Miners will generally operate when their revenue is greater than their loss in term of electricity bill(if they're not being parasites). Let calculate the reve

Miners will generally operate when their revenue is greater than their loss in term of electricity bill(if they're not being parasites). Let calculate the revenue a miner would get if they win all the blocks: 50 bitcoin every 10 minutes mean 300 bitcoin, which mean 7200 bitcoin a day. The price of a bitcoin at the time I am writing this is 6.54 USD, so the total revenue is 47,088 USD per day. That mean, assuming a miner can win all blocks at 50 bitcoin forever assuming the price and difficulty stay the same, he can afford to pay an electric bill of 47,088 per day assuming that the price hold stable. This equals to 17,187,120 USD in revenue every year. The market cap of bitcoin is only 60,951,475 USD, which mean miner revenue equals 28% of the market cap. However, most economic activities are certainly not miners, but actually trading bots and speculators across exchanges. In the MtGox USD alone, about 8.7 million dollars worth of bitcoin were exchanged in 30 days. You can also add in the various economic activities other than trading currencies, such as virtual goods, hosting, ice creams, t-shirts, porn, illegal drugs, etc. You can conclude that the miners are guaranteeing more than just revenues for themselves, as the various activities in the bitcoin economy which actually probably dwarf what the miners are making in term of revenues. As far as being actually efficient method of security for a currency? Probably not right now, assuming that security measures actually scale economically. It could be that the actual electricity cost grows slower as more and more efficient method of hashing are found.