I can answer from experience rather than comparison or speculation because I have done the very thing you describe, starting with a proof of concept using a window unit. This perhaps should be a comment; however, answering is the path to gain reputation to do that so here you go...
Verifying effectiveness on actual equipment
Other answers have covered instantaneous measurement in detail. Since evaporation happens rather quickly, just turning off the water supply while the compressor is running should be an effective measurement strategy. My observation in the POC was that the compressor current draw was most affected (more below).
For a season-long assessment — There are plenty of home metering systems for around $100 that use current transformers to measure individual circuits, showing consumption online or within an app. For example (not a recommendation)...
Your local historical weather data is readily available from wunderground.com or your own PWS if you have one. Your utility bill may also include "cooling degree days" or an equivalent standardized measurement of temperature over time.
I set up a very simple solenoid valve connected to the city water supply on my split system after the proof of concept. (An 18k unit, well out of warranty so I don't care as much if it caused rapid failure.) Recycling condensate posed too many practical challenges in my situation.
The somewhat surprising result — including to a professional HVAC tech who saw it — was that there has been no obviously accelerated deterioration of the condenser coil as a result of the water. It has been in place over more than four years. I clean it once per year without special equipment or chemicals, which is adequate but leaves room for improvement.
With chlorinated city water there was nothing additional needed to keep things from growing any more than they would without it. Commercial systems, I believe, use some kind of biocide. For this sort of setup also consider the common wisdom that sunlight is the best disinfectant (though algae like it).
Validating what (seems to be) a good idea - the POC
In the POC, dripping or spraying water on the condenser resulted in approximately 10% reduction in power draw from the window A/C unit. That was essentially current draw, with slightly improved power factor. I measured using an AC clamp meter, and also with a kill-a-watt. A relatively small amount of water was all that was required, and more did not result in greater gains, even when it was essentially flooded.
The window unit makes a good POC because:
- It did not incorporate condensate recycling / evaporation
- It runs flat-out, i.e. 100% power all the time
- Power requirements at ~500W are easy to measure (kill-a-watt)
- The size makes experiments easier
Some other notes:
- I made measurements in a variety of conditions (all within normal operation) and found essentially the same results.
- Water spray made essentially no change in the evaporator (inside) temperature. Presumably that was because the a thermostat attached to the evaporator coil switched the compressor on or off.
- The particular unit had a fixed opening (i.e. capillary tube) rather than more sophisticated modulation of refrigerant flow based on pressure or temperature, so there were no effects from that. (The TXV in a conventional split system modulates refrigerant flow based on pressure and temperature.)
There may be some differences for a rotary vane (scroll) compressor vs. the reciprocating compressor in the window unit, but that is beyond my knowledge.
Could this put you ahead by consuming water to reduce electricity?
A relatively small flow of water is required to keep the condenser wet, and it only flows while the unit is running. City water here costs $4.32 per 1,000 gallons. Electricity, as consumed, is about 11 cents/kwh (that means all per-unit charges: generation, transmission, delivery, metering, tax, others like fuel adjustments).
For a very rough estimate, assume:
- 0.1 gpm flow for the spray
- 10.4 RLA 240v compressor (actual 18k BTU compressor rating)
- 25% duty cycle (compressor runs 25% of the time)
- No compressor modulation (on or off, full RLA)
- Ignore blower and other consumption
- VA == watts (not true, but makes for easy math)
In 30 days, there will be 180 hours of compressor runtime, which will consume 411 kWh of electricity and will cost $45. In the same time, 0.1gpm will use 1080 gallons of water that will cost $4.67.
In sum, a more or less even trade of $4.67 worth of water to save $4.50 worth of electricity. I showed all of the math so anyone can repeat them with costs for their location and consumption for their equipment.
That number is useful input to calculations for re-using condensate where pumps consume electricity and chemical treatment is necessary to prevent scaling, corrosion, biocontamination, etc. Hopefully that will inform further measurements of flow rate, effects in other conditions, etc.