"Deemed" heat generation

[pdf27]

Question for those who know about these things. I'm thinking about fitting solar thermal, but the RHI payments are pretty much going to define whether or not I can afford to do it for quite some years. There are loads of examples out there saying that they are typically deemed to supply 60% of the hot water supply, worth x amount of money per year for a particular example - but I can't find any way of working out what it will be for me. Hence the following queries:

1) Is 60% of the hot water supply a typical figure? Will this be affected by panel orientation/angle, or is it just a blanket figure for all solar thermal installs?
2) What determines your deemed demand, the size of house/hot water tank or the number of people living there? At the moment there are just two of us, but as we're likely to have kids in a few years our actual hot water demand will go up. What if we built a loft conversion for more bedrooms?
3) Since the payback is mostly determined by the subsidy, which is IIRC for 20 years (rather than until a certain date), the cost advantage of getting the panels installed in time for the summer isn't all that high. Do installation costs vary from month to month (I'm guessing most people do want to get them in for the summer), and if so by what sort of percentage?

[Ted]

As the RHI hasn't been finalised yet then this is all based on guesswork.

1) 60% is a reasonable average, given that you do not want to oversize the panels in order to have them contribute greatly to winter heat demand.
2) Not yet clear, but the RHI consultation used a SAP calculation as an example of how the deemed heat might be calculated. This would be based not on your house as it is but on the heat demand it might have with cavity wall insulation and a minimum of 125mm of loft insulation. Annex 2 of the consultation doc contains some example figures for different house types.
3) Price is likely to reflect seasonal demand.

Note if you have a system installed before the RHI actually comes into operation (expected June 2011) then you need to meet the transitional requirements, as contained in the consultation document - so the first thing you need to do is obtain quotes from two MCS installers, who will also tell you what they calculate your deemed heat demand to be and what % the solar would be able to meet. The lower of the two figures would be the one that is used.

Here is the full example calculation given in the doc:

Example of calculating RHI entitlement

A household’s useful energy demand for heat averages 15,000 kWh per year. The
property is a three bed semi-detached house with cavity wall type construction.
A switch from current gas use to a combination of biomass and solar thermal is
being considered. Under the proposed deeming approach based on an on-site
assessment by an accredited installer, the process for determining the level of RHI
compensation could be as follows.

The installer determines that a reasonable space heating requirement for this
property is 10,000 kWh, taking into account some straightforward energy efficiency
measures the household could implement, for instance installing loft and cavity wall
insulation. Hot water will require approximately 3,700 kWh/year. The total deemed
heat load in this situation would therefore be 13,700 kWh/ year.

He might conclude that solar thermal panels would provide 60% of the hot water
requirement (2,200 kWh), with the biomass boiler providing the rest (1,500 kWh) as
well as the space heating requirement (10,000 kWh).

In this case the RHI entitlement would be:
2,200 kWh x 18p = about £400 per year for 20 years
11,500 kWh x 9p = £1,035 per year for 15 years

Total RHI payments would be over £1,400 per year for the first 15 years (and around
£400 per year for the following 5 years). This amount would be paid as a fixed
(deemed) annual amount regardless of actual energy use (subject to the terms set
out by the RHI such as continuing to use the equipment).

[pdf27]

That's really useful, thanks.

[fittoburst]

Doesn't this do the job of giving you your expected kWh/annum and would be suitable for 'deemed heat'?

http://valentin.de/calculation/sap/

/Fitto