Hi Derek,

Yes, we're very fortunate in that we have a big 'swiss style' roof that faces directly south and is at 30 degrees so absolutely ideal for solar panels - so our generation seems to be a decent chunk above any estimates (today was 30kw+ despite it raining for the first half of the day and being cloudy for the second chunk).

We've got a Givenergy 5kw hybrid inverter and 2 X Givenergy 9.5kw batteries - yes our system can cover power cuts also, haven't set that up but is equipped for it. Let me know if you need any other details and happy to share!

Good shout re diverter - I've got an Eddi on order, so this will take solar excess and heat water, just not sure how much it'll need and may need to divert as you describe.

The other option is I have home assistant running all my devices across the house - so could likely do it without a diverter and do it smart (ie take energy generation, subtract battery max and level and then smart select other devices to run to use up excess - eg underfloor if temp under 16C, air con if temp over 23C, car battery if level below X% etc etc)

Definitely keen to get another inverter though as this would unlock our maximum draw to a more comprehensive level.

Will look at A2A though, that seems sensible addition.

Likewise will have a nose at powerpod too! Ta for tips!

The power diverter will have a Current Transformer (CT) with which it monitors your electricity supply from the grid. If it senses that you are exporting power, then it can divert the excess to a designated load, normally up to a maximum of 3kW. If your solar PV is producing say 5.5kW, and your normal house load at the time is 0.5kW, then the diverter could send 3kW to say your immersion heater, which would leave 2kW being exported back to the grid.

If you now switch on a 3kW kettle, 3kW would go to the kettle, but only 2kW to the immersion heater. When the kettle is switched off it would revert back to the above example.

Dependent upon the power rating of your electric UFH, you could have it as a second load connected to your diverter, so the diverter would heat the water first then supply UFH. Alternative you could have your control system monitor the power flow, and when there is sufficient excess generation it could switch on your UFH as a direct feed and allow any excess up to 3kW to be diverted to your immersion heater. In that way you could heat both your water and your home at the same time.

I hope that my explanation makes sense.

Posted by: @kev-m

With a COP of 3 and a price per kW of 54p,an ASHP will cost 18p/kWh. With a 90% efficient boiler and £1/litre, oil will cost about 10.5p/kWh. Sadly oil is a lot cheaper at the moment.  

I still don't understand why the government doesn't extend a cut in levies and duties on HVO to bring it more in line with kerosene. 88% less emissions than kerosene and a great way to cut CO2 emissions for something like 1.8 million rural homes running their central heating on oil. 

With a COP of 3 and a price per kW of 54p,an ASHP will cost 18p/kWh. With a 90% efficient boiler and £1/litre, oil will cost about 10.5p/kWh. Sadly oil is a lot cheaper at the moment.  

I still don't understand why the government doesn't extend a cut in levies and duties on HVO to bring it more in line with kerosene. 88% less emissions than kerosene and a great way to cut CO2 emissions for something like 1.8 million rural homes running their central heating on oil. 

Yes, we're very fortunate in that we have a big 'swiss style' roof that faces directly south and is at 30 degrees so absolutely ideal for solar panels - so our generation seems to be a decent chunk above any estimates (today was 30kw+ despite it raining for the first half of the day and being cloudy for the second chunk).

We've got a Givenergy 5kw hybrid inverter and 2 X Givenergy 9.5kw batteries - yes our system can cover power cuts also, haven't set that up but is equipped for it. Let me know if you need any other details and happy to share!

Good shout re diverter - I've got an Eddi on order, so this will take solar excess and heat water, just not sure how much it'll need and may need to divert as you describe.

The other option is I have home assistant running all my devices across the house - so could likely do it without a diverter and do it smart (ie take energy generation, subtract battery max and level and then smart select other devices to run to use up excess - eg underfloor if temp under 16C, air con if temp over 23C, car battery if level below X% etc etc)

Definitely keen to get another inverter though as this would unlock our maximum draw to a more comprehensive level.

Will look at A2A though, that seems sensible addition.

Likewise will have a nose at powerpod too! Ta for tips!

Looking at my data from last December my 4kW system was averaging 1kWh to 1.5kWh, so you could probably expect twice that amount. November and January were just over double the December figure. Our panels are East facing so you may get more if you have better orientation.

I would be interested in the details of your batteries and inverter as I am seriously considering installing a system. Do you know if your system is capable of operating in 'Island Mode' in the event of a power cut?

I think most power diverters can connect to up to two individual loads of up to 3kW. So you could have a power diverter that feeds the immersion heater as its primary load and then feeds your electric UFH as a secondary load, obviously via a thermostat. In that way you can utilise excess solar generation without importing power when the clouds appear. For several years I have been using a 2kW convection heater in the hallway, fed via the power diverter, to help reduce or gas burn.

For A2A think AC Unit, in fact that is what it is, but can run in reverse to heat.

I looked at Tesup wind turbines, but I seem to remember someone on the forum said they had received bad reports. I was only looking yesterday at a unit called a Powerpod, which is being developed by an American company. They are not yet available, but look very promising, and may not require planning permission since they don't have external blades.

- solar carried away - yes, totally - I've used forecast APIs to model this - expect 5-9kw in winter. Ie better than nothing, but not transformational - think the batteries and cheap energy will be more useful in those months!

- batteries - actually these are 100% DOD ones, so should get more from that. Think 4% is min boundary. Pondering a third or fourth battery though - even now producing way more than I can store...

- electric storage - interesting ideas, hadn't thought of that, I'll take a look. We've actually got electric underfloor in the hall and kitchen which is slightly related - ie thought in spring/ autumn we could turn this on to take the edge off the oil heating. The hall is under the central column of three stories, so heat could flow upwards.

- air to air - hadn't thought of this either, I'll have a look at this too. Need to understand how this works, but could be a useful add-on also!

- octopus agile export - yes, you can't be on octopus go at the same time unfortunately on this, so have opted for go for now. As octopus let you change after 30 days without penalty, I might look at this in spring as the generation ramps again!

Thanks again for all the tips!

Finally I'm looking at wind generation with some of these vertical turbines from firms like Tesup that might be interesting to test!

The first thing that I would say is 'don't get carried away with your present solar generation, you will get nothing like that amount in the Winter when you need it'. Having said that there are various ways in which you can better use what solar energy is available.  Do you have a hot water cylinder with immersion heater? If so then have a power diverter installed to provide hot water from Spring through to Autumn.

Your 19.5kWh of batteries will provide approximately 15.6kWh of usable energy at a maximum Depth of Discharge (DoD) of 80%.

Have you considered electric storage radiators to store some of your excess solar energy? It may be possible to use them to provide a portion of your heating, particularly during Spring and Autumn.

Another way would be to install an Air to Air (A2A) ASHP rather than a Air to Water (A2W) one, to be used in a similar way as electric storage heaters to help reduce the use of your oil boiler. I have a small A2A heat pump which provides all our heating and cooling needs from Spring to Autumn, and also assists our gas boiler on sunny days in the Winter. I estimate our gas consumption has been reduced by approximately 40%.

I commend you on installing solar and batteries, we have had a 4kW solar system for over 10 years, which has reduced our electricity consumption by approximately 50%. After further insulation, a battery storage system will be our next priority.

Finally, since you are with Octopus you should ask them about selling your excess solar, they have I think it is an Agile export tariff, about which maybe someone on the forum will be able to provide more information.

Please ask any further questions.

Then with octopus energy I can get electricity at 7.5p per kW - the limit here is inverter and batteries.

So as a curious thought experiment - I could double my inverter and batteries to give myself another 20kw of energy @ 7.5kw and then top up with solar to aim for 5-10 KW extra per day (obviously way more in hot months, but less needed).

So if I got 30kw of free or cheap energy, then that might bring an ASHP to a more reasonable frame. Naturally there's investment costs there (around 6-7k), but it'd pay off in longer term. 

The obvious challenge to this is swapping oil for petrol - ie we have a Leaf and also a Model Y on order, so thisll give us a other 100kwh of batteries to fill up, so if we pointed free energy at heating, it might then require high cost energy to charge cars.

Oh the conundrums! I can't quite tell if ASHP are the future or the 'WAP' of mobile internet - ie we'll intentioned, but destined to be replaced by a better solution....!

With a COP of 3 and a price per kW of 54p,an ASHP will cost 18p/kWh. With a 90% efficient boiler and £1/litre, oil will cost about 10.5p/kWh. Sadly oil is a lot cheaper at the moment.  

I am a fan of ASHP and have one but you can't use them like a boiler very efficiently. They are better than the naysayers say though and you can set back temperatures and use TRVs and other room heaters.