Thats a lot of extra math...and I can say right off the bat that 1 litre is not equal to 2/3rds of a cubic meter, as your 5.5kwh/litre vs 8.8kwh/cubic meter suggests. 1 cubic meter is 1000 litres, so it would be 0.0088 kwh/litre.
Quote:
One GGE of natural gas is 126.67 cubic feet (3.587 m3) at standard conditions. This volume of natural gas has the same energy content as one US gallon of gasoline (based on lower heating values: 900 BTU/cu ft of natural gas and 115,000 BTU/gal of gasoline).[16]
One GGE of CNG pressurized at 2,400 psi (17 MPa) is 0.77 cubic foot (21.8 liters or 5.75 Gallons). This volume of CNG at 2,400 psi has the same energy content as one US gallon of gasoline (based on lower heating values: 148,144 BTU/cu ft of CNG and 115,000 BTU/gal of gasoline.[16] Using Boyle's law, the equivalent GGE at 3,600 psi (25 MPa) is 0.51 cubic foot (14.4 L or 3.82 actual US gal).
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and 1 US gallon of gasoline contains 33.40 kwh of energy.
250 bar is pretty close to the 3600 psi mentioned (3625), at which it takes 3.82 gallons of CNG to equal the 33.40 kwh. You have "only" 3 gallons, so 3.82/3 = 0.78534 multiply that by 33.40 kwh and you have 26.23 kwh in your tank. (~2.3kwh/litre @ 250 bar - or 9.2 watt-hours per litre at 1 bar vs the 8.8 watt-hours per litre of our original math. The 0.4 difference is either from using a different value per litre, or from rounding too much in the math)
Now you see why CNG vehicles have such big tanks...