ok lets use some maths
EDIT(after writing the below maths): seems the mining community have had a discount on mining costs compared to last time i calculated it in the summer.. so read on and enjoy
numbers we know..
hashrate: 250petahash(avg for month)https://blockchain.info/charts/hash-rate
1terrahash miner: 1kwatt PSU
transactions: 486 a block(avg of month)https://blockchain.info/charts/n-transactions-per-block
so lets do this:
486tx x 6 =2919tx per hour
250,000kwatt per hour
250,000/2919=85kwatt/hour
lets say it is 20c per kwh
=$17
which does roughly correspond to:
https://blockchain.info/charts/cost-per-transaction?timespan=180day ($18-$20 recently once you add on the physical cost of a rig)
now lets average cost per bitcoin
(17 x 486)/25reward= $332 per btc(my maths)
(20x 486)/25reward=$388 bc.info's high estimate for today
seems cheap to mine right now...as you can see by the link above the cost estimates were alot higher during summer
but atleast it proves one thing that i have been harping on about for the last year. that no one wants to sell below cost of mining and that this price point should be used as a resistance point.
now how to do the maths real easily.. in the future so you have our own indicaters
take the cost/tx
https://blockchain.info/charts/cost-per-transaction?timespan=30days for today (far right point of graph is ALWAYS the most recent day)
take the transactions per block
https://blockchain.info/charts/n-transactions-per-block?timespan=30days for today (far right point of graph is ALWAYS the most recent day)
multiply them together. then devide that by the 25btc reward per block
again today 20cost x 486tx=$9720
$9720/25=$388/btc
now you know the resistance point miners wont want to sell below as its costing them money. ill leave it to you lot to find ways of getting raw data of the numbers in API format so that you can make your 'instant' calculators of btc cost price (resistance point)
Possible miscalculation
Some research will help before deriving 0.2 kwh. Work out again the maths and you can find out how low it can go.
EDIT: Rephrase the statement