The Sogeti problem

Heating energy consumption variation

The client has an apartment house with high variation in energy consumption for heating during the year.

The client is interested in why and which factors that can inflect on the heating energy consumption?
The client is also interested if he could forecast heating energy consumption?
The client would also appreciate if you could calculate the coefficient of determination or any other similar value for the forecast?

The client has provided us with the heating energy consumption for the last 3 years and also how many apartments that exist in the house and how many persons that lived/live in the house, all this information is supplied per month.