Where I live, it's still hot enough outside to require running the A/C, so until it gets colder outside, any additional heat produced by my rig is an additional cost, because the A/C has to run that much more.
Anyway, the question was more general than that, I'd simply like to know a way to calculate this additional cost using a rule o thumb of some sort.
We used in the past a induction current meter to monitor our airco as it's hooked in with a 3 phase cable to the office. It sucked so much power we decided to junk our old servers and replace them with low power units.
Observe your consumption for a day without computer equipment running and compare it to day with the equipment running at the same weather conditions?
In general I'd just note the meter for the entire setup (including miners, airco and house hold consumption) and if you can't break even, it's not worth it. That is not taking in account the writeoff cost of the mining equiment, purly the power consumption. If you can't break even (including the household consumption), you are making a loss.