I remember reading at least one case in the US where a home grow operation was discovered by the amount of power it was using, too -- the electric company noticed.
As part of my law degree I spend some time in courtrooms, just watching cases being heard, many of them about small time growers. Many people caught growing at home are punished twice: first through criminal law, and then through civil claims like from energy companies and banks or landlords. Very often these people have no other option then to start growing again, pushing them into a vicious circle. (They made the initial choice themselves, not spinning a sob story here - just saying that the stories behind these cases are quite interesting from a human interest point of view).
Why would the energy company sue you if you paid your electricity bill? Same for landlords or banks, if you paid your rent, who cares?
Essentially yes if you pay your energy bill, there's no problem, but almost always the electricity is taken illegally (by bypassing the meter, or otherwise tampering with it) because otherwise the energy provider will notice straight away that your usage is abnormally high (this is the relation to the GP that prompted me to write this). Also, for a proper growup you need (much) more power than a residential supply line provides. The energy supplier then starts a civil procedure to get their money back, sometimes going after the landlord if that landlord could/should have known something was up (some case law includes that neighbors complained about abnormal heat in the house; the landlord should have followed up and is therefore liable for damages, as well as criminally (!) liable for aiding and abetting.
Land lords: they will cancel rental agreements the moment they find out you're growing weed, whether you pay rent or not. Banks will cancel mortgage agreements when a plantation is found. Furthermore, mayors can close a house for (IIRC) 6 months; meaning nobody can live there, but if the land lord doesn't cancel your rental agreement, you're still liable for the rent. Plus you have to live somewhere, because generally you won't be in jail that long. And if you have a family or live in your parent's house - let's just say that there are some heartbreaking cases arising from this.
(also, I should have specified probably, this is in the Netherlands. People think that NL is an anything goes area when it comes to marijuana, but it's really not - if you're caught with a sizeable (i.e., not just for personal use) grow op, you're royally fucked, especially if you don't have much money to begin with - which is generally why people do it at all, because it's not like you'll be a millionaire from stuffing your attic or basement with a couple dozen/hundreds of plants).
Heck, I remember cases where they've used thermal imaging from helicopter to identify such buildings.
Presumably this stuff is only checked post-suspicion, and not automated and parallel construction stuff done.