Some of the water in the brew kettle is lost to steam during the boil. This changes the final volume of wort, and must be accounted for in your recipes. Every brewhouse and set of equipment has different boil-off rates, and it should be measured each time you change equipment.
The factors affecting boil-off are:
Boil rate – How vigorous your boil is. A slow boil loses less to steam, a rapid boil loses more. A rapid boil extracts more hop compounds.
Heat source – A stove generally has lower boil-off rate than a propane burner, and the type of propane burner has a big effect.
Kettle shape – A tall, narrow pot has a lower boil-off than a short, wide pot.
Covered pot – A covered or partially covered pot with lose less water than an uncovered pot, but the trade off is
DMS.
Ambient air – More water is lost in a dry environment, windy environment, and low temperatures.
Altitude – Altitude affects the boil temperature, which can change the other factors.
A good starting estimate for new equipment is 1.25 gallons/hour.