Nel Hydrogen of Norway, founded in 1927, state having the world's most efficient and reliable electrolyser: https://nelhydrogen.com/product/atmosphericalkalineelectrolyseraseries Electrolyser DC power consumption: 3.8  4.4 kWh/Nm3 Nm3 = Normal cubic meter  Temperature 0C, Pressure 1 atm Using the ideal gas law: PV = nRT n = (101,325pa x 1m3) / (8.314 x 273.15K) = 44.618 mol Hydrogen has a molar mass of: 2.01588 gmol1 1 Nm3 of H2 is therefore 44.618 mol x 2.01588 gmol1 = 89.945g Hydrogen higher heating value 286 kj/mol converted to kWh = 39.4 kWh/kg HHV 1 Nm3 of H2 has therefore an energy content of 3.544 kWh 3.544 / 3.8 kWh/Nm3 = 93.3% efficient 3.544 / 4.4 kWh/Nm3 = 80.5% efficient Inferring from the following cost graphic, their electrolyser cost at scale (50MW) approaches ~520 €/kW (atmospheric pressure) or ~610 €/kW (15 bar). 
A specialist in PEM and alkaline electrolysers, fuel cells for vehicles & stationary applications, and powertogas. All figures below from: PEM Cell Hydrogen Output: 620 Nm3/h x 3.544 kWh/Nm3 2197 kW / 3000 kW = 73.2% efficient Alkaline HySTAT10010: 100 Nm3/h x 3.544 kWh/Nm3 354.4 kW / 500 kW = 70.9% efficient Presentation quotes efficiencies as high as 89.5% (39.4/44) for PEM electrolyser by 2020 and 82.1% (39.4/48) for alkaline electrolyser by 2025. With central efficiencies of 83.8% (39.4/47) and 78.8% (39.4/50) respectively. (Source: FCHJU, Development of Water Electrolysis in the European Union, February 2014) Electrolyser efficiency increases with higher current density and average cell voltage. Stack efficiency of 3MW HyLYZER 600 is 80% HHV @ 2MW, 78% HHV at 2.5MW and 75% HHV at 3.0MW, see 3MW HyLYZER 600 Brochure Costs (Development of Water Electrolysis in the European Union, February 2014):
Costs (PowertoGas Roadmap for Flanders):
Cell stack replacement costs are expected to fall to less than 300 €/kW by 2030 and less than 200 €/kW by 2050. Alkaline @ 20MW scale in 2025:

The hydrogen production rate (40,000kg) and system power (100MW) on the ITM Power HGAS product page suggests an efficiency of 66% for their PEM electrolyser. However their technology updates suggest higher efficiency at >77%. ITM Impact Report 2015 (Stack efficiency 84%, 2015) 13th June 2016: Thüga Group: PowertoGas technology performance update (over 70% efficiency) Cost projection to 2024 suggests less than 500 €/kW at 100MW scale Interim Results January 2019 