Power and Energy
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- Last Updated: Wednesday, 26 February 2025 15:36
Power
Many people have difficulty in comprehending the difference between power and energy. This page attempts to explain this and the simple math behind the two. Once familiar, and knowing the specifications of an EV under consideration, its easy to work out likely range and charge times for various situations.
Horsepower and Watts
This is just a measurement of capability or rating. One can have: A 100 Watt light bulb, A horse, Car, or battery, but unless actually in use, no energy is used or provided. A horse eating in a paddock might be one horsepower, but unless put to work it does not produce any useful energy. A 100 Watt light bulb only provides light (energy) when power is applied.
There is a conversion factor between horsepower and watts, but its not that useful in comparing an ICE engine with an EV. This often leads to economy quoted as MPGe, but still not very meaningful when energy prices are taken into account. In comparing an ICE and and EV its more useful to take the cost of a unit of petrol and that spent on electricity and to determine the distance traveled for each.
For example, an ICE car might travel 10 km on one liter of petrol at a cost of $3.00. That same amount could buy 10 units (kWh) of electricity, and if the EV uses 0.15 units (150 watts) per km, the distance is 66 km. So 10 km versus 66 km for the same $. If one recalculates this using imperial gallons, the cost of one gallon is close to a full charge for an EV.
Energy
This is the production or use of power measured over time, typically in hours. It can be in watts or in amps for a given voltage, such as a battery, which could be a dry-cell, lead acid, nickle cadmium, lithium or many other types. Many quote capacity or total energy in mAh or Ah units mainly specified for rechargeable types. An AA battery may quote from 600 to 2000 mAh and a 12 volt lead acid car battery might range from 30 to over 100 Ah. Multiplying this value by the voltage converts it to watt hours, which eliminates the need to quote a voltage and is why electric vehicles quote watt hours.
To make things even easier in determining range, charge-time, and costs, the common units are Kilowatt hours. One kWh is the same as a unit on your electricity bill and which typically costs from 15 to 45 cents depending on connection type, supplier, time of day, etc.
So for a given amount of energy its easy to determine how long it will last for a given power usage. For one unit, a 100 Watt bulb lasts 1000/100 or 10 hours, but a 2000 watt fan heater 1000/2000 or half an hour.
Therefore, knowing the battery capacity of a EV, which can vary from 10 to 100+ kWh, its easy to determine the time needed to fully charge, based on the power supplied by the charger. By knowing the power your EV uses to travel a given distance at a specific speed, or an average, (most EV's show both on their screen), its maximum range can be determined.
Chargers come in several types defined as Level 1 to 4. AC chargers are actually built into the car itself and convert the 240 AC to 400+ volts DC.
AC Level 1
Power comes from a standard 3 pin 10A wall outlet which most EV's support and come with a cable to do this. For safety reasons the rating of the socket under continuous load is usually limited to 80%. Typically this results from 1.3 to 2.0 kW and for a 65 kWh battery a full charge would take about 32 hours. Its not likely that one would need to do this unless your daily commute was above 100 Km, as most EV's have over 300 Km of range and only 1/3 of the 65 kWh would have been used. This brings the daily charge time to full back to 10 hours. Many EV owners are able to exist using just Level 1.
AC Level 2
Power comes from a heavier rated 3 pin outlet typically rated at 15 amps. Depending on wiring, just swapping out a 10 A outlet to 15A for a few $ maybe all that is needed. However, the cable needs to have a 15 A plug and usually a small control box at this end. This can increase the charge power to close to 3.0 kW giving a near 150 Km daily commute when combined with a 10 hour charge time.
AC Level 3
This requires a permanently wired unit fitted by an electrician, although some new dwellings are being built with these. They are relatively small wall units, about shoe box size, and have a heavy cable with a Type 2 connector attached. They can be quite smart and take advantage of cheap power rates and even support a phone app to override as desired. For home installations, the maximum currently supported on a fuse box or breaker cabinet is 32A, giving a power rating of around 7.5 kW and offers an 8 hour charge time for a 65 kWh battery and easily done over-night using reduced rates. The purchase and installation costs though might run to $3000 depending on charger box itself, distance from power cabinet etc. Definitely get some quotes and advice first. If contemplating two EV's this can be cost effective as its unlikely both would need a full charge at the same time. Some charger boxes support two EV's at once and can load share for best efficiency. (They talk to each vehicle and provide most power to the one with the lowest charge until they are balanced.) Tesla supplied wall boxes with every car for a while, and its worth talking with the car dealer about such offers prior to purchase. About $850 from Tesla.
Level 3 is also supported commercially and by using 3 phase power levels up to 22 kW are supported, although most cars limit the maximum on AC to 12 kW. These are generally known as destination chargers, and found at hotels, motels, camp grounds, and free use at many shopping malls.
DC Level 4
All AC charging is actually done and controlled by the car itself using its on-board AC to DC converter that communicates via the connecting cable to determine max charge power permitted.
DC charging is the reverse, where the charger cabinet might be rated from 50 kW to 350+ kW, but the car determines what is safe maximum for its battery at any particular charge state and temperature, and adjusts the charger's output in real time. The standard connector in N.Z is CCS2 which is just a type 2 with an extension underneath for two fat DC connector pins. The main and important point is that if your car is rated for 120 W max, connecting to a 350 kW charger won't make it faster and its quite likely to cost more as well. These charge cabinets are not cheap, and there are time costs as well as for power used and a 350 kW unit may cost 65 cent per kW compared to 30 cents for a 50 kW cabinet. Also, many are limited to only charge up to 80%. The reason being that the car will reduce the rate gradually after the charge reaches about 50% and getting from 80 to 100% can take longer than 10% to 80%. This will greatly annoy any other people waiting who only want to plug in for 10-15 minutes... (Its not unusual to find oneself just a few % short of charge short to make it home and a few minutes is all one needs!)
Efficiency
This has been ignored in the above calculations. Generally between 5 and 10 % is lost during charging and use. Because EV motors use multi-phase AC power and the battery is DC, this must be converted by electronics which produce heat and some energy is lost in the process. The same happens in reverse during regenerative braking which may not always give the best result. On level ground it might be better to coast to a halt for traffic lights. In practice this is difficult and annoys following traffic so probably not worth it. On hills there is a huge advantage to regenerative braking compared to an ICE vehicle, which can never put fuel back into the tank once burnt. (This can also annoy ICE drivers as EV owners may find that going a bit slower down hill generally gives a higher recovery rate and for longer.)