There are 32MegaJoules of Energy per Litre of Petrol (~ same for diesel)
Therefore: 100litres of fuel = 3.2GJ
How to turn Joules into Watts – calculation:
The power P in Watts (W) is equal to the energy E in Joules (J), divided by t – the time period the Energy is consumed, in seconds (s):
P(W) = E(J) / t(s)
100 litres of fuel would typically last 10 hours (36,000 seconds) in a motor vehicle (Internal Combustion Engine) – cruising at say 110 kph
Hence the energy output of that 100 litres of fuel might be considered thus: 3.2GW/36000 seconds = 88.8 kW. However, the efficiency of ICE vehicles is, at best, 33% (so only approximately 29 of those kW are being used to propel the vehicle against the various mechanical/aerodynamic drags) the rest leaves mostly as heat through the radiator (and some noise!)
Note: Whilst fuel tanks can hold Gigajoules of energy, Electric Vehicle (EV) battery’s are generally expressed in KWh – don’t forget efficiency!
EV’s typically use 1/3rd of the Energy of Internal Combustion Engines (ICE) – no radiator not only means EV’s have lower drag it also means that energy is not being shed to cool an engine!
Electric vehicles not only have slightly less aerodynamic drag, they can recover energy from downhills and slowing (hence they can be considered as having effectively around 30% less drag) – with the result that a typical 85 KWh battery will propel an EV at the same speed (110kph) for just over 4 hours (i.e. 440 km). This comes from the fact that the EV, which is very efficient at turning battery energy into kinetic energy (~98%), would, following the above logic, need just 20.3 kW to cruise at 110 kph!
Of course headwinds, terrain and towing can all effect these figures (as they also do for ICE vehicles, too).
As you can see, there are many factors at play that need to be considered when trying to make efficiency comparisons.
However it is perhaps easier to look to the economic story!
100 litres of fuel costs ~ $150 – giving 1100 km of travel. At 26.5 cents per kWh (typical price hereabouts in The Northern Territory – 1100 km of travel would cost (from the above energy needs of an EV) $56 – very much cheaper still if you use your own solar PV!
Is it perhaps worth looking at this yet another way?
100l of fuel = 3.2GJoules = $150.That exact same amount of energy can be had in electrical form for $848 from your local power utility (or for FREE from YOUR OWN solar PV!). Of course, if you have an EV you’ll only actually need to use 212.5 KJ of energy to travel the same distance! Still. it’s informative to look at the cost of energy produced by burning fuel.
It is interesting to consider that the Power Utility puts about $698 of margin on the cost of that 100L of fuel (presumably to cover the costs associated with the inefficiencies of burning the fuel, servicing the IC engine and generator, distributing the electricity over the grid and paying for the management and staff that make this all possible (oh, and amortising the original investment in the publicly-owned Grid and Generation assets – if that’s not already done). Still, fossil-fuelled electricity looks pretty expensive – with fuel having 365% on-costs!
Of course – if the Power Utility used solar PV there’d be some solar panels, batteries and controllers that would have to be amortised (say over 10 years – ‘though panel warrantees are now 25 years – so they could be amortised over a longer period if so desired) and of course there’s the grid to amortise, maintain and operate (management and staff to pay, etc.) but the zero-input cost on fuel (which effectively negates any photon to electron conversion inefficiencies) would make a significant saving ($150 in our above worked example) – likely enough to fund the operation given that the PV, controller and batteries don’t need a lot of maintenance, or management (they’re intelligent devices) – and never need an oil change!
Interestingly – leaving all other PowerWater costs/margins in place and simply removing the cost of diesel (subsidised) the price per kWh could be $0.219. That would assume all staff remain in place (and it doesn’t factor-in oil changes, spare parts, etc. which are all costs avoided when going solar)
Power utility staff could perhaps be retrained to become solar-technicians and electrical engineers, tasked with installing extra capacity and upgrading the grid as appropriate to the likely increase in consumption that would inevitably result from the reduced price per kWh and the move to EV’s that will come sooner than most expect.
