OK, so you're talking about running air conditioning? You need a generator putting out 115v AC (alternating current) with sufficient power (in watts). For your size boat, you probably have one 12,000 BTU air conditioning unit, or similar, right? If so, a 2000 watt generator should be sufficient. Depends on other loads too. Major ones, albeit for a short term, would be recharging batteries, microwave, hair dryer, etc. Inventory your needs and quantify the required load, then determine what kind and size generator you'll need.