For almost a decade, commercial-fleet managers have acknowledged that electric truck are much cheaper to run per mile than the diesel equivalents they now use.

But justifying the purchase of electric trucks has been a challenge because their upfront costs have been considerably higher.

That may now have changed, according to no less a fleet operator than United Parcel Service.

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Its tens of thousands of big brown trucks, largely powered by diesel engines, will soon be joined by 50 Workhorse plug-in hybrid delivery vans.

And, UPS said, those new vans cost it no more than diesel versions with the same capacity.

That's a big advance for trucks that plug in, and may well encourage fleet managers to take a closer look at trucks that plug in to provide part or all of their range.

Workhorse N-Gen electric delivery van

Workhorse N-Gen electric delivery van

In a press release issued in late February, UPS said the 50 plug-in electric delivery trucks it will deploy "will be comparable in acquisition cost to conventional-fueled trucks without any subsidies."

That achievement is, it said, "an industry first that [breaks] a key barrier to large-scale fleet adoption."

Indeed, it is the high cost of lithium-ion battery packs to provide adequate range in large, heavy delivery trucks that has proven to be the hurdle to wider adoption thus far.

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Workhorse said it had worked "closely" with UPS over four years to refine its vehicles based on lessons learned by UPS over millions of road miles and thousands of packages delivered.

The startup truck company said the cab-forward Class 5 delivery trucks it provides to UPS will travel roughly 100 miles between battery recharges, which UPS expect to do overnight at its depots.

Neither UPS nor Workhorse provided specifications or images of the vans to be acquired, however—so readers should note the images in this article are of different types of delivery vans.

Artist's rendering of possible future UPS plug-in electric delivery truck

Artist's rendering of possible future UPS plug-in electric delivery truck

The trucks from Workhorse will join the UPS "Rolling Lab," an expanding alternative-fuel and advanced-technology test fleet that now numbers more than 9,000 separate vehicles.

UPS is not the only large delivery fleet testing a variety of plug-in electric trucks.

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Federal Express has tested the all-electric Nissan e-NV200 small delivery van, and the French Postal Service has tested an electric Renault Kangoo ZE van with a hydrogen fuel-cell range extender.

The German postal service has even gone into the business of manufacturing and selling its own electric delivery vans, after purchasing a small startup in that business several years ago.