How much heating power is delivered to a load?

There is no specific ratio, as it depends on the pressure and temperature of the steam.

Steam is merely water vapor, at a temperature above the boiling point of water (212 degrees F., 100 degrees C.). When water boils, and turns to steam, the steam will occupy a volume 1846 times that of the water that was converted -- at atmospheric pressure, and at the boiling point of water.

Such steam is useful in cooking, for example, to steam vegetables. But in general, it is not very useful at this low pressure. Steam used in industrial processes or to drive engines or turbines, is at much higher pressures. Those pressures are obtained by vaporizing water in a closed vessel, so that the steam is allowed to occupy a volume much less than the 1846 times ratio just noted.

Steam in closed vessels can build up to enormous pressures, which ultimately can rupture the vessel. Fragments ejected as shrapnel from such an explosion can have more striking power than a bullet or a cannon shell. Perhaps worse, the released steam contains enormous energy, which can for example scald a human to death almost instantly.

For that reason, steam generators are provided with valves which release the steam when a desired pressure is reached. They are provided also with blowoff valves, which open at a somewhat higher pressure, in the event that the orifice or pipe through which steam is withdrawn may not be of sufficient size. Such blowoff valves vent the steam chamber into the atmosphere, or into a condensation chamber. The steam vessels are designed to withstand a pressure usually twice or more the level of pressure that triggers the blowoff valve; this is a safety measure.

Because the ratio of water to steam varies, according to pressure and temperature of the steam (and whether it is "wet" steam or "dry" steam), it is more common to speak of pounds of steam. This is the weight of the water converted into steam (of the desired pressure and temperature) in a given time, such as pounds per hour. Water, as you may know, weighs 8.33 pounds per gallon.

Knowing the weight of the steam is important, for example in running a steam turbine. The weight of the steam, and its velocity, determine the momentum of the steam; and that can be translated into the force or torque applied to the turbine rotor, thus the output of the turbine. That in turn translates into how many kw. of electricity the turbine-generator can produce per hour. Such computations actually get pretty complicated, but you can see that there is a relationship between the amount of water converted into steam, and the output of an engine or turbine or generator which is driven by that steam.

There is a specific field of engineering (and physics) which deals with steam. It is called Thermodynamics, It is essential in designing steam boilers and steam-powered equipment.