Is there a cost benefit to homeowners with solar panels using storage heaters?

[albertross]

Before anyone tells me that Economy 7 only operates during night-time off-peak hours I am asking if there is a cost benefit to using any electricity generated from solar panels which currently is unused by the homeowner during the daytime to heat up storage heaters for night time use.

My somewhat limited understanding of solar generated electricity is that the homeowner, under current arrangements, gets paid for all of the electricity his system generates and gets free electricity (ignoring the cost of buying the solar panels) from his system for as long as it is being produced. I also have read that the average use by the homeowner of the total electricity generated by his system is around 40%-60%. So, I am thinking that if the remaining electricity is free and the homeowner gets remuneration for producing it regardless of who actually uses it then is there a saving to be made on central heating costs during a night time by using storage heater(s) heated on free electricity generated during the day.

I realise that there is the capital cost of the storage heater(s) but I don't know how much energy is required to heat one up and whether or not a reasonable sized solar panel system would provide sufficient energy from that currently unused (by the homeowner) ,i.e. 40%-60% of what is generated. I also don't know if it would be possible to switch off the heater(s) automatically if the energy generated is insufficient and the consequence is that electricity is drawn from the grid at premium rates (i.e. daytime rates).

 

[FB.]

Storage heaters vary from single-element "bathroom-sized" around 0.9kW consumption per hour for about six hours, to triple- or even quadruple-element "large lounge-sized" around 3-4kW consumption for the same six-hours.
In theory, there's no reason why not to charge one from solar PV, other than solar PV output is very variable from day-to-day and even from hour-to-hour.
This erratic production from solar PV would mean there will be times when the panels are producing very little, and the storage heater would need to draw some or all its power from the grid.

If you're on an economy-7 tariff, you'll be paying twice as much for electricity between 6am and midnight (give or take an hour) as you will during the small hours of the morning. A friend of mine is with Southern Electric and pays about 7.5p per kWh during the small hours while in bed, and about 15p per kWh during waking hours.

So you'd need to be confident of of managing to at least meet half of the storage heater's demands from solar production. Therefore a lower powered heater would be more likely to not eat into too much higher-rate mains electricity.

Note that just because a solar array is 3kW capacity, it will spend most of its time operating well below that potential, affected by the position of the sun in the sky, and by any cloud cover, or the time of year, or even air temperature and cooling breezes.
My array, for example, is 3.75kW capacity, and generated a total of 425kWh during the whole of August, which averages about 13.7kWh per day, which averages about 1kWh of generation per hour of daylight. But even that doesn't tell the full story of the fluctuation at different times of day


The following pictures of my inverter's screen are an excellent indication of how the real world of solar PV generation is.
Along the left axis are just-visible notches marking each kW of output. the maximum output is 3.6-3.7kW, with each notch being 1kW.

First image: a day of occasional sunny spells, with more sun in the afternoon than the morning, peaking around 2kW production shortly after lunchtime.

Second image: a day of mostly sunshine, with a few cloudy spells. Several hours close to full 3.6kW capacity, but with several brief drops in output to 0.5-1kW as dark clouds passed over the sun.

Third image: a day of mostly sunshine, holding near peak 3.6kW output for several hours, but with a couple of hours of cloud at lunchtime which dropped output to 0.75kW.

Notice that my array faces SouthEast, so its peak generation is skewed towards mornings unless the sun is only out in the afternoon.

.

 

[yellowvanman]

Using something like the immersun could be used to divert excess energy to heaters. But because of the variability just described I wouldn't go out and buy some storage heaters to provide primary heating. If storage heaters are already in place then the immersun can 'top up' the heaters during the day and provide more heat for the evening and maybe some latent heat within the heater by the time Economy 7 switches on means that less is taken by the heater(s) during the night.

 

[provr6]

My understanding is that storage heaters are wired on a seperate circuit to an Economy seven meter. That means the storage heater will only come on and charge up when the economy seven hours are available (say midnight to 7am).

No power is available outside these hours as the circuit is off which prevents you from using daytime electricity at a higher tarriff.

If you were to use excess solar power to top up during the afternoon, how would you get that to the heater as it's not connected to the main meter?

 

[yellowvanman]

If you were to use excess solar power to top up during the afternoon, how would you get that to the heater as it's not connected to the main meter?

There is a changeover switch within the immersun. If you connect the immersion to the common, normally closed connected to the LD terminal of the immersun and bring a supply from the off peak CU to the normally open. During the day the normally closed is used and will feed the heater with the excess, and if you program the timer in the immersun then you can switch the relay over to the normally open contact when the econmony 7 kicks in!

Forgot to mention, if you have RCDs and you take 2 different neutrals to the immersun you could get RCD tripping!

 

[JulianC]

If you were to use excess solar power to top up during the afternoon, how would you get that to the heater as it's not connected to the main meter?

By using a relay, or my old favourite: a cunning series of levers and pulleys. I guess for this application a relay may be the better option...

 

[SolarCity]

It's a shame, 'cos I'd love see to the levers and pulleys option ;-)

 

[provr6]

Can the immersun divert the electricity to more than one appliance. i.e can you connect it to your water cyclinder/element and a storage heater and switch where you want the excess power to go?

 

[yellowvanman]

Can the immersun divert the electricity to more than one appliance. i.e can you connect it to your water cyclinder/element and a storage heater and switch where you want the excess power to go?

Yes, but the relay is only rated to 16A so you need to be careful what is turned on. If more than one device turned on then naturally they will all take longer to heat up, as compared to being on in isolation.