How do I estimate the leakage of single-pane windows?

Hello,

My parents’ gas bill went up tenfold for some reason, and while the two boilers almost certainly acted up (we think they kept starting and stopping), they live in a Tudor Revival with nasty single-pane windows and now we have an excuse to rip them out. Given the temperature difference in Centigrade and total area of the windows in square meters, can I do any back-of-the-envelope calculations?

With values from the website 8. Heat Transfer through Windows - Mypdh.engineer (found with DuckDuckGo; sqm = square metres)

The heat transmission of a 3mm-thick glass pane is about 300 W/sqm.°C. This goes down to about 3 W/sqm.°C for basic double-glazed glass (and even further if at least one pane has anti-radiative coating).

Thus, single pane glass has a heat resistivity of about 0.003 sqm.°C/W and double-glazed about 0.333 sqm.°C/W. Together with an estimate of the ambient air resistivity of 0.15 sqm.°C/W, this is a total system resistivity of 0.153 sqm.°C/W with single pane and 0.483 sqm.°C/W with double-glazed.

Finally, taking reciprocals again, this is a system transmission of about 6.6 W/sqm.°C for single pane glass and about 2.1 W/sqm.°C for double glazed.

At 10 sqm of windows and a 15°C temperature difference on average, over 24 hours, this is an energy bill of about 16.8 kWh per day for single pane glass – and only a third, down to 5.6 kWh per day, with double glazed.