said by Jack_in_VA:If you don't open it 8 hours is no problem.
For most refrigerators this is correct. Modern high-efficiency refrigerators tend to have better insulation and may stay cool even longer if the doors are kept closed.
The more important difference between old and new refrigerators (and freezers) is the time it takes to cool them back down. High-efficiency refrigerators have compressors that are sized to optimally maintain a cold temperature but take a very long time to cool down once the temperature inside has risen. Older (or current non-HE) refrigerators tend to have more powerful (less efficient) compressors that cool quickly.
This does have an impact on how you use your generator. The HE refrigerator needs you to run the generator most of the day but because of its smaller compressor can be used with other loads concurrently even on a small generator. The non-HE refrigerator has a significant start-up current but will be happy with a few hours of generator runtime allowing long pauses in between runs.