What is the highest efficiency of HHO generators you have come across?
Efficiency is the energy input divided by the energy output multiplied by 100
You can test the input energy by measuring voltage, and current across the entire system in volts, and amps respectively, and timing how long the system is on from time = 0 seconds being the time the system is turned on, and simply multiplying them by each other; Energy = (voltage)*(current)*(time). (The greater the time measured the greater the accuracy).
To simultaneously get the energy output you need to hook up the generator in such a way that you can measure how much HHO is being generated. The amount of HHO you generate after you turn the timer off, say in Liters, can be converted to energy because there are 55.5 mols of water per liter of water, and 286kJ*55.5= 15873kJ are needed to generate 55.5 mols of HHO gas. Multiply your moles by 286kJ to get your energy output (how much energy you'll get back from combusting the HHO back into water).
Then use the efficiency equation I stated above
It should look something like this
Energy input =24V*40A*300s=288kJ
Energy output = 15 milli Liters * (55.5 mols/ Liter) * (286kJ/mol) = 238kJ