Here is part of my calc on BTC at time of March 2015, maybe expert people do more research into this, from my perspective need of fully POS coins is given by math.
I updated current Bitcoin electric energy consumption, calculated like here:
http://www.reddit.com/r/Bitcoin/comments/2hlwpy/5_terrawatt_per_hour_bitcoin_network/cktv3xl
BTC global hashrate:
360.000.000 Megahash/Second
Source: https://blockchain.info/charts/hash-rate
Current mining efficiency:
Optimistic: 0.51 Wh per Gigahash/s
Source: https://bitcointalk.org/index.php?topic=902305.0
Consumption of the network per year:
(360 000 000 / 1000) * 0.51 = 183 600 kWh per hour
183 600 x 24 x 365 = 1 608 336 000 kWh p.a. = 1.6 Terrawatthours per year
Please correct some numbers you know better, even if 10% is real, too much carbon is burning in this moment, possible >1,6 million tons of coal a year, by a software with only 3 billions market cap, what else can go wrong?