Drone-based deliveries have been promised for years now, and are just now starting to (no pun intended) get off the ground. There are plenty of reasons to be excited at the prospect of flying robots bringing your latest online delivery, but one that is frequently mentioned is the idea that drones will remove a certain number of delivery vehicles from the road, leading to a positive environmental impact.

Not so fast, claims a recent study carried out by a researcher at Martin Luther Universitat in Halle-Wittenberg, Germany. Having run the numbers, Thomas Kirschstein, from MLU’s department of Production and Logistics, thinks drone deliveries could actually wind up consuming a whole lot more energy than alternative vehicular options, especially in dense urban areas. Kirschstein’s simulations suggest that drones could use around 10 times as much energy as electric vans, and significantly more than diesel vans (which use twice the energy of electric vans).

“The results of the simulation study provide some evidence — provided that all the technical parameters are sufficiently precise — that parcel deliveries by drone are more energy-intensive than truck deliveries, particularly in densely populated areas,” Kirschstein told Digital Trends.

Kirschstein’s models weighed up energy consumption for drones and trucks, based on various parameters. These included things like technical specifications (a drone’s air drag coefficient and number of rotors), along with environmental conditions, travel speed, and more. It also took into account factors such as the ability of a truck to make multiple deliveries in a single journey, compared with drones’ ability to serve only one customer at a time.

It’s not all bad news for drone deliveries, though. “The more [dispersed] the customer area, the longer the truck tours become,” Kirschstein said. “Thus, in rural areas, it is possible that drone deliveries consume less energy than truck deliveries. From an environmental perspective, the results indicate that. in the considered parcel delivery setting, using electric trucks can reduce energy consumption and related greenhouse gas emissions compared to diesel-powered trucks, provided eco-friendly electricity is provided. Furthermore, in urban settings with high population density, drones are likely to consume more energy than electric trucks, incurring higher emissions. In rural settings, the results indicate that drones may consume less energy than electric trucks and might be a more eco-friendly parcel delivery technique.”

Advancing technologies will also change this equation over the coming years. For instance, larger battery density for drones or lower weight designs could dramatically improve the efficiency of drones. There are also plenty of scenarios in which drone deliveries might be far preferable for other reasons, such as delivering vital medical supplies in parts of the world where road infrastructure or traffic makes this otherwise difficult.

In other words, don’t dismiss the potential of drone deliveries based on one study. But also understand that just because something is shiny and new doesn’t guarantee that it is more efficient.