Get a 24 or 48 hour period, this should be the worst case time for an outdoor freezer, and it will surely draw more during the day than night. If you can't do an exact time period then a longer time is better than shorter.
Freezers are one of my most important items as I but meat by the case (Costco, USDA PRIME sirlion, under $7 a pound, less than half the cost of the USDA choice shit they sell at the grocery store) and get venison and sometimes other grass fed beef from a client also.
Bookmarks