How are storage units cooled if they don't have electricity?

The building does the cooling, not the unit. Climate controlled units sit inside a building whose heating and cooling system runs on the facility’s own power, so your unit doesn’t need an outlet for it to work.

Extra Space explains that climate controlled storage means “the building’s HVAC system” keeps the space between 55 and 80 degrees. It adds that this “does not mean the storage unit has an electrical outlet.” Public Storage describes it as a building with an air system that may cool in warm weather, heat in cold weather, or both, depending on the facility.

Units that aren’t climate controlled usually aren’t cooled at all. U-Haul says its drive-up units are “typically not climate controlled, so temperatures and humidity inside the unit can change with the weather.” Some leases say it outright. One Virginia facility’s rental agreement states that “no heat or conditioned air will be provided.”

So there are two kinds of unit:

This is where power matters. If you want to run a fan or a dehumidifier in a unit that isn’t climate controlled, you need an outlet, and most units don’t have one. Ask the facility whether the unit is climate controlled, what range it aims for, and whether you can plug anything in.