There are three key differences between NHER and SAP:
1) NHER uses degree day data to give regional variation in energy demand, fuel costs etc. - SAP is location independent;
2) NHER includes all energy usage, so it is heating, hot water, cooking, lights & appliances, so giving total fuel costs;
3) NHER permits (at Level 3) matching to fuel bill data to give a good sense of how households are actually living - making it useful as a research tool for fuel poverty.
However at the moment NES (National Energy Services), operators of the NHER scheme, are concentrating on developing the next version of SAP, and the updating of NHER will only take place when that has been completed.
Hope this is useful.