Power consumption will vary based on usage on modern machines and Google could do automatic boot/shutdown of machines to match demand. But the marginal cost in electricity alone should still be pretty small.
According to this link someone else found [1], they claim to use 0.0003 kWh electricity per amortized search, which at e.g. 0.06 USD/kWh (typical [2]) is 1.8*10^-5 USD, or 0.26% of search costs (according to the estimates in the article).
(In a related estimate, 0.0003 kWh/search means 54 CPU-core-seconds of indexing per search, assuming (without any justification and probably wrongly) that (i) most electricity consumption is by CPUs during indexing and (ii) they are using x86 machines with about 20W/core power consumption. I'm sure someone else here can fix/improve this estimate.)
There has been a form of innovation - in some server farms that consist of thousands of interchangeable computers, they simply don't try to cool them, and instead just exchange air with the outdoors (possibly with a bit of filtering). If your software doesn't mind having nodes frequently drop out due to overheating, then it apparently is cheaper to replace melted CPUs than to keep them all cool.
Which is also why more data centers, including Google data centers, are being built in cool climates. If you're exchanging for cold outside air, you get to keep more of your servers from melting down without paying more for cooling.
I dont think it works that way - in centralised cooling spaces, they have to keep their cooling machines running. So, it costs the same whether you are generating that amount of heat or not.
To cut down on cooling, you need to architect your farm to have smaller, independent cooling systems. However, your system must also be architected such that weird things like "only machine is active in my cooling cluster, so I cant shut down the cooling for that area" cannot happen.
I'm not sure if things like that have been solved.
Correct, HVAC systems are designed to handle variable capacity and to stand down as demand lessens in the same way as a data center is. It's such long standing practice that most recent homes even have variable speed compressors on the condensing unit and multi-speed fans on the air handler.
I think pedrocr means that the baseline would be moved down, so that less cooling in total would be required, not that they reduce the amount of cooling used during usage dips.
