How much power for mining single Bitcoin or bitcoin cash?
Here's the quick math I did:

According to Blockchain.info, the total hash power is about 20.1 million TH/s.
If this is all done by the most efficient AntMiner S9 hardware, at 13.5 TH/s, then there are around 1.5 million devices.

Each device draws around 30KWh each day, and they are probably paying around 8 cents KWh, for a daily cost of about $2.40 USD each.

So:

$2.40 per day * 1.5 million devices = $3.7 million daily electricity cost


How much is being earned?

There are around 144 blocks mined every day, with a value of 12.5 BTC, which currently goes for about $9,000.

So the total income is about $16.2 million every day.

How low could prices fall before miners can't even pay their electrical bill?

144 blocks * 12.5 BTC * $X cost == $3.7 million electricity cost
solve for X:
X = $2,055


So, when the price of a BTC gets close to $2,000, miners will no longer be able to even pay their electrical bills.  Then there is hardware, staff, maintenance, overhead, profit, etc.

Side issues:

I assumed the most efficient S9 hardware, but some devices are more wasteful.  It is possible that more efficient miners will be coming out too.

I used some of the lowest US costs for electricity.  Its likely that costs in other countries are less.  I also assumed perfectly efficient power supplies, but checking on some online posts, it seems real-world power consumption may be 30% higher than what I estimated.