There’s really a lot of variables that you need to test, so I doubt you can run a successful test over just one summer!
Weather
Wind
Humidity
Machine Efficiency (power, clean, age…)
Temps vary, too: machine temp, ambient temp, apparent temp,
Not suggesting that you don’t try, but suggesting that you try to isolate one or two obvious variables and then test them exclusively over a longer period. With day to day fluctuations in humidity/temperature/wind, it’s hard to see what might just be an occasional blip.
Other factors you may want to look at: eliminating or reducing unnecessary heat sources within the apartment from: sun-exposed windows or walls, cooking, hot light bulbs (esp. spotlights with 50 watts or more), keeping doors closed, eliminating draughts, etc.
Ultimately, though we just replaced an older pair of a/c machines with Daikin machines and that cut our bills considerably! Cleaning the machines also helps, not just cosmetic cleans, getting rid of the gunk from inside them! Our school bills went from $44K for 1 year to just over $27K, and we are NOT shy about running our a/c. That alone was worth the money we saved. The machines (approx. 110K) will probably pay for themselves in additional savings in just 6.5 years!
But also don’t overlook other electricity savings from non-AC sources, as well. Lights that are on for extended periods can really push electricity bills into higher tariffs… 😀