The water intensity of US power generation averages 21 liters per kWh (5,600 gallons per MWH), but 95% of this total comes from evaporation at hydro reservoirs. Excluding hydro power, good estimates are that nuclear power uses 2.1 liters/kWh of water, coal power uses 2 liters/kWh and CCGTs use 1.2 liters/kWh, or less in some configurations.
On some cuts of the numbers, power generation accounts for 20-60% of all US water use. It depends a bit on how you define ‘use’. But the US electricity sector most likely uses 21 liters per kWh that is generated (or 5,600 gallons per MWH). So is this an objection to expanding the grid, or just watery whataboutism?!
95% of the average water intensity of power generation quoted above – i.e., the 21 liters/kWh – is attributable to hydro-electric plants, which comprise just c5% of the total US power generation, but use 200x more water than thermal generation sources, as 350 l/kWh of water evaporates from large reservoirs upstream of hydro-electric dams.
Excluding hydro power, good estimates are that nuclear power uses 2.1 liters/kWh of water, coal power uses 2 liters/kWh and CCGTs use 1.2 liters/kWh, all mainly for cooling/condensing low pressure steam, downstream of the steam cycle, and thus thermal power plants are c3% of US water use. The numbers vary, as shown in this data-file, which aggregates 5-7 case studies in each category.
Gas-fired power generation can also get down to 0.1-0.2 liters/kWh, either in an OCGT (there is no steam cycle); or in a CCGT with dry-cooling, although this latter configuration comes with a c1-2% energy penalty, and 3-4x higher costs for these cooling systems, potentially adding $50-300/kW to the costs of the fully installed gas turbine.
Solar and wind hardly use any water either. Solar maybe uses 0.1 l/kWh for occasionally washing panels. Wind turbines do not use water, but do use lubricant, typically 80-300 gallons per turbine per year, perhaps as much as 800 gallons on older, smaller, geared turbines; but on average this still means that lubricant use is 0.00001 l/kWh.
Finally, definitions are difficult. Water is evaporating from lakes and rivers all the time, or flowing from rivers into the sea, making it hard to set a baseline. Some sources say thermal power plants account for 40% of all water withdrawals, but 97% of this total is simply used for cooling and then discharged, i.e., it is not used up.