This thread has been interesting. I am currently a Energy Tune-uP inspector from CMC Services.

It is important to know that HERS rating apply to NEW construction only and do not relate to EXISTING construction. The training from a HERS provider is invaluable in understanding building science that contributes to Energy Usage Issues in an existing home.

As a home inspector, you may wish to look into Tune-uP training. To become a HERS rater follow the instructions as described at the RESNET website. Be prepared to invest in training, equipment purchase (blower door and duct tester), software, signing a contract with a HERS provider, insurance.... Again for new construction.

Choose your target market - exsiting homes or new construction - do your business case and go for it.