PHPP is just a big spreadsheet and is fairly easy to understand in a general kind of way. But IMHO it's quite complicated to understand and use it to produce a detailed model. So I paid an architect to use it on my behalf, and a certifier to audit the results. I restrict myself to tweaking various bits to see what happens. It is interesting to have a PHPP model of my house and it does seem to be pretty accurate. Some other people on here have used PHPP themselves, so I suspect opinions vary.

The UK is developing a model called the Home Energy Model (HEM), which will run alongside SAP starting some time next year and eventually replace it. It's another thing that may be worth following.