Best Roof Boxes for Electric Cars: How Much Range Do You Lose?

Roof boxes and EVs are not natural companions. A petrol car loses some fuel efficiency; an EV loses measurable range — and range anxiety makes that feel worse. But the penalty is predictable and manageable if you choose the right box and adjust your speed expectations. Here is what to know before buying.

How much range does a roof box actually cost?

Aerodynamic drag increases with the square of speed. At motorway speeds (70mph), a large roof box can add 15–25% range loss compared to driving without it. At 60mph, the penalty roughly halves. In town at 30mph, it is barely measurable.

The drag coefficient of the box matters more than its size alone. A long, narrow, aerofoil-shaped box like the Thule Motion XT creates less drag than a short, boxy one at the same volume. For EV owners doing long motorway runs, box shape and keeping speed to 60mph are the two most effective levers.

If you are doing 200 miles of motorway with a 300-mile range EV and a roof box at 70mph, plan for an extra stop. At 60mph on the same car, you likely make it without a stop or with one quick top-up.

Hard shell vs soft bag: which is better for EVs?

For motorway trips in an EV, a hard shell box is the better choice. Hard shell aerofoil designs have a lower drag coefficient than soft bags, which tend to billow and create turbulence. The difference matters most above 60mph.

Soft bags (also called cargo carriers) attach with straps and do not require a roof rack. They are cheaper, easy to store, and fine for low-speed trips: going to a campsite via A-roads, moving house locally, or carrying kit to a day event. If your journey is mainly motorway, spend the extra on a hard shell.

Do I need a roof rack?

Hard shell roof boxes require a roof rack (also called roof bars). Most EVs have fixing points for aftermarket roof bars; check your car's handbook or a fitting guide. Soft bags attach with straps directly to the roof and frame — no rack needed. If you already have roof bars from a previous car, check the width and load rating before fitting a box.

Our picks

Best hard shell for motorway EV trips: COSTWAY 400L

The COSTWAY 400L is a decent mid-range hard shell that fits most roof rack systems. At around £250, it is a fraction of Thule pricing without sacrificing the key benefit of a rigid aerofoil shape. Dual-side opening makes loading easier when parked parallel to a wall or fence.

Best budget: Amazon Basics rooftop cargo bag

If you are doing a one-off move or a slow-road trip and do not want to spend £250 on a hard shell, a soft cargo bag is the pragmatic choice. No roof rack needed, folds flat for storage, and waterproof for UK weather. Just keep your speed down.

Flexible soft bag: Rooftop Cargo Carrier

This reinforced cargo bag sits between a cheap supermarket bag and a full hard shell in both price and protection. Suitable for camping gear, sleeping bags, and bulky items you want to keep separate from the boot.

Tips for using a roof box on an EV

• Drive at 60mph instead of 70mph — the range saving is significant and the time cost is usually 10–15 minutes on a two-hour journey
• Remove the box when not in use — even an empty box adds drag
• If you have a range estimator, check it after 5–10 miles with the box fitted to calibrate your actual loss
• Load heavier items low in the boot rather than the box — EV range is also sensitive to total weight
• Pre-condition the car before setting off on cold days so you are not using heater power on top of box drag