How much electricity can a tram store

The estimated energy use for a diesel bus is 0,080 kWh per place kilometer, while a tram only uses 0,017 kWh per place kilometer. Battery buses are better than diesel buses, but do not come near the efficiency of a tram. When braking, modern trams can also feed electricity back in the grid, that other trams can use.

Because the length of trams can vary, a single carriage, which is also comparable with the weight of a double-decker bus, has been used in these calculations. In reality, a three-car tram can carry as many as 140 passengers, standing and seating, compared with the new electric London buses, which propose to have a capacity of 90.

On average, for the life of a battery the charge and discharge cycles are estimated to be approximately 80% efficient. That means using a battery adds in losses of 20% to our equation. 80% x 90% gives a drive efficiency of 72% for an electric bus. A tram is effectively plugged into the mains and avoids these storage losses.

Trams are a seemingly expensive outlay for a city. They require permanent tracks and overhead wires. At least that's how the figures can often initially appear, but there is more to it than basic installation cost. What of the energy required to operate, maintain and recycle our transport network.

What of the energy required to operate, maintain and recycle our transport network. Once installed trams are one of the lowest energy and most sustainable mass transit systems a city can operate. Much lighter than trains, without the need for often cost prohibitive and energy-intensive underground tunnels that a metro system requires.

A diesel combustion engine loses 60% of the energy in the fuel (energy density 10 kWh per litre) as heat before it is converted into motion. That means the bus above, needing 24.9 kW of power to keep moving at 30mph, actually needs 62.2 kW of fuel. An electric bus has an electric motor, just like a tram, so benefits from the improved efficiencies.