array size, otherwise you just have a job managing it and it will be wasted.  Look at it as a back filler for your normal loads only. Anything above G98 limits can be full time job in summer.

My heat pump is air-to-air, the water is simply heated via immersion in a UVC. So the array is just going to boost that when there's enough sun. There are two immersion elements on the tank, I'm not sure how best to set everything up to take best advantage of the diverter. I guess that ideally I would have the immersion powered by the diverter set to a higher temperature, and the one powered by the grid would be set lower as a backup. But of course one of them is half way up the tank.

 

My thinking with the panel heater would be simply to dump excess power, but I'd really want it to operate only from the solar. A very crude way of doing that would be to simply fit a timer, so that it is always off overnight. But it would be far better to set it up to only come on when the diverter had already fully boosted the UVC. I'm not sure how to do that.

[TedM]

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and I presume that its own thermostat is enough to prevent overheating the property?

Ummm .... not really!  Well, yes it has but the heater is located effectively at the bottom of the stairs which lead to an open plan kitchen and living room and it never gets hot enough downstairs to trip the thermostat - and, as it's there to dump excess power I don't really want it to switch off. 

There is a bit of an exception to that though  The upstairs has quite a lot of south facing glass (inc. floor to ceiling sliding doors) so it benefits from a lot of solar gain if the sun's out.  This is great generally but if we're not there [the doors/windows are all closed] and it's a very sunny day the combination of the solar gain plus the extra heat from the PV dump heater downstairs can push the temperature upstairs to over 30c 😬
I fixed that last year by adding a relay in the feed to the PV dump heater that disconnects it from the Immersun if the upstairs temperature gets too high - surplus generation is then sent out to the grid.

 

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My understanding (and I could be wrong) is that a grid tied inverter feeds in a higher voltage than the grid

Is that a thing? Maybe.  My inverter is currently outputting 260v but it ultimately all feeds into the house consumer unit which is running at 250v.  Screenshot below shows the current readings from the power going into the CU from the PV and the small amount being exported by the Immersun:

 

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There are two immersion elements on the tank, I'm not sure how best to set everything up to take best advantage of the diverter.

If your diverter has dual outputs (i.e. Immersun or the Myenergi Eddi) you could set the top element to be heated first and then the bottom element - but TBH I'd just go with the bottom element (hot water rises anyway) and then a heater wired directly to the second diverter output.

 

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I could leave a panel heater switched on and it would take from either the grid or the panels depending on how sunny it is. Obviously not a very clever way to do things...

... Correct - it's not a clever way to do things as the heater would always be on (thermostat dependant) 🤦‍♂️

 

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My thinking with the panel heater would be simply to dump excess power, but I'd really want it to operate only from the solar.

Two solutions:
1) Wire it directly (and exclusively) to the diverter then anything that would have been exported is sent to the heater.... even if it's only 100/200/300W

2) Put a wireless plug on the heater and have it switch on then the exported power is over a certain amount.  The issue with this method is that the heater will draw the maximum it's set to so it may well end up drawing a load of power from the grid if the exported power is not monitored constantly.  For example you have a heater that on it's lowest setting draws 1kW - you could set up a system that would switch that heater on if > 1.2kW was being exported (gives a little headroom), but if a cloud comes over and the generation drops then the heater will continue to pull 1kW unless it's switched off by your monitoring system.

I use Shelly power monitor and wireless relays to control various devices now as they don't need an internet connection to operate - signals can be sent via your house LAN - and they constantly check the amount of power going into or out of the house and react very quickly.

Edited 12 hours ago by TedM

[Crofter]

I looked up the Eddi and the ability to power two different devices sequentially looks absolutely perfect. Is this a common thing on diverters?

 

What should I be considering when choosing the heater? As it happens I never removed the old storage heaters when I put in my A2A, so it would be tempting just to wire up the one that's in the hallway, which would add a nice bit of background warmth to the whole house. But I don't know what wattage is appropriate for this, e.g. should it be sized to take account of the array output?

[JohnMo]

32 minutes ago, Crofter said:

Eddi and the ability to power two different devices sequentially

This does as well

https://coolenergyshop.com/products/cool-energy-solar-power-diverter?srsltid=AfmBOoqhPEOh5honAnsCyw0EYcVA8sIS74Zkpt_ENnLL5pn1vo1dzyRM

 

Most don't do two outputs

 

Spending big on diverters is a waste of money, so buy carefully. Pay back can be huge, especially if spending £400 on an Eddi.

 

If you are thinking about a battery, a diverter make no sense and can it other ways more efficiently.

[Crofter]

41 minutes ago, JohnMo said:

This does as well

https://coolenergyshop.com/products/cool-energy-solar-power-diverter?srsltid=AfmBOoqhPEOh5honAnsCyw0EYcVA8sIS74Zkpt_ENnLL5pn1vo1dzyRM

 

Most don't do two outputs

 

Spending big on diverters is a waste of money, so buy carefully. Pay back can be huge, especially if spending £400 on an Eddi.

 

If you are thinking about a battery, a diverter make no sense and can it other ways more efficiently.

I know diverters aren't the right answer for everybody. 

My interest in this was actually sparked by seeing a cheap used diverter for sale locally.

The unusual thing about my use case is that, being a holiday house, I'm not in a position to load shift. Guests are going to use electricity when they want to, not when the sun is shining.

[-rick-]

1 minute ago, Crofter said:

I know diverters aren't the right answer for everybody. 

My interest in this was actually sparked by seeing a cheap used diverter for sale locally.

The unusual thing about my use case is that, being a holiday house, I'm not in a position to load shift. Guests are going to use electricity when they want to, not when the sun is shining.

 

Would have thought you could load shift the hot water and bulk of heating, especially as you are targeting most usage during summer when heating less needed?

[JohnMo]

I would

 

Self install the PV 

Pay octopus to do self installation export, which is £250. Then not worry if you are using or exporting.

 

You need an electrical.install certificate, as well as G98 etc, you may need a structural certificate depending how you mount the panels.

 

Keep it all simple.

[TedM]

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I looked up the Eddi and the ability to power two different devices sequentially looks absolutely perfect. Is this a common thing on diverters?

I don't think so - and the external boost [by applying any voltage to a special 'port') is a game changer and, I think, limited to Immersun and the Eddi.
(Immersun came out quite a long time ago but went out of business but you can still find the units for sale on eBay.  Myenergi bought Immersun and based the eddi on it - it's extremely similar but has a couple more bells and whistles)

 

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What should I be considering when choosing the heater?

Literally any cheapo electric convector heater (without a fan) will do.

 

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But I don't know what wattage is appropriate for this, e.g. should it be sized to take account of the array output?

The Immersun and Eddi can output a maximum of 16A (you can actually throttle the eddi back if you wanted) so pretty much any heater will do.

[TedM]

1 hour ago, JohnMo said:

Pay back can be huge, especially if spending £400 on an Eddi.

Yep, I've never paid more than £100 on any of mine - all bought on eBay.

(although if you have an EV then the eddi works in conjunction with the zappi charger to utilise your PV generation even better - I think this adds value to the eddi)

Edited 8 hours ago by TedM