I have a few general questions about central AC and some specific ones about pets.

I have a tenant who's complaining that her electric bill is too high.  Her monthly electric bill is around $150.  She said she spoke to friends and they live in houses and the bill is not that high.

The property is an older multi-family built in the 1950s, concrete block structure with a flat roof.  So I know as far as insulation it's far from perfect.  Furthermore I am in South Florida and it's HOT out.  Today is probably like 90 degrees with 200% humidity.

So here are some additional information.

My own house, about twice it's size, I am paying only $100 electric.  Although my house is well insulated, with a pitched roof and an insulated attic, and I set my thermostat such that it's 80 degrees during the day when no one is home, and 76 degrees in the evenings.

Her building has three units, her unit is an end unit, facing south.  The other units are similar sizes.  Her next door neighbor's bill is about $100 a month in the summer.  Although her next door neighbor is a single person, and does the same thing I do, set to 78 during the day, and 76 when home in the evenings.

The unit who is complaining about high electric bill, she sets her thermostat to 72 all day long.  Her and a boy friend, with two dogs, both 50 pounds.  One is an Australia Cattledog and the other a Labrador.  It is a two bedroom unit about 750 SF.

I went to the unit and the AC gets down to 74 and it can't seem to go lower any further, and hence the unit is running all day.  When they get home at night, its gets warmer.

The AC unit is a 2 ton unit, which according to the sizing chart, is appropriate for that size apartment.  Unit was installed in 2004, so about 11 years old.  Last serviced in May 2015, refrigerant fully charged then.

I looked up the inspection report I did eight months ago when I bought the unit, and inspector noted the temperature difference is 16 degrees and unit is operating as expected.

I called up the AC company who service all my units and they said about 18-22 degree difference is what should be expected on a brand new system, and the efficiency drops as the system gets older and the coils get dirty, or there is a leak in the refrigerant line.  When they serviced it in May, there was no leak.

Spoke to the tenant about raising the temperature being set to 72 all day, she said they feel 72 is not that low, and they have lived in places where they set it to 68 or lower and no issue.  As for leaving it at 72 all day, it is because she doesn't want her big dogs to get too hot.

I change the filters every month.  I can tell their filter is at least SEVERAL TIMES dirtier than the other tenants with no pet.  Often time the filter is matted down with pet hair.  All units in this building have the same brand/model units installed at the same time.  So we are comparing apples with apples.  I wonder how much the efficiency is affected by a dirtier filter due to the two big dogs?

The other issue is with a two bedroom apartment of 750SF, with two adults and two 50# dogs, is this oversized for this size unit?  Is it almost like 4 adults living in a two bedroom in terms of requirements for heating and cooling?  I normally do not allow pets but this tenant was inherited from the last owner so I made an exception on allowing the dogs.

My inclination is to advise them to try and optimize their thermostat during the days, and perhaps try changing out the filter every 15 days, and see if that makes a difference.

What I couldn't believe is how much dirtier the filters are with the two dogs.  I actually changed the filter one day went back after 7 days and checked it and it has MUCH MORE dirt and hair on it then a three month old filter in the other no pet apartment.

Also, are the modern day units engineered differently?  I seem to remember old units I have in the 80s and 90s were able to cool to 65 degrees when the outside is 95 degrees, but newer units I kept hearing all they can do is cool 18-22 degrees.

Sorry to be so long winded.