If you want to compare 2 water heaters, let them work at the same time, one near the other, but not to close! so one won't be a "wind shield" to the other.
Give them clean flat surface in the front 5 Meters, to avoid non-equal wind disturbances.
Of course, no shadowing on them while testing.
Mount them on the same declinment and direction to the sun, as recomended in your area.
At the begining, feel their tanks with cold water.
At the end, mix the water in every tank and mesure average temperature.
Initial cold water temperature minus the average temp' at the end is the comperative factor.