I’m sure some of you remember the days when an air conditioner was considered a luxury, and even if you could afford one, it was often used frugally (mom yells, “who left the friggin’ AC on??”). Well, thanks to the steady advancement of science, air conditioners are not only more affordable than ever, they’re becoming increasingly efficient to run.
Unlike colder, resource-rich countries, most Japanese homes don’t have central heating. In fact, it’s practically unheard of except the far north of the country (Tohoku, Hokkaido). We rely primarily on space heaters to keep our rooms warm during the winter.
Space heaters come in many shapes and sizes, and can run on kerosene, natural gas (toshi gas; 都市ガス), or electricity (resistance or heat pump). All 3 have been known to be cost-competitive with each other at various times due to price-fluctuation of the energy in question. The exception to this is the electric heat pump.
Heat Pumps and Modern AC units
Conventional air conditioners and heaters use heat pumps that utilize the thermal properties of refrigerants to extract or reject heat for the purpose of heating or cooling an interior space. We often use the term COP, or “coefficient of performance”, to describe the ratio between heat supplied or removed to the work required.
Simply put, normal kerosene or gas heaters have a COP of 1 – the most basic form of heating where 1 unit of heat produced requires 1 unit of work. Not very efficient at all. Heat pumps on the other hand, equip a modern air conditioner with a COP of 3 to 6, depending on outside temperature.
Initial Cost Vs Long-term Running Cost
As wonderfully efficient these machines are, the initial purchase-cost put them out of reach for many lower income households until fairly recently. Not to mention, older heat pump COP figures didn’t have the impressive numbers they have today, making them that much less attractive for folks contemplating an investment approach (forking out initially in the hope of making back the cost in energy-savings).
However, today the scales are certainly tipped in favor of electric air conditioners – in more ways than one too.
– Cheaper to run. Superior COP values all but eliminate the initial cost factor due to the literal guarantee (new AC units generally come with 5, sometimes 7 year guarantees) you will make your money back in energy savings.
– Cleaner in terms of interior air quality. No more having to crack the window for ventilation so you don’t accidentally kill your whole family.
– No more weekly trips to the gas stand to buy 20 jerry cans of kerosene, as well as having to refill the heater tanks.
– Neater and less cluttered, as the AC can be mounted on the wall as opposed to a kerosene heater sitting on the floor for people to trip over.
It wouldn’t be fair to list only the pros, so for the purpose of comprehensiveness I’ll list the 2 main things that take away from electric ACs:
– If used for extended periods of time, it can make the room quite dry. Running a humidifier helps.
– Can struggle to heat the floor level of the room during colder temperatures due to its location and the propensity of heat to rise, causing the feet and lower leg area to remain cold even when the upper body area might be at a comfortable temperature. Using an electric carpet or underfloor heating along with your AC will solve this problem.
Ways to Calculate COP and Energy-efficiency of an AC unit
To determine COP, divide the rated output capacity (能力; nouryoku) by the rated electric consumption value (消費電力; shouhidenryoku).
Another method utilizes a label that proactively displays the total number of kilowatt-hours used in a typical year (期間消費電力), based on the national industrial standard (日本冷凍工業規格). This number sometimes specifies the heating and the cooling value separately, making it easier to run fairly accurate calculations based on your own situation and needs.
(Note that this label isn’t always on display or readily available, leaving you with the first method above.)
This number is pretty straight-forward, and can be simply multiplied by your current electric power rate (per kWh) to give you an idea of how much it’ll cost you throughout the year. Just keep in mind that the heating and cooling functions generally have different consumption values.
The national industrial standard uses the following criteria:
Interior temperature setting: 27 degrees for cooler; 20 degrees for heater.
Seasons: May 23rd to October 4th for cooler; November 8th to April 16th for heater.
Hours: 6:00 to 24:00. 18 hours/day.
Structure: Generic wooden house, facing south as per JIS C9612.
Room size: The size that matches the AC unit being used. AC units in Japan are rated as “for X number of mats” (for example, 6畳用, or for 6 mats).
The rated output capacity (in kWs) of an air conditioner is directly related to its “mat size”. Also, rated capacity will differ between heater and cooler functions. A 6 mat AC’s rated capacity will go something like this: Cooler capacity (冷房能力): 2.2 kW; Heater capacity (暖房能力): 2.5 kW.
APF (annual performance factor)
The annual performance factor is a number used to evaluate and rate an AC unit’s energy-efficiency over an annual period and has only been in use here in Japan since 2007. It is defined as the quotient of the required annual energy output and the rated annual consumption (期間消費電力) – both in kWhs.
The required energy output is determined by the rated capacity (能力) of the AC in question, as its capacity is directly related to the area it can heat or cool. This capacity is used to calculate the amount of energy that is then required to heat or cool said area to a predetermined temperature over an annual period.
In short, the APF is a slightly more complicated version of COP, but it is useful in that the calculation has already been done for you.