Nice savings! Tho no ingenuity is needed to calculate them as there is actually an established process used in the energy saving business. It uses "degree days" that are derived from temperature measurements, rather than using temperature measurements directly.
Degree days work better in regression as, assuming the correct base temperature is chosen/calculated, heating degree days are directly proportional to heating energy consumption (including being zero when it is warm enough that no heating is needed) and cooling degree days are directly proportional to cooling energy consumption.
Degree days work better in regression as, assuming the correct base temperature is chosen/calculated, heating degree days are directly proportional to heating energy consumption (including being zero when it is warm enough that no heating is needed) and cooling degree days are directly proportional to cooling energy consumption.
More info and data here: https://www.degreedays.net
including an article explaining the process typically used for before/after calculations:
https://www.degreedays.net/calculate-energy-savings
Disclaimer: I work for the company behind that site, and yeah I know it looks a bit dated :D