What does the y-offset of the best fit to the performance curve mean?

The y-offset of the best fit to the performance curve is an indication of the energy used in the building that is not a function of external temperature. It is usually water heating and any other gas appliances such as cooking that have a low or zero external temperature dependence. It is measured in kWh/m2/week.

The mean value of y-offset is 2.1

50% of buildings have a yoffset value less than 1.6, 30% less than 0.7 and 10% less than 0.2

y-offset > 1.6 worse than average tend to use 37% more than average
y-offset < 1.6 better than average tend to use 37% less than average
y-offset < 0.7 much better than average tend to use 48% less than average
y-offset < 0.2 excellent tend to use 53% less than average