The answer to this question will help you decide what size dehumidifier do I need. If you have extremely high dampness levels in your basement, you will definitely need a larger dehumidifying device than if you have very low humidity levels in your basement. Also, keep in mind that air temperatures can vary greatly from day to day and even from season to season. A basement that has no ventilation at all is likely to have extremely high humidity levels, so you will want to get a dehumidifier that has a high-efficiency rate and is powered by a high amperage electrical system.