Most EV owners who track charging costs do it the same way: kilowatt hours in, times the price per kWh on the bill, done. It is simple, and if you charge on a time of use tariff it is wrong, sometimes by a factor of three.
The rate depends on the hour
Plenty of tariffs now charge a standard rate through the day and a much cheaper one overnight. Plug in at seven in the evening and you pay full price. Plug in after midnight and the same electricity can cost a fraction of that. A log that uses one flat rate averages the two away and tells you nothing useful about either.
A worked example
Take one car doing 3.9 miles per kWh and a single 30 kWh top up. That charge is worth about 117 miles whatever time you plug in. The miles never change. The cost of those miles does, depending entirely on which rate was live when the charge started.
Cost per mile is the number that matters
Price per kWh is what the energy company sells you. Price per mile is what the car actually costs to run, and it is the only figure you can fairly set against a petrol car or a fuel card. Getting there takes two things: pricing each charge at the rate in force when it began, and dividing by the miles the odometer actually recorded rather than what the dashboard estimated.
Why it is worth doing properly
Once you can see the real per mile figure, the case for moving charging into the cheap window makes itself. So does the case against topping up on a public rapid charger out of habit.
VoltLog is an Android app that prices each charge by the time it started and divides by the miles you actually drove, so the log ends with a real cost per mile instead of a flat rate guess.