Reefers are refrigerated containers. There are numerous methods for providing the refridgeration ability within the footprint of a standard container, the most common being an electrical connection. This requires the container ship to provide a power plug, typically 30 amp 440-480 volt service, per reefer container. Where ventilation is not an issue, small diesel electric generators may be fitted in lieu of the electrical system. For ships without sufficient service or ventilation for these methods, for example, below decks, water or gaseous (cyrogenic) cooling may be employed, however these are less common. The carriage of reefers is appealing to owners, as refrigerated cargo tends to be particularly lucrative.