payback on proportional heating controller

[yellowvanman]

Would anybody like to share there calculations on the payback of the immersion controllers?

 

[FB.]

Well, if a 3kW PV array generates 2700kWh in a year, and most households manage to use one-third of the power generated (900kWh), then the remaining 1800kWh which would normally go to the grid is worth about 13p per kWh and would be worth £234 in potential bill savings.

In reality, since there will be times when the immersion is fully heated, or times during winter when there isn't surplus power, it is unlikely to make the full theoretical saving.
I'd guess more likely half to two-thirds of the surplus power can be used, making it worth about £117 to £156 per year for a 3kW system, which agrees with the claims of 3-year to 5-year payback if it costs about £400 for the unit and £100 for someone suitable qualified to install it.

 

[BruceB]

It might be worth that if they were going to heat the water by electricity, but if they were going to heat the water by gas or oil then the cost of that alternative should be factored in.

 

[FB.]

It might be worth that if they were going to heat the water by electricity, but if they were going to heat the water by gas or oil then the cost of that alternative should be factored in.

Yes, the savings from hot water gas v electric would be less. After heat losses for burning the gas in the boiler, and for having to use a 100W pump associated with the boiler, the cost of gas is about 60% that of electric. Making the savings more like £70 to £100 per year from a 3kW array.
Still, the £100 per year saving just about makes the five-year payback.

So I think the manufacturer payback claims are reasonable.

 

[Gavin A]

we're using the sap calculators for solar water heating, and assuming the Immersun will generate around the same amount of energy to the hot water system as a 2 panel, 4-5m2 swh system. You need to know how many people are in the house, how big the tank is, floor area, and how many bathrooms they have to be able to do the calcs, as well as what fuel type you're replacing IIRC.

 

[Gavin A]

I think it works out at around £50-60 a year payback for gas, £70-80 for oil, and £90-100 for electric. depending on consumption levels, but I'm assuming that any house that can have a 3-4kWp solar system fitted is going to be a reasonable sized house with several bedrooms, and relatively high hot water consumption.

If people are using electric showers though, then this will seriously reduce the savings, probably to the point where it's not worth it, unless they only use the electric shower as a back up shower etc.

We'll be fitting extra cylinder jackets at the same time as the Immersun, which will save in the region of £30-50 a year as well though for older tanks with just a 25mm insulated layer, or even a single jacket... a lot more if there's no jacket fitted, which is still occasionally the case. So the combined benefits around more like £100-£150 a year.

 

[bommels]

My (homebrew) proportional controller has been running since May. Amongst other things, it logs the energy dumped in the immersion. The readings of somewhere between 2.8 and 4 kWh per day do correspond very well to the observed reduction in consumption meter counts (our DHW is heated electrically over summer)

So far, it has been saving us about 100kWh per month, or about £13,- in electricity.
Of note is that our shower is still a stand-alone electrical unit. Should this be supplied from the immersion tank as well, the savings could be doubled, as our household burns off 3kWh/day on showers, on average.

Guessing 75% boiler efficiency, and a kWh of gas costing 4.2p, 100 kWh of hot water would cost you £5.60 per month.