That's a great question that we get a lot here. We recommend running a light bulb test. What this does is it helps you find what amount of wattage to run for the volume of space that it's heating. It's a cheap way to find the right amount of wattage.
Start by using a 40 watt light bulb and see if it reaches your desired temperature inside your incubator, if not move up the wattage until you find the right amount that doesn't shoot much higher than your desired temperature.
I don't recommend using a light bulb in your incubator though. They are bulky and they tend to have a higher heat density than other heat sources available. This means it stays hot longer after turning off and takes longer to heat up. This creates a wider temperature range, which generally leads to lower hatch rates.
For this reason many people prefer our
IncuKits. There are different models with varying wattage and all use elements that use low heat density heating elements. The IncuKit DC for example has 4 10-watt ceramic heaters. It's great for smaller incubators. It heats up quickly and cools down quickly, creating a tight temperature range which helps to increase your hatch rate. You can read more about them
here.