Q: How does the energy use per ETH vary as the Blockchain lengthens?
The energy use per ETH should theoretically not differ as the blockchain lengthens. The main factor that will affect energy use per ETH is the difficulty.
This difficulty is adjusted so that blocks on the Ethereum blockchain are produced on average every 15 seconds. If more miners mine the Ethereum blockchain, the difficulty will adjust upward so each miner has less chance of mining a block. More information at How is the Mining Difficulty calculated on Ethereum? .
Q: I'd imagine these values are dependent on hardware, ambient temperature and the characteristics of the blockchain. Have I missed anything?
Regarding ambient temperature, your GPU chips should perform the same number of calculations per unit of electricity consumed. The GPU fan however will have to work harder to move the heat away from the GPU chips if the difference between the ambient temperature and your GPU chip's temperature is smaller.
Q: Does anyone have data on the typical kWh required to produce 1 ETH?
Currently my 126 MH/s will mine a block on average every 3.81 days (from https://etherscan.io/ether-mining-calculator). 5 ETH will take 3.81 days x 24 hours / day x 0.680 kW x 2 = 124.36 kWh.
1 ETH will take 24.87 kWh.
Note that Ethereum hashrates are not directly comparable to Bitcoin hashrates as the hashing algorithms used are different.
I'm running 2 x the rigs listed under My Setup in What's the best Hardware for Mining Ether? . Each of these rigs have 2 x Gigabyte 8GB R9 390X G1 Gaming PCI-E VGA Card GPUs.
Here's the statistics from my monitoring command of my rigs:
00:36:33.202 63.486 MH/s 78C 76C 1150:1150 1150:1150 MHz R solo
00:36:33.613 63.559 MH/s 75C 74C 1150:1150 1150:1150 MHz M solo
The rigs are producing about 63 MH/s each, and the GPU temperatures currently range from 74C to 78C. On warmer days, it ranges between 77C to 84C. My rigs are overclocked to 1150 MHz each.
(Yeah, my rigs run hot as they are enclosed in a case, but it keeps my cat and dog away from the computer parts. I have had to create a script to shut down
ethminer when the temperature touches 92C to avoid burnouts.)
My room temperature is comfortable, on the cool side.
And each rig draws around 680 watts.
(And I'm using my rigs to warm my house as it's getting cool in Sydney now.)