HOW MUCH DOES ENERGY WEIGH?

PROF M. GOOSEY

​Energy is generally considered to have so little mass that, for all intents and purposes, it is negligible.  It can of course be considered in the context of Einstein’s famous law, which defines the relationship between energy and mass as follows;  where E is energy, m is mass and c is the speed of light.

Using this law and taking the speed of light as being 3x 108 m/s, a single joule of energy has an equivalent mass of around 11 femto grammes, which is indeed very little.

However, if we start to think about how much energy weighs in real life applications, it is not the weight of the energy itself that is important but the medium that is used to store it.  In the world of transport this was once, for example, the weight of the coal that was used to boil water in steam engines or, more recently, the petroleum-based liquids that power our ubiquitous internal combustion engines.  In the last few years, we have become accustomed to the use of new power sources and technologies, such as the lithium ion batteries that are increasingly used to store and provide energy for powering electric vehicles.  In these cases, a key consideration is not just the amount of energy stored but the overall weight.  This is very important for transport applications, as moving a large mass of batteries consumes a substantial amount of energy.  As a result, this aspect is typically reported in terms of energy density, i.e. the amount of energy stored, or provided, per unit mass and it is where comparisons of the different approaches start to get both interesting and more complex.

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For example, if one considers the lithium ion batteries that Tesla has previously used to power its electric vehicles, such as its BT85 85 kWh unit, this is reported as weighing around 540 kg.  Thus, the energy density is approximately 0.16 kWh/kg.  This figure is a little low by latest technology standards, with more recent battery energy densities of 0.25 kWh/kg or more now being achieved.  By contrast, the energy density of conventional petrol is quoted as being approximately 13 kWh/kg.  Thus, it only needs 6.5 kg, or approximately two gallons of petrol to provide the same amount of energy as a more than a half-tonne of battery pack.  This is just a simplified comparative example and ignores the weight of the petrol tank, but is does help to highlight the need for much improved lithium ion battery efficiency. 
 
In this context, it is therefore fair to say that current electric vehicles still have a long way to go to match the capabilities of their conventional counterparts powered with internal combustion engines, and that is even before one makes comparisons of refuelling and recharging times!  In fact, the Argonne National Laboratories in the USA have concluded that, when it comes to driving long distances, it will not be until around 2045 that electric vehicles will be ‘comparable to conventional ones in terms of the energy spent at the wheel per kg of the powertrain mass’.

In support of electric vehicles, energy density is just one small part of the story and many more factors need to be considered when comparing different types of energy sources for vehicles, including cost, energy conversion rates, refuelling times, environmental impacts and infrastructure costs etc.  For example, hydrogen fuel cell technology can offer energy densities far superior to those of current lithium ion battery technology, but it means driving a vehicle with a tank storing hydrogen under a pressure of several thousand pounds per square inch.​
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When it comes to electric vehicles, the question of ‘how much energy weighs’ is just one important aspect of a much more complex set of considerations determining the best choice of power.  While good progress is being made, there is still work to be done and many choices need to be made before the newer forms of transport propulsion achieve true parity and ubiquity with the convenient and, still evolving, internal combustion engine.