I am struggling to get my head around what you guys are talking about. If you connect a 100W light bulb to a service for 10 hours, that kWh is consumed. You cannot "recycle" that energy or "return" the power that the light is consuming to the service. I guess you could set up a PV module and inverter to harvest a little of the light bulb's output and return it to the grid, but it would be a minuscule amount compared to the consumption by the light. What's the point?
The issue is that this is a test environment where you want to move full power through a device that doesn't actually consume much power.
If you were testing lightbulbs, then your point is spot on. You don't actually _need_ the light being produced, you just want to demonstrate that the bulbs produce so much light given so much energy input for so many hours. In theory you could recycle some of the output, but as a practical matter it isn't worth it.
Again, this is only applicable in a situation where power moves through a device, but you don't actually _need_ the output. You need to run the power through the device to test it, and your concern is properly testing the device. You are not an end user with the device in service.
If you are testing something that is very efficient, and the output is some form of high quality energy, then there is reason to 'recycle'.
For example you want to test out a battery system. Your job is to charge the batter up and measure how much energy is required to do this, and then you have to discharge the battery and measure how much energy it can supply. The simple way to do this is to use your standard wall powered charger to store the energy, and then use a resistor as a load for the discharge. You don't actually need the output of the battery, you don't need the heat that the resistor would produce, you just want to test the battery.
Recycling this 'test' energy is the point of the discussion.
In any situation where the device under test doesn't actually _use_ most of the power being moved through the device, and you don't actually need the output of the device, then recycling the output to the input is possible and potentially valuable.
For another example, imagine that you need to test a 75kVA 480:208V transformer. For purpose of the test you have to supply 75kVA (and 65kW) to some load, but it can be any load of suitable characteristic. You only care that 75kVA and 65kW move through the transformer from primary to secondary. You don't need the output of the transformer, in fact you prefer an artificial regulated load that maintains exactly the required experiment values. You will save lots of $$ if this artificial load is some gizmo that returns that 65kW to your 480V supply bus.
-Jon
