Cooking In SAP

This is a note almost to myself but I thought I’d record it here as it took a little while to find the information earlier today. One of the spreadsheets we have setup at work is designed to help with analysing the energy usage, and hence carbon dioxide emissions, of a new development (specifically of a residential housing scheme). An accepted way of estimating such things is to perform a series of SAP calculations for each unit type (or at least a representative number of them). That produces an energy usage for such things as hot water production, heating, ventilation, etc. What is often required in addition, mainly due to planning guidance, is an estimate of the “unregulated” emissions (or energy use). This includes the energy used by appliances (fridges, washing machines and the like) and, subject of this post, cooking.

In the past we have used BREDEM-12 (on which SAP was originally based—sorry, can’t find a link to a downloadable without login copy) calculations to estimate these energy demands. This has a few disadvantage though. First BREDEM-12 gives a usage for appliances and lighting, lumped in together with no way to separate them. As SAP also calculates a usage for lighting it is important not to “double count” the usage. There are two obvious approaches to this: One could simply discard the SAP calculation and take the BREDEM figures; or, the approach we tend to, one can subtract the SAP calculated lighting usage from the BREDEM-12 produced figures (neither of these is particularly compelling in terms of being a convincing measure if I’m honest). Secondly, BREDEM-12 is quite dated by now, and likely to get further out of step, particularly as SAP now has a method for calculating appliance and cooking usage (at least for Code 6 housing, in Appendix L). Which is where I finally get to the point. Instead of using BREDEM-12 I thought I’d update the spreadsheet to give the option of performing a calculation as per the SAP Appendix L.

This works quite well and simply for appliances, and means we no longer have to worry about double counting lighting (note to self: check how close the SAP appliance plus lighting figure is to BREDEM). Cooking usage isn’t so straight forward though. SAP helpfully gives an equation for calculating the emissions, 119+24N (kg CO2/annum), N being the calculated occupancy. That’s fair enough but at least some London Boroughs talk in terms of energy use (not emissions) and it’s often interesting (or useful) to look at a breakdown of energy/emissions by fuel type (basically gas or electricity in this case).

The question arises then, what is the split of fuel usage and, working from that, what is the energy use for cooking. It took me a while to find the answer (it took longer because the office internet was down mind!) The answer seems to be contained in the SAP Technical Document STP09/AUX01 [PDF]. Reading from that it turns out that SAP is assuming a 50% split between electrical and gas cooking, with the energy use for each being 275+55.0N and 481+96.3N (kWh/annum) respectively. This makes sense; if one multiplies the variables by the current emission factors (0.517 & 0.198 kg CO2/kWh and averages then the original SAP equation is reached.

So there we have it. Energy use for cooking, according to SAP (sort of), and of course hence emissions.

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