Every fleet controller knows the cost per mile for their diesel vehicles. It's simple math: total fuel cost divided by total miles driven. Your fleet card gives you the fuel cost. Your telematics gives you the miles. Done.

For electric vehicles, the same calculation is straightforward in theory but nearly impossible in practice — unless you have the right data infrastructure in place.

Why EV cost-per-mile is harder than it looks

The diesel cost-per-mile formula is:

Total fuel cost / Total miles driven = Cost per mile

For EVs, you'd think it's the same:

Total electricity cost / Total miles driven = Cost per mile

But here's the problem: "total electricity cost" is scattered across multiple sources, each with different data quality:

1. Public charging (fleet card)

Your fleet card statement shows a dollar amount, but no kWh. Without kWh, you can't calculate a rate, and you can't verify you're being charged correctly.

2. Home charging (utility bill)

Drivers who charge at home incur costs on their personal utility bill. You need to know: how many kWh went to the fleet vehicle (vs. the household), at what rate, and during which hours (time-of-use rates vary).

3. Depot charging (facility meter)

If you charge at a depot, the electricity cost is buried in your facility's utility bill. You need sub-metering or smart charger data to attribute costs to specific vehicles.

Step-by-step: Calculating true EV cost-per-mile

Step 1: Gather your charging data

Collect charging session data from every source:

Fleet card statements (public charging costs)
Charging network portals (kWh, session details)
Telematics platform (miles driven, energy consumed)
Smart charger data (depot/home kWh)

Step 2: Match and enrich

For each fleet card transaction, match it to the corresponding charging session to get kWh delivered. This is the hardest step if done manually.

Step 3: Calculate blended rate

For each vehicle, calculate the blended electricity rate across all charging types:

Blended rate = (Public cost + Home cost + Depot cost) / Total kWh

Step 4: Calculate cost per mile

With total cost and total miles from telematics:

Cost per mile = Total energy cost / Total miles driven

Step 5: Compare against diesel

Now you can make a true comparison:

Diesel Vehicle D-0015: $0.142/mile
EV Vehicle E-0042: $0.038/mile

That's a 73% reduction in fuel cost per mile — but you can only prove it with clean, reconciled data.

The bottom line

Calculating EV cost-per-mile isn't a math problem. It's a data problem. The formula is simple; getting the inputs is hard. Fleet finance teams need a systematic way to collect, match, and normalize charging data from every source before the calculation becomes meaningful.