Merida’s heat has been building, and lately our upstairs bedroom/solarium has been getting hotter every night. March in Merida generally kick’s-off the annual HOT season here, with temperatures rising through April, to hit 104º – 115º F peaks in May. Mid-June often brings a little relief with the start of Rainy Season – as the daily afternoon rains come in as ~ cool fronts ~ which is a great contrast to Washington DC and the Eastern Seaboard, where afternoon summer showers mean steamy muggy nights of heat intensified by humidity.
Anyway, while lying in bed, sweating … trying to decide whether to turn on the clima, (AC), I started running the numbers in my head for ~ how much “AA” (“Aire Acondicionado“) costs to cool down our bedroom for one night. ~
Short Answer: As little as $4 pesos a night . . . or over $100 pesos a night, ~ depending on your bedroom, ~ your air conditioner, and ~your lifestyle
For better or worse, figuring out your PPN ($pesos por noche) means we have to know either 3 things, or a whole bunch of things:
~ At bedtime: Shut off everything in your house, go out and read your electric meter – XXXX kW-hr. Write it down. Turn on your A/C and run it in the usual way.
~ Next morning: Go read your electric meter – XXXX kW-hr. Write it down.
~ Subtract the bedtime reading from your morning reading to get you kW-hr per night.
~ Multiply that kW-hr of electricity used per night times your average CFE electricity rate ( $ pesos per kW-hr ) for that month to calculate how much it cost for that night of air-conditioning.
The Alternate Approach –
Let’s estimate typical A/C electrical usages from common factors: (It’s worth reading this method at least once, so you have an idea of just why your A/C bill is particularly large … or small => to give you pointers on how to get the most out of lowering your cooling bills.)
~ e.g. Just how big an effect is it to cool to 20ºC vs. 25ºC?
~ Is it worth retiring an old low-EER air conditioner, to buy a new high-EER “inverter” air conditioner? How long will it take to recover the cost of the new A/C?
First, we must consider the key factors – via answering some questions:
~ How many hours will you run the AA por dia?
~ How cool do you want the room? a.k.a. What temp to set the A/C?
~ How big is your room and your AC / AA ?
~ What season is it? ~ Hot season (April – Sept) has lower CFE rates, while CFE raises rates for the “Cool Season”. Hot season CFE rates for Merida can mean $0.6 peso per kW-hr rates versus $2.8 peso cool season rates => a 4X difference in what you pay to cool the bedroom . . . .
~ Do you live at the beach with cooler temps but higher CFE rates (less subsidies) “Tarifa Zona 1B” or in Merida’s hotter “Tarifa Zona 1C” with lower CFE rates ? This can add at least ~ another 2X higher rate difference to run the A/C at the beach than in Merida. ~
~ Do you live like a church-mouse, using very little power per month, staying in the cheapest rate category: Basico ($0.78 pesos/kW-hr) ? or do you use lots of power and pay high “DAC” (De Alto Consumo) rates for every kW-hr ($2.8 pesos/kW-hr)
(Sharp readers will note that high energy users lose the 78% government subsidies, and pay 4X more for the same power.)
~ How efficient is your AA ? and How big is your AC ?
Is it a big old inefficient ~ 2,600Watt ~ 12.2A ~ 24,000 BTU unit ?
This points to the Energy Efficiency Ratio (EER): How much cooling (BTUs) do you get for every Watt of electricity? Efficient modern AA’s have EER’s of “14” or higher.
A quick EER calculation of our old split A/C ~ 2,600Watt ~ 12.2A ~ 24,000 BTU unit, gives an EER of a very shabby ” 9.2 ” EER. ~ bleah ~ This means our old A/C uses 62% more electricity to get the same cooling as a newer 15 EER unit.
Continuing with the questions to know how many PPN ($Pesos Por Noche) a little relief will cost:
~ What duty cycle does your A/C use to keep that room cool for your current outside temperatures + window & door quality?
Duty Cycle is just a fancy way of saying that out of every 10 minute period, how many minutes is your A/C compressor running? The A/C fan only uses about 75W – 120W, so we really only car about the big 2,000 Watts that the compressor uses when running.
If your thermostat is set to a high temperature (like 25º C = 77ºF) then the Duty Cycle is likely short ( e.g. => 0.20 ) then your A/C compressor is only running 20% of the time.
If your thermostat is set to a low temperature (like 20º C = 68ºF), then an A/C in a poorly insulated room may have a high Duty Cycle (e.g. => 0.8) then your A/C compressor is running 80% of the time.
Notice that lowering the thermostat to 20ºC can quickly cost you 4X more than keeping it at 25ºC.
Long Story Made Short:
Our Data for Our Home and Our Style:
Our big old ~ 2,600 Watt ~ crummy low-EER A/C runs at a ~ 100% duty cycle ~ for about 1 hour to cool our hot 2’nd story solarium bedroom (Temp set at 23ºC), and then I get up and raise the thermostat to 26º C to get a modest ~ 25% duty cycle ~ for the rest of the night.
We turn on the A/C at 10:00 PM and then off at 7:00 AM => 10 hr of operation.
We live like church-mice – so we rarely go into the high rate “Excedente” and never into the “DAC” rate category, so our typical CFE rate is about $0.8 pesos per kW-hr.
=> 1 hr @ 100% x 2.6 kW x $0.8 pesos/kW-hr = $2 pesos (about 15 cents)
=> 9 hr @ 25% x 2.6 kW x $0.8 pesos/kW-hr = $4.7 pesos (about 36 cents)
=>$6.7 pesos a night to modestly cool our bedroom to 26ºC (79ºF).
=>$140 pesos a month to modestly cool our bedroom to 26ºC (79ºF).
VARIATIONS ~ Cooling to lower temperatures (20ºC) at higher CFE rates:
Dropping the thermostat to 20ºC increases the duty cycle by at least 4X.
Using more than the 175 kW-hr/month low-usage CFE category bounces the rates up 3.5X higher.
4X higher Duty Cycle x 3.5X higher category CFE rates => 14X higher nightly costs:
=>$94 pesos a night to heavy cool our bedroom to 20ºC (68ºF) at high CFE categories.
So, savvy readers realize that cooling to 20ºC by ~ heavy-energy user homes ~ during the cool season ~ can cost over $100 pesos a night . . . .
While it can cost ~ as little as $4 pesos a night ~to cool a bedroom with ~ a high EER A/C ~ set at 26ºC ~ for low-energy user homes ~ during the hot season …
Hint: Running a simple 75W floor fan that circulates air directly onto you while sleeping can allow you to set the air conditioner at 26ºC (78ºF) ~ and have it feel like 23ºC (73ºF) ~ while using 50% less electricity.
. . . And now you know what chemists think about as they drift off to sleep…
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Feel free to copy while giving proper attribution: YucaLandia/Surviving Yucatan.
© Steven M. Fry
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