"We presently own and operate four primary campus locations (“Primes”), encompassing 12 active multi-tenant data center facilities with an aggregate of up to 14 million gross square feet (GSF) of space. Our existing facilities are equipped to provide up to 490 megawatts (MW) of power, with the potential to scale to more than 1,300 MW upon full build out of our existing footprint."[0]

It sounds like Switch Inc. advertise their datacentres by Wattage. I am not sure how common that is.

[0]https://investors.switch.com/company-profile/default.aspx

Very common.

Do you buy by the watt though?

It also advertises the square footage but you don't buy by the sq ft either

> Do you buy by the watt though?

Usually in larger DC deployments, yes, you are quoted, metered, and billed by total power. The amount of space is (usually) an afterthought. There is more space than you can fill and remain under the power commit, which also correlates to total cooling load too.