kWh to MJ Converter for Comparing Fuel Energy Density

Energy Density: What a Litre or a Kilogram Actually Carries

Energy-density literature is written in megajoules. Petrol is quoted at about 34.2 MJ per litre, diesel at 38.6, hydrogen at a spectacular 120 MJ per kilogram. Electric storage, meanwhile, is quoted in kilowatt-hours, because that is what meters, chargers and utility tariffs speak. The two vocabularies describe the same physical quantity, so any honest comparison between a tank and a battery starts by putting them into one unit.

Conversion factor: 1 kWh = 3.6 MJ exactly, because a kilowatt sustained for 3 600 seconds is 3 600 000 joules. A 50-litre tank of petrol at 34.2 MJ/L therefore holds 1 710 MJ, which is 1 710 ÷ 3.6 = 475 kWh of chemical energy.

The Three Numbers Behind Any Fuel Choice

Volumetric Density (MJ/L)

How much energy fits in the space available. Ships, tankers and the fuel bay of a delivery van are volume-constrained, and this is the column that decides whether a tank fits under the floor.

Gravimetric Density (MJ/kg)

How much energy you carry per kilogram of dead weight. Aircraft live or die by this figure, which is why kerosene at roughly 43 MJ/kg has been so hard to displace.

Conversion Efficiency

The share that reaches the wheels or the propeller. Density alone ranks the fuels; efficiency decides the outcome, and the two carriers at opposite ends of the density table also sit at opposite ends of this one.

Setting a Fuel Tank Beside a Battery in Megajoules

Most comparisons start with one figure in the wrong unit — a battery in kWh next to a table of fuels in MJ, or the reverse. Convert first, interpret second.

Enter the electrical figure in kWh

Type the kilowatt-hour value straight in. A comma decimal works as well as a dot, which matters when the figure was copied out of a European datasheet written as 0,26 kWh/kg.

Attach the per-litre or per-kilogram basis yourself

The converter handles energy, not density, so keep the basis in your head or in the neighbouring spreadsheet column. Converting 0.264 kWh/kg gives 0.95 MJ, which you then read as 0.95 MJ/kg.

Reverse it for a published MJ table

When the source already reads 34.2 MJ/L, the swap arrows turn the pair around so megajoules drive kilowatt-hours — or just type into the megajoule side and let the other field follow.

Move the value into your own table

The copy control above each field yields a plain number, which keeps a density comparison sheet clean when you are filling in a dozen carriers one after another.

Check the heating-value basis first: published densities may be lower heating value or higher heating value, and for hydrogen the two differ by about 18 %. Converting units never resolves that ambiguity — mixing an LHV figure with an HHV one produces a comparison that is wrong before any arithmetic happens.

Transport Fuels and Carriers by Litre and by Kilogram

Representative lower-heating-value figures for the carriers that actually compete for space in a vehicle, with each one expressed both ways.

Energy carrier	MJ/L	kWh/L	MJ/kg	kWh/kg
Diesel	38.6	10.72	45.4	12.61
Jet A-1 kerosene	35.0	9.72	43.0	11.94
Petrol / gasoline	34.2	9.50	44.0	12.22
LPG, liquid	25.3	7.03	46.4	12.89
Ethanol (E100)	21.4	5.94	26.8	7.44
Hydrogen at 700 bar	4.5	1.25	120.0	33.33
Lithium-ion pack	1.8	0.50	0.95	0.26

Two things jump out. Hydrogen tops the mass column by a factor of nearly three over diesel, yet at 700 bar it is the worst carrier in the volume column — a tank eight times the size of the diesel one for the same energy. And lithium-ion trails everything on both counts, which is a real disadvantage for aircraft and a much smaller one for a car, for the reason set out below.

Working Through a Density Table

Datasheet Values Convert Either Way

Sources mix the two units freely, so neither field is fixed as the input. Type on the side your source uses and the other resolves immediately, with the swap arrows there when you change sources mid-table.

Plain Digits for a Comparison Matrix

Copying a field, from the button or with Ctrl+C inside it, produces the bare value — ready to paste as one more cell in a carrier-by-carrier density sheet.

Gigajoules and BTU Are One Search Away

Energy-density sources are inconsistent; both dropdowns search all 23 units, so a figure quoted in GJ per tonne or BTU per gallon can be pulled onto the same scale without leaving the page.

Small Per-Kilogram Values Keep Their Digits

Battery figures land below one, and results carry up to eight decimal places before scientific notation takes over, so 0.26 kWh/kg does not collapse to a rounded 0.3.

Energy-Density Questions That Trip People Up

If a battery is so poor per kilogram, why does an electric car use less energy per kilometre?

Because density measures what you carry, not what you use. A petrol drivetrain turns roughly 25–30 % of the fuel's energy into motion; the rest leaves as heat and noise. An electric drivetrain delivers about 85–90 %. Take that 1 710 MJ tank: at 25 % it puts around 428 MJ, or 119 kWh, on the road, and an electric car would need only about 137 kWh from its pack to match it. The mass gap stays enormous, but the useful-energy gap shrinks to something a car can live with — which is exactly why the argument still defeats batteries in aviation.

Which column should I judge a carrier by, MJ/L or MJ/kg?

Whichever one your application is short of. An airliner carries fuel as payload it must lift for hours, so mass rules and MJ/kg is the deciding figure. A container ship has room in its double bottom but cares about cargo space, so volume rules. Road vehicles are somewhere in between, and stationary storage barely cares about either. Quoting a single "energy density" without saying which basis is the most common way these comparisons go wrong.

Are published density tables on lower or higher heating value?

Transport and automotive sources almost always use the lower heating value, because an engine or turbine exhausts water as vapour and never recovers the condensation heat. Heating and gas-industry sources often use the higher value instead. For petrol and diesel the two differ by around 6 %, for hydrogen by about 18 %, so a table that silently mixes them can move a conclusion. If a source does not say, its hydrogen row usually gives it away: 120 MJ/kg is the lower value, 142 MJ/kg the higher one.

Why is hydrogen's headline 120 MJ/kg so misleading?

Because the number is quoted per kilogram of a gas that is extraordinarily hard to keep in a small space. Compressed to 700 bar, hydrogen still stores only about 4.5 MJ per litre, roughly an eighth of diesel — and the pressure vessel that holds it adds mass that the clean 120 MJ/kg figure ignores entirely. Liquefying it to about 8.5 MJ/L helps the volume but costs a large fraction of the energy in the process. On a system basis, tank included, the advantage over other carriers is far smaller than the headline suggests.

What does 3.6 megajoules look like in everyday terms?

It is one unit on a household electricity bill: a 2 kW kettle running for half an hour, or a 60 W lamp left on for about 17 hours. In fuel terms it is about 105 millilitres of petrol — barely a teacup — which is a useful reminder of just how much energy is packed into a full tank, and of how much of it a combustion engine throws away as heat before any of it reaches the road.

Kilowatt-hours to Megajoules