Tesla has remotely unlocked the ability for Florida drivers fleeing Hurricane Irma to get an extra 30 to 40 miles in an electric car without recharging. It’s a temporary measure that highlighted an usual element to the battery capacity.

Early Model S and Model X Tesla cars came with a standard 75 kWh battery pack, but users had the choice of whether to pay an extra fee (of between $4,500 and $9,000) to ‘unlock’ the full capacity. For those who didn’t, a software lock would restrict it to 60 kWh.

The idea was to get the economic benefit of offering two models – getting the maximum revenue both from people prepared to pay the full amount and those only willing or able to pay less – without the need to physically produce two different batteries. Owners who chose the cheaper option could upgrade later on.

It’s that upgrade that’s allowed the temporary range boost. Tesla says that because of the “exceptional circumstances” in Florida right now, it’s issued an over-the-air update that will allow all drivers to access the full 75 kWh until September 16. The move reportedly follows a driver contacting Tesla to say that the usual 200 miles range would leave him 30 miles or so short of leaving a mandatory evacuation zone.