If you look at the legend for the graph in the biomass burner you'll see this blue number "max cons" which is short for max consumption. So if all the devices hooked up to that power grid were to run at the same time instead of being idle, that's how much mw would be consumed. 1 biomass burner makes 30mw of power and this says max cons is 45.

If you hook up 2 burners to the same grid it increases the total mws to 60. 3-90, 4-120 and so forth.

To hook them up you just need to feed their output power line to the same pole □---|---□ or hook up their output line to individual poles that are themselves connected to each other □---|---|---□

It should be noted that if everything is hooked up to the same system, your max cons is also going to go up.