When purchasing an outdoor heater, make sure it will be powerful enough to heat your space. Outdoor patio heaters that run on propane have a BTU number, used to measure thermal (heat) energy. You’ll need to know the number of square feet you want to heat.
According to homeair.org, you can calculate the number of BTUs (British Thermal Units) you need with this formula: The area in square feet times the current temperature in Fahrenheit minus the desired temperature. That gives you the minimum BTUs you should have. Electric heaters measure heating power in watts. One watt of electricity produces 3.4 BTUs, so 10 watts of power equals 34 BTUs. (Yes, there’s math involved, but it’s easy!)
This powerful kerosene heater projects 23,800 BTUs in all directions, making it a great choice for heating large open-air areas.