Experimented today with a 4.0 Ah 18 volt Li-ion battery, a 12 volt/10A DC/DC inverter powered by the battery and a 9-litre 12 volt fridge-freezer to see how long it would run on the most energy-intensive setting. The temperature started at 14ºC and within 25 minutes had dropped to its coldest, -20ºC. At this point it was empty. I then added 3 litres of milk (one litre being at room temperature - safe, it's a UHT product) and ½ a litre of ice cream. The temperature increased to -13ºC before stabilising at -14ºC. Two hours after being connected the battery was discharged. If I chose a lower input power setting (I had been using Max instead of Eco) and only try for refrigeration temperatures instead of freezer ones, I could reasonably expect 6 to 7 hours of battery life.