It would only need to be 4.7 to beat a gas/oil system that is 100% efficient. In reality, a very good gas based system that is well specified and installed mig

It would only need to be 4.7 to beat a gas/oil system that is 100% efficient. In reality, a very good gas based system that is well specified and installed might reach 80% as a whole. Most UK gas/oil heating systems are significantly lower.