If the car can compute/track EV miles, which I think every EV can, then a difference of EV miles at time point and at another, with the cumulatie charge inbetween, is really like a "fill up". If the site tracks overall miles and EV miles, then as long as the user can report kwh and cost for power once a month it is consistent with the model of mpg.
I don't see the problem the FAQ is suppose to show. Filling up every day still fills the gas tank up to full and provides a difference from full to calculate MPG.
While you can't calculate MPG per fill-up, the aggregate MPG is still correct. It's becomes similar to how the overall average MPG is calculated, just for a shorter interval.
Anyhow, please be specific about the problem(s) you see with tracking kWh. I keep getting vague answers like, the rhythm is unknown or tracking kWh is a different beast. If there is a real problem, a specific hypothetical example exists to illustrate the problem. A well-defined/specific problem can be understood and solved while a vague one cannot.
I've tried my best to be clear, ryogajyc and I feel like I've been very patient explaining our side of things. We're not going to see eye to eye. We've had a good discussion here hashing out some of the issues.
We're going to take our time with this, and we might not add kWh tracking. Fuelly is a free service that we run on the side because we believe people should think about their fuel usage. We can't be all things fuel economy to all people. Some folks are going to disagree with how we do things—some are going to want more features than we offer. We're ok with that.
If it at all helps, I drive Outlander PHEV. A scientifici way to approach this is to track everything in MPGe that is convert all fuel types to essentially kWh and then compute efficiency from there. This is very confusing but it does provided consistent comparison of different fuel types. E.g. this is done by EPA in the USA and in the UK by the household utility providers both gas & electric prices are in kWh (with conversion factors for gas).
But this does not answer the question of when and how to record fuel-ups. Driving a hybrid - I do not care which fuel i've used (even if my car tracks it), all I care about is how much, on average, a mile costs me to travel. Thus I track fuel-ups with the odometer as needed and get the crazy amazing mpg values.
Next I take the total cost of charging per month (my car info screen tells me that by calculating charging kwhs and I had to put in my electric price per kwh) and record it. And I do it every month as I share my household bills with flatmates, such that they are not paying for my car.
To figure out my cost per mile - i look up total cost for a time period, add total cost of charging, and then look up miles driven between that time period to figure out final cost per mile.
Thus cost for electricity charging should be possible to track in fuelly just like any other fixed costs and expenses associated with a car. E.g. yearly service, road tax, monthly/queterly/yearly insurance cost, electric/hybrid car battery lease payments, and the car lease / car price itself. Obviously every item like that has a different amount and time period it is applicable at, but it would help to calculated total cost of ownership and total price per mile over a year.