https://en.bitcoin.it/wiki/DifficultyThe difficulty is set such that the previous 2016 blocks would have been found at the rate of one every 10 minutes, so we were calculating (D * 2**48 / 0xffff) hashes in 600 seconds. That means the hash rate of the network was
D * 2**48 / 0xffff / 600
over the previous 2016 blocks. Can be further simplified to
D * 2**32 / 600
without much loss of accuracy.
That's what I meant - your numbers are off. Slightly off, but still, they are wrong. Considering that ASICs do cost (and hash) different magnitudes than CPUs back when this Wiki was written, this can amount to more than just a few fractions of a cent over time.
Correct is: (2^48/65536) instead of 2^32
What might be helpful too is to help people come up with their real energy costs: I pay less than 10 EURcents per kWh for the energy itself, but that's only about a third of the actual price I pay (the other thirds are roughly: network fees like maintenance and so on as well as taxes for subsidizing green energy + the 20% flat tax). What might help is to let people enter the following data from their last electricity bill:
"My last electricity bill total kWh used"
"My last electricity bill total amount of currency paid"
From that you can calculate the real costs (including taxes, fees, fixed costs and whatnot) of electricity for a user, which probably should serve as an upper bound (though some companies seem to charge more per kWh if you use more to incentivize saving) but at least as a more realistic projection than some numbers you might find on a web site of an electricity provider.
