Solar PV - are they worth fitting?

[Jude1234]

We are currently planning/costing our build with our builder.  As we have an l-shape design house which is facing south we do have lots of roof space that could be used for solar panels.  When we suggested it to our builders, who are  knowledgeable about renewable energy etc, they said that since the FIT has reduced to only 4p it isn't worth it anymore.  Just wondered what your thoughts are?

 

FYI we are building a well insulated house with MVHR and ASHP (although we do have access to mains gas and electricity). We are a family of 2 adults and 2 children and we both work so the house is unoccupied most of the day.

 

[ProDave]

I have stated my view several times, that I think it is worth it as a cheap DIY install and not claiming the FIT. I am no longer convinced it is worth paying the premium for an MCS registered company to install them in order to claim the FIT.

 

My model relies on you being able to use near 100% of what you generate, be that gig appliances in the daytime, excess dumped to water heating or even battery storage.

[Jude1234]

We are not DIY builders so would need to hire someone to install anyway.  have considered something like a SunAmp to use excess electrictiy during the daytime but may not need it with the ASHP

[Jeremy Harris]

It's not wholly true to say the FIT has reduced to 4p per kWH.  The current rate for sub 10 kWp  systems is  3.93 per kWh generated, plus an export payment of 5.03p per kWh for half the generated energy, unless you have an export meter fitted.  With an export meter you still receive 3.93p per kWh for every kWh generated, plus 5.03 kWh for every kWh exported to the grid. 

 

This can alter the dynamics of the payback significantly if you export a high proportion of your generated energy.

 

It's also worth noting that FIT payments are indexed linked to the RPI, not the CPI, for the guaranteed period.  This means that the 3.93p rate will increase by 4.1% in April.

Edited January 18, 2018 by JSHarris

[SteamyTea]

Glad you mentioned the export rate and the index linking, people seem to forget that bit.

It is odd how people still think that the FiT rate should be the old high rate of 41p, that was 6 years ago and only lasted just over a year.

 

As you have an ASHP, having PV could work to your advantage as any power generated can be absorbed by the ASHP and/or an immersion heater, so the 50% deemed export could work to your advantage, rather than an export meter.

All you have to do is make sure that your domestic hot water (DHW) is heated up either side of Noon Universal Time.  That is when, on average, you will get most power.

Then you fit your lifestyle around that.

 

You could get a more complicated system with a power diverted and use anything over a fixed amount to help heat your DHW.  Storage for DHW is your cheapest option to reduce exports.

 

Also make sure you size your heat pump correctly to run at no more than 40°C.  You get a much better coefficient of performance (CoP) then.

[TheMitchells]

Sounds like a job for @Nickfromwales.  Jude - he's the man for your design!  Give him a PM.

[Stones]

@Jude1234  All you can really do is determine whether the cost of an installed system (be it MCS or DIY) and the payback period is acceptable to you.

 

I had initially planned an in-roof Solar PV, but the local installer wanted £6.5K for an MCS install (when everywhere else in the country was sitting at £4.5K).  I did look at getting an installer from elsewhere but by the time travel, ferry and accommodation was taken into account, it wasn't significantly cheaper.  I worked out the break even point to be around 18 years so decided it was a non starter.

 

I then looked at a DIY install, but even then, the payback was significant - around 17 years.  To be able to get that period down, you have as @ProDave says, be able to use near 100% of what you generate.  Typically, an average household will use 20% - 25% of what they generate from a 3.68kWp system over the course of a year.  If you have a heat pump that figure could increase to 40%-50%.  You need to be storing excess generation to maximise your useage (and therefore reduce the payback period) and the only options are for DHW (relatively cheap) or batteries (expensive).  Bear in mind that in summer, even with excess generation diverted to storage there will be days when you generate far more than you can use.  

 

Some information on DIY installs:

 

 

 

[Jude1234]

Thanks for the advice.  Will look into it some more over the weekend

[richi]

20 hours ago, JSHarris said:

The current rate for sub 10 kWp  systems is  3.93 per kWh generated, plus an export payment of 5.03p per kWh for half the generated energy...

 

Don't forget to include the savings from offsetting the energy you won't be buying. That could be more than 20p/kW·h on a no- or low-standing-charge tariff, which dwarfs the FIT's 6.445p.

[AndyT]

On 1/18/2018 at 20:25, ProDave said:

I have stated my view several times, that I think it is worth it as a cheap DIY install and not claiming the FIT. I am no longer convinced it is worth paying the premium for an MCS registered company to install them in order to claim the FIT.

 

My model relies on you being able to use near 100% of what you generate, be that gig appliances in the daytime, excess dumped to water heating or even battery storage.

 

Agreed

[AndyT]

On 1/18/2018 at 20:32, Jude1234 said:

We are not DIY builders so would need to hire someone to install anyway.  have considered something like a SunAmp to use excess electrictiy during the daytime but may not need it with the ASHP

 

:-)

[AndyT]

Just wanted to add my agreement to self-consumption and storage. 

For readers, I recently came across a diversion controller called the  "eddi" £365 inc vat from myenergi who have a feature described on page 56 and 57 of their manual (attached) as Priority control for a heat pump which states;

 

In this application the water/PCM is heated by eddi with the immersion heater when there is a relativity small amount of surplus power (e.g. < 1kW).

But when there is enough surplus the heat pump is used to heat the water/PCM instead.

 

This configuration takes advantage of the higher efficiency of the heat pump compared with the immersion heater, whilst ensuring no surplus power is wasted.

For this application the heating element (4) is wired to the eddi Heater 1 output. The water heating timer control (10) for the heat pump (8) is connected to the Relay 1 normally open contacts of the Rely & Sensor Board (6).

 

Most heat pumps require control of the immersion heater in the hot water cylinder, this is to periodically heat the water to a higher temperature in order to kill any Legionella bacteria that may have built up in the stored water. This is not required with PCM as water is only heated on demand.

 

Remember it is not permissible to 'back-feed' the eddi device; a voltage supply must not be connected to the heater outputs. Because of this, the heat pump immersion heater relay (9) needs to be connected to the eSense input of the Relay & Sensor Board (6) instead of the immersion heater element (4). In this application, the eddi is configured to respond to the eSense signal by fully switching on Heater 1 output regardless of available surplus. Installation of the temperature probe (7) is optional; the heaters' mechanical thermostat can be used instead if temperature reading is not required.

eddi_manual_v1.2.pdf

[Jude1234]

@AndyT I really wish that I understood your post. Perhaps you could translate it for those that aren't as familiar with heating systems 

[SteamyTea]

I think that @AndyT is saying that you use the immersion heater when the PV is only producing a small amount of excess energy, when the PV produces more it disconnects the immersion and helps run the heat pump.

But you can only use either the immersion or the heat pump, not both.  This is because there is a secondary electrical element in the heat pump and it may overload the eddy box of tricks, or kill the CoP as the stored energy is at a too higher temperature to get a decent CoP from the heat pump, which would cause the heat pumps internal immersion to kick in.

 

Ideally you never want to use the heat pumps internal immersion heater.  If you are using a thermal store then the water that you use (what comes out the tap) and the stored energy (which may be a brick, PCM or a fluid) can be separated with a heat exchanger.  This gets rid of the Legionella problem (not that it is a real problem anyway, but that is another debate).

[ProDave]

That's a very good point made by @AndyT

 

My heat pump wants control of the immersion heater for the ant legionella cycle.  I am planning my own home made immersion dump controller, and it will include switching of a changeover relay so the dump controller takes control of the immersion and disconnects it from the heat pump for that time.

 

That eddi device looks interesting but at that price I will stick with my home made one, which will be a derivative of the one I used at my last house using an arduino as the controller.

[willbish]

Hi @Jude1234 I am also considering the  economics of PV.

 

As others have said, particularly @ProDave if you can spread your PV panels onto different elevations you have a better chance of using all of the generated power. With your I-shaped build you might be fortunate enough to have East South and West facing elevations.

 

I am currently looking at installing 2kWp on my east and west elevation and at a later date installing another South facing array on an outbuilding.

[ragg987]

All good points, an additional factor is that roof integrated panels reduce the surface area to be tiled so there is a small saving here too

 

In our case, 4kWp south facing with ASHP and immersun to divert to DHW. 2017 generation was 4MWh, measured export 128kWh. So we are consuming 97% of what we generate. I estimate our return from the PV is around £600 per year, less than half of which is from FiT and Generation subsidies. Based on our electricity costing 12p per unit.

[Jude1234]

4 hours ago, ragg987 said:

All good points, an additional factor is that roof integrated panels reduce the surface area to be tiled so there is a small saving here too

 

In our case, 4kWp south facing with ASHP and immersun to divert to DHW. 2017 generation was 4MWh, measured export 128kWh. So we are consuming 97% of what we generate. I estimate our return from the PV is around £600 per year, less than half of which is from FiT and Generation subsidies. Based on our electricity costing 12p per unit.

Wow that is a decent return.  If we had them would plan to have them integrated for aesthetics as well as the saving on roof tiles.  How much approximately is a 4kwp PV panel and the immersun?

 

How do you use 97% of what is generated?    Is somebody at home during the day to use the electricity?

[Jude1234]

16 hours ago, SteamyTea said:

I think that @AndyT is saying that you use the immersion heater when the PV is only producing a small amount of excess energy, when the PV produces more it disconnects the immersion and helps run the heat pump.

But you can only use either the immersion or the heat pump, not both.  This is because there is a secondary electrical element in the heat pump and it may overload the eddy box of tricks, or kill the CoP as the stored energy is at a too higher temperature to get a decent CoP from the heat pump, which would cause the heat pumps internal immersion to kick in.

 

Ideally you never want to use the heat pumps internal immersion heater.  If you are using a thermal store then the water that you use (what comes out the tap) and the stored energy (which may be a brick, PCM or a fluid) can be separated with a heat exchanger.  This gets rid of the Legionella problem (not that it is a real problem anyway, but that is another debate).

Thanks for the explanation, it makes sense now!  I am on a very steep learning curve and just want to make the correct decision

[SteamyTea]

I might be wrong as I do not know the finer details, so that was just generic advice really.

The main thing to always remember are the laws of thermodynamics, no one ha managed to break them yet.

[willbish]

39 minutes ago, Jude1234 said:

How much approximately is a 4kwp PV

 

Around £1900 at the moment

[PeterW]

That’s just for the panels. On top of that you have the inverter and all the fixings so you can probably add £1200 to that as a minimum. 

 

80p/kwp is the cheapest I’ve seen so far 

[SteamyTea]

21 minutes ago, PeterW said:

kwp

It is kW_p really

[Jude1234]

Another newbie question what are the differences between immerson and Sunamp? Pros and cons?  By the sound of it to my basic understanding they both use excess solar PV to supplement DHW heating.

[Stones]

Also have to think about an inverter replacement at the 10 year mark as well of the cost of getting someone to fit them (if you are not that way inclined or qualified on the final electrical side). 

 

As well as orientation, where you live also plays a big part on the output you will get.  

 

http://re.jrc.ec.europa.eu/pvgis/

 

is a great resource to work out what you would generate from any given size of installation.  

 

In our case, expected output for a 3.68kwp system was circa 3200kWh/yr, quite a bit lower than where we previously lived in Perthshire, which in turn was a lower figure than could be achieved elsewhere.  That lower output increased the payback period for us (including the saving made by not having to import a certain % of our electricity) to the point where there was no real benefit in us fitting panels.  I hope to revisit the subject int he future, but that really depends on the price of battery storage coming down.