Electric-car drivers are saving the planet, right? Their vehicles produce none of the pollutants that dinosaur-burning, fossil-fuel-powered machines do. That is the standard view, and governments around the world provide incentives to encourage the uptake of this new technology.

That is why a Tesla owner got a rude shock when he went to import his vehicle into Singapore — the first person to do so. Instead of an expected rebate of around S$15,000 (US$10,800) he received a fine of the same amount for being a gross polluter. The Tesla Model S is a 100% electric vehicle. It does not have an exhaust to emit from. So what happened?

Instead of an expected rebate of around S$15,000 (US$10,800) he received a fine of the same amount for being a gross polluter.

The Singapore authorities calculated the ‘carbon cost’ of generating the electricity that will be used to charge the car. This is the elephant in the trunk of electric vehicles. Where and how the power is produced is not often considered, but perhaps it should be. Let’s move the elephant up to the passenger seat and address it directly.

First — this specific case.

The authorities in Singapore apparently found the Tesla in question consumes 444 watt-hours of electricity per km (Wh/km) in tests. Tesla has disputed the rating.

“The Model S that our customer imported into Singapore left our factory in 2014 with energy consumption rated at 181 Wh/km,” the company said in a statement sent to BBC Autos. “This qualifies as the cleanest possible category of car in Singapore and entitles the owner to an incentive rather than a fine.”

Without wanting to get too maths-heavy, the number of 444Wh/km does seem high. In our non-scientific testing of the Tesla Model S (ie: driving with a lead foot and having fun) we recently achieved around 3 miles per kWh in a Model S P85D. That is 4.83 km per 1000Wh, or 207Wh/km, which is much closer to Tesla’s claimed figure.

Should we be factoring in the emissions of power stations when working out how green an electric car is? The logical answer is yes.

“We are having cooperative discussions with the [Singapore Land Transit Authority] to ensure a proper understanding of these issues and to make sure that they are correctly testing our customer’s Model S,” says the company. “Based on the positive nature of those discussions, we are confident that this situation will be resolved soon.”

“Their numbers are completely wrong,” agrees Professor Dan Sperling, director of the Institute of Transportation Studies at the University of California Davis.

But what about the bigger picture — should we be factoring in the emissions of power stations when working out how green an electric car is? The logical answer is yes. Emissions shifted elsewhere are still emissions, and CO2 impacts the global atmosphere wherever it is released.

"Based on the average current mix of renewable and non-renewable electric power sources in the US, the average of well-to-wheels greenhouse-gas emissions for current battery-electric vehicles is 214 g/mi,” explained Dr Michael Sivak, Director Sustainable Worldwide Transportation at the University of Michigan, who has studied this issue.

“In comparison, the average for current gasoline-powered vehicles ranges from 356 g/mi for direct fuel injection to 409 g/mi for traditional fuel injection."

(It is probably best not to ask about cars with a carburetor if you ever want to enjoy a drive in a vintage vehicle without being weighed down by guilt.)

If you get an electric car running on electricity made from coal, its impact would probably be about the same as a gasoline car. If you run on anything else it gets much better.

That's good news for the electric elephant then, at least in the United States. The argument that power stations can be more efficient and cleaner when it comes to burning fossil fuels seems to hold true. Additionally, emissions away from cities and homes have a lesser impact on people’s health, and if the electricity is generated through renewable sources then the environmental impact can be minimal.

However a deeper dig into the numbers reveals some local differences.

“If you get an electric car running on electricity made from coal, its impact would probably be about the same as a gasoline car,” says Prof Sperling. “If you run on anything else it gets much better.”

Your personal calculation on whether an electric car would make you an eco-warrior will therefore depend on how your electricity is generated locally. That is not always easy to figure out, but there are some tools available online, including climatecentral.org's interactive map and ucsusa.org's emissions calculator.

(The calculation holds true even when complete lifecycle carbon emissions are considered btw. A common criticism of electric cars is that making the batteries is energy intensive, but they still produce less global-warming pollution overall, according to the Union of Concerned Scientists.)

To come back to our initial question though about whether electric cars should be incentivized or penalized, the view of most experts is that rewarding people for driving them is the correct approach. Not only are they more efficient now, in most cases, they should become even more so in the future.

“Electric vehicles are being promoted because of their long term potential,” says Prof Sperling. “The electricity system is going to decarbonize — almost every single country in the world has that as a policy and a strategy. So comparing it to what it is today it's really not the point.”

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