Excuse my use of metric units - I have no idea how much energy a cubic foot of gas has.
In ontario, canada has a newer (late 90s, early 90s) relatively well build 2000 sq house with a natural draft furnace (spark ignition @ 68% afue) may use 2000 m3 of gas per year excluding water heating.
Upgrading to a 95% afue unit should save around 29% or 580 m3 per year.
1 m3 of gas has 36000 BTUs in it; one kwh has around 3400 btu/hr...
Furnace upgrade savings: 6141 kwh per year.
Energy produced by 1kw of pv per year: 1100 kwh (
http://pv.nrcan.gc.ca/pvmapper.php?L...llement&lang=e)
So in one year a high eff furnace can save as much energy as a 1kw pv system can produce in 5.6 years. (I exaggerated in my previous post)
Now, late 80s/early 90s homes aren't energy pigs. In old homes energy efficiency upgrades can cut consumption by 50%+ as you know (getting rid of ancient < 60% afue oversized furnace, air sealing, blowing insulation into framed walls/attic), at a similar cost to a 1.5-2kw pv system.
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As for determining the roi of efficiency upgrades, it's as simple as analyzing utility bills.
Software which can simulate the impact of efficiency upgrades is also available - for example, hot 2000.