I assume you understand...
So perhaps then we won't specifically make those assumptions you suggested i.e.
a) That I am heating my house every day for the whole year
b) That I am using an electrical radiant heating system
c) That I have the heating on for 12 hours every day
d) That I am running the heater on peak electricity rates
e) That the price of energy is 20c/kWhr
How about this for more realistic assumptions:
a) That I heat the house for half the year
b) That I am using Natural gas heater or a high-efficiency RCAC
c) That I run the heat on average for 8 hours a day (2 hours in the morning and 6 hours in the evening)
d) That I am using today's energy prices. (Natural gas at 5c/KWhrfor or off-peak electricity at 7c/kWhr).
e) That the price of peak rate electricity is 17c/KWhr
The energy savings for double glazed windows would be $267 per year. However, I will be generous and double that saving to $500.
Now because you actually sell the stuff I will assume that the cost you quoted for 50m2 double glaze retrofit is actually $15000 (labour not included).
Therefore by my reckoning the energy payback period for investing in DG will take 30 years.
Now we could speculate on all sorts of things e.g. the price of energy doubling in 10 years time or even the price of DG halving!! I'd actually to see both happening so that the market forces come more into line with Europe. With plenty of cheap and dirty coal and a relatively mild climate there is little motivation for Joe Average to be energy conscious - he will just crank up his heater to compensate for his inefficient house.
I am being the devil's advocate here but a customer might decide that for an extra $500 yearly energy bill he isn't going to bother to put 15K worth of windows in.
Even if your assumptions were "realistic" which they are not, and which I never claimed mine to be...
If we assume that we need windows anyway, then the cost of double glazing is maybe $5000 above the price of single glazed windows (read my other posts). Most retrofits are to replace windows because they need replacing, if the additional cost is $5,000 then it's a no brainer... Replacement windows for energy costs alone are the toughest to justify as the cost of the labour is probably as much as the windows.
And regarding double glazing in general... it's like wearing a nice coat, nobody dies of hypothermia without a coat even on a cold day here, but most people wear one for comfort...
Ed