how accurate are SAP Calcs?

[Mike_scotland]

Hi Group,

Just wondering how accurate SAP calcs are?  basically the SAP is saying i will require 8500kwh per year for my Heating and my Hot water..... thats not including TV,dishwasher etc i dont think so im wondering how accurate they are?

we are building a 230m2 house so thats what the SAP is based on i suppose along with the insulation and all that. 

i also noticed they have made the ASHP (Which we are using to heat our home) at 1.7 efficiency (Which i think might be actually a little low im hoping for atleast x 2 efficiency but maybe more on beter days)

with the rising costs in elec im a little worried if the SAP is accurate or not? were going to be paying about 22p per KWH but its looking like the CAP will rise again so who knows the the costs will be.

Thanks in advance - any additional info anyone needs just let me know  

[Mr Punter]

I think they are fairly good but you need to make sure the correct values are input, such as the actual ASHP efficiency, air leakage, cold bridges and u values.

[Mike_scotland]

  On 10/01/2022 at 11:55, Mr Punter said:

I think they are fairly good but you need to make sure the correct values are input, such as the actual ASHP efficiency, air leakage, cold bridges and u values.

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Ok Mr Punter Thanks,

 

I am pretty sure the ASHP will be more efficient than 1.7 as the SAP assessor has put in..... what happens once the house has been completed do they re evaluate the SAP or does it become a EPC? 

The assessor also has no heating June July August and Sept - not sure how accurate that would be in September but maybe... 
 

[Mr Punter]

  On 10/01/2022 at 11:58, Mike_scotland said:

what happens once the house has been completed do they re evaluate the SAP or does it become a EPC? 

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Yes they do an AS BUILT SAP and this will allow them to make any changes you tell them about and input the airtightness test results before producing the EPC.  Make sure they amend any errors or oversights.

[Mike_scotland]

  On 10/01/2022 at 12:03, Mr Punter said:

 

Yes they do an AS BUILT SAP and this will allow them to make any changes you tell them about and input the airtightness test results before producing the EPC.  Make sure they amend any errors or oversights.

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and how do we find out the efficiency of the ASHP? manufacturers data sheet or something?

[jamieled]

Remember that SAP is short for Standard Assessment Procedure. It may be relatively good for some people, but the process needs to make a lot of assumptions about how you use the house - do you heat it permanently, to what temperature, your domestic hot water use pattern etc. Particularly in the case of having primary and secondary heat sources SAP makes an assumption about the proportional use which is unlikely to reflect reality.

 

So I'd say if you want to understand how accurate it is, you need to 1) understand the assumptions its making about the big energy uses and 2) ensure it has the most representative information on your construction and DHW/heating.

[Mike_scotland]

  On 10/01/2022 at 12:11, jamieled said:

Remember that SAP is short for Standard Assessment Procedure. It may be relatively good for some people, but the process needs to make a lot of assumptions about how you use the house - do you heat it permanently, to what temperature, your domestic hot water use pattern etc. Particularly in the case of having primary and secondary heat sources SAP makes an assumption about the proportional use which is unlikely to reflect reality.

 

So I'd say if you want to understand how accurate it is, you need to 1) understand the assumptions its making about the big energy uses and 2) ensure it has the most representative information on your construction and DHW/heating.

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makes sense but i will be unsure what the how water will be until were in i suppose, im just hoping its relatively accurate for the KWH usage, but i have no idea what the monthly KWH would be for the ovens, hob,TVS etc which i will all need to add into the mix.

[ProDave]

Even my as built SAP over estimated my heating energy by about 3 times.  It had all the correct information about the building structure, windows, heating system, actual air test etc.  I found Jeremy's simple heat loss spread sheet a far more accurate heat loss calculator than the SAP.  I often ponder how much better the SAP rating would be if you could somehow input real life measured heat input rather than the theoretical figure it calculated.

[Ferdinand]

8500kWh for a house that size sounds OK. But does that price change calculations around eg solar pv or battery storage?

 

IME SAP is a decent guide to a 'normal' house, and as your house becomes more extreme in any way it becomes less reliable. If the SAP numbers were up in the 80s or 90s I would treat it as at best an approximation.

 

I had a free trial copy of the SAP software from Stroma and did my own calculations to understand the workings. This may still be possible.

I would suggest building a heat model of your house using the spreadsheet available here, as a supplement to SAP Do you know about this? 

HTH.

 

Ferdinand

[Mr Punter]

  On 10/01/2022 at 12:43, ProDave said:

Even my as built SAP over estimated my heating energy by about 3 times.  It had all the correct information about the building structure, windows, heating system, actual air test etc.  I found Jeremy's simple heat loss spread sheet a far more accurate heat loss calculator than the SAP.  I often ponder how much better the SAP rating would be if you could somehow input real life measured heat input rather than the theoretical figure it calculated.

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That is very odd.  Although the calculation is a bit black box, the input data is available.  The only way your heating load could be 3 times higher would be for the heat losses to be 3 times higher.  Did they supply the input data?

[A_L]

  On 10/01/2022 at 12:26, Mike_scotland said:

but i have no idea what the monthly KWH would be for the ovens, hob,TVS etc which i will all need to add into the mix.

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The energy use of cookers and appliances is not included in SAP

 

Here is a decent overview, https://ggbec.co.uk/sap-an-overview/

[A_L]

  On 10/01/2022 at 12:04, Mike_scotland said:

and how do we find out the efficiency of the ASHP? manufacturers data sheet or something?

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The SAP software uses data from the Products Characteristics Database (PCDB)

 

https://www.ncm-pcdb.org.uk/sap/searchpod.jsp?id=17

[Mike_scotland]

  On 10/01/2022 at 12:49, Ferdinand said:

8500kWh for a house that size sounds OK. But does that price change calculations around eg solar pv or battery storage?

 

IME SAP is a decent guide to a 'normal' house, and as your house becomes more extreme in any way it becomes less reliable. If the SAP numbers were up in the 80s or 90s I would treat it as at best an approximation.

 

I had a free trial copy of the SAP software from Stroma and did my own calculations to understand the workings. This may still be possible.

I would suggest building a heat model of your house using the spreadsheet available here, as a supplement to SAP Do you know about this? 

HTH.

 

Ferdinand

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Hi Bud,

im looking at solar! i roughly worked out using 60% of the solar to my house and 40% to the grid at 4p - roughly it would take around 7 years to pay back based on 6k installation fee of a 5kw system producing 4400kwh a year (according to various solar calculator's i have used for my area thats the average) 

im unsure if im being too nice saying i would use 60% or being too harsh i just thought it would be a achievable number i thought.

[Ferdinand]

  On 12/01/2022 at 14:03, Mike_scotland said:

Hi Bud,

im looking at solar! i roughly worked out using 60% of the solar to my house and 40% to the grid at 4p - roughly it would take around 7 years to pay back based on 6k installation fee of a 5kw system producing 4400kwh a year (according to various solar calculator's i have used for my area thats the average) 

im unsure if im being too nice saying i would use 60% or being too harsh i just thought it would be a achievable number i thought.

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You run the numbers, then use your skill and judgement.

 

 

(Except you aren't guessing where the ball is; you are guessing where the judges think the ball is.)