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Oxford study proves heat pumps triumph over fossil fuels in the cold
(www.nationalobserver.com)
This is a most excellent place for technology news and articles.
I don't know that we really needed a study for this. You can find the COP vs temp vs capacity curves for every heat pump out there. This will tell you exactly how many BTUs of heat the pump will produce given a watt of electricity input. I guess they were just validating that the curves were accurate?