How much power for mining single Bitcoin or bitcoin cash? Is there a fixed amount, or maybe just the average, power used to mine one single bitcoin coin or single bitcoin cash coin? 

How low can the price fall until miners start red numbers?
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.