Hydrogen

• Enthalpy of combustion - 286kJ/mol - [1].
• Heat of combustion (per weight) - Compare to gasoline or diesel - enthalpy of combustion of hydrogen is about 3x as large - [2]
• Hydrogen has about 1/8 and 1/3 the volumetric density, resepctively, compared to propane and natural gas.
• Let's look at basic calculations.
• Compare to energy fuel efficiency of gasoline engin - 200 g/kWh, best marine diesel - 150 g/kWhr - [3]

Pressurized storage of hydrogen: molar mass of hydrogen molecules is 2 g/mol. 1 mole is about 5 gallons. Pressurized to 100 atm (under 1500 PSI) - readily doable with high pressure electrolyzer - you get 200 grams per 5 gallon container. Max practical for volumetric reasons would be say up to 100 gallons of 20 cu ft volume in cylindrical tanks- say 2 meters by 1 meter wide (base of a single person car). Thus, 100 gallons volume at 100 atm gets you 4 kg of hydrogen.