I use about 1kWh per 8h of fridge. So two fridges for 40 hours would be about 15% of battery, and 2 gallons is about 15% of your tank. But average power usag

I use about 1kWh per 8h of fridge. So two fridges for 40 hours would be about 15% of battery, and 2 gallons is about 15% of your tank. But average power usage of your Mac Pro would have been a lot higher since it would use power a lot more steadily than the fridge. And unplugging the fridge for a bit doesn't substantially change its power consumption. So you did better than me by however amount your Mac Pro used. Biggest advantage I had was I didn't have to open my garage door while it was powering the fridge.