EV vs PHEV vs Petrol Car: Comparing Energy Costs

Why energy cost comparisons get confusing

Many drivers want to know whether an electric car is really cheaper to run, where a plug-in hybrid fits, and how both compare with a petrol or diesel car. The difficulty is that efficiency is expressed in different units, electricity prices vary greatly depending on where you charge, and a PHEV mixes electricity and fuel depending on how you use it. Put the raw numbers side by side without accounting for those differences and the conclusion can easily be wrong.

This guide explains how to line up efficiency units, how to handle charging losses, how to weight prices by where you charge, how to include a PHEV's electric share, and why comparisons must use the same conditions. Electricity and fuel prices change with country, region, time and tariff, so treat every number below purely as a hypothetical example.

Matching units: km/L, L/100km, mpg, kWh/100km and km/kWh

Fuel economy is shown as km/L (distance per litre), L/100km (fuel used per 100 km) or mpg (miles per gallon) depending on where you live. With km/L and mpg, higher is better; with L/100km, lower is better. You convert between km/L and L/100km by dividing into 100. For example, 12.5 km/L equals 100 divided by 12.5 = 8 L/100km. Be careful with mpg: a US gallon is about 3.785 litres and an imperial gallon about 4.546 litres, so the same mpg figure means different things.

Electric cars work the same way, with kWh/100km (energy per 100 km) and km/kWh (distance per kWh). For instance, 16 kWh/100km equals 100 divided by 16 = 6.25 km/kWh. In the calculator you can enter whichever unit you already have and it will convert internally, so there is no need to risk a manual conversion error. Official ratings and real-world figures differ with season and driving style, so your own records are usually more realistic.

Battery-based vs grid-based consumption: count losses once

EV consumption comes in two flavours. The figure on the dashboard usually shows energy drawn from the battery, which is battery-based. Your electricity bill, however, is based on energy taken from the grid through the charger, which is grid-based. Some energy is lost as heat while charging, so the grid draws a little more than ends up in the battery.

If you use a battery-based figure, add a charging loss. If your figure is already grid-based, such as charger logs or a rating that already includes losses, do not add the loss again. Adding it twice inflates the EV's cost. For example, at 12,000 km a year and a battery-based 16 kWh/100km, the battery needs 1,920 kWh. If you assume the grid must supply 10 percent more than that, your billing basis becomes 1,920 x 1.1 = 2,112 kWh.

Weighting home, public AC and DC fast charging

What you pay per kWh depends heavily on where you charge. Home or workplace charging, public AC charging and DC fast charging usually have different prices. Rather than using a single price, it is more accurate to weight each price by the share of energy you get from that source.

For example, assume 60 percent at home at 0.20 per kWh, 20 percent public AC at 0.30 and 20 percent DC fast at 0.35. The weighted price is 0.6 x 0.20 + 0.2 x 0.30 + 0.2 x 0.35 = 0.12 + 0.06 + 0.07 = 0.25 per kWh. Multiplied by 2,112 kWh, annual charging costs 528. If those public prices are member rates that require a subscription of 10 a month, add 120 a year for a total of 648. A subscription swaps a lower unit price for a fixed fee, so it may not pay off if you rarely use public chargers.

Under the same conditions, a petrol car at 12.5 km/L with fuel at an assumed 1.70 per litre uses 12,000 divided by 12.5 = 960 litres a year, costing 960 x 1.70 = 1,632. Again, these figures only illustrate the method.

For a PHEV, the electric share decides the result

A plug-in hybrid runs on stored electricity for a certain distance and then uses its engine. The share of total distance driven on electricity therefore drives the result. If you charge at home every night and have a short commute, the share is high; if you rarely plug in or mostly drive long distances, the car behaves much like a fuel car.

For example, assume 50 percent of 12,000 km is electric. The 6,000 electric kilometres at a grid-based 18 kWh/100km need 1,080 kWh, which at the weighted 0.25 costs 270. The other 6,000 km at 16 km/L need 375 litres, which at 1.70 costs 637.50. The total is 907.50. Try different electric shares in the calculator to see how much a PHEV makes sense for your own driving pattern.

Compare at the same distance and over the same horizon

When comparing vehicles, keep annual distance and ownership period identical. Entering 20,000 km a year for the EV and 10,000 km for the petrol car makes the comparison meaningless. The same goes for time: looking at one car over three years and another over seven distorts resale value and maintenance patterns.

Energy is also only part of the total cost. Purchase price, insurance, tax, tyre wear, maintenance items (an EV has no engine oil but still has other consumables) and residual value all matter. Even when energy is cheaper, how many years it takes to recover a higher purchase price depends on how far you drive, so set both cars up side by side in the calculator under the same conditions and check.

Energy cost comparison checklist

Check these points before comparing so you do not misread the numbers.

• Did you pick the correct efficiency unit for each vehicle?
• For mpg, did you confirm US or imperial gallons?
• Is EV consumption battery-based or grid-based, and is the charging loss counted only once?
• Did you enter your home, public AC and DC fast charging shares and prices?
• Did you include fixed costs such as charging subscriptions?
• For a PHEV, is the electric share realistic for your routine?
• Are annual distance and ownership period the same for every vehicle compared?