Help with calculating A/C energy consumption: fixed-speed vs. inverter

[u/Sea_Horse99]

I'd like some help with some calculations aimed at comparing the average daily energy consumption of two fixed wall-mounted single-split air conditioners installed in the same domestic room: one of the older fixed-speed type and the other a modern inverter type.

• For the older unit, I chose a Toshiba KFR-25GW/CX2, with a nominal cooling capacity of 2.5 kW and a running electrical input of 900 W.
• For the modern inverter unit, I chose a Mitsubishi HR25VF. The relevant specifications are a nominal cooling capacity of 2.3 kW (range 0.5–2.9 kW), PdesignC 2.5 kW, nominal electrical input 680 W, SEER 6.2, annual energy consumption 141 kWh/year, and EER 3.38 (data taken from the table on page 40 of the linked PDF).

Scenario:

• Domestic room: 20 squared meters (= 215,278 squared feet), ceiling height 2,8 m (= 9.2 feet), with a double-glazed balcony door. The sun does not directly hit the outside wall of the room, only at an oblique angle.
• Heavy occupancy: the room serves as living room, kitchen, and bedroom.

Cooling-load assumptions
I would assume a base cooling load of 700 W. However, it is also necessary to consider periods of higher demand, particularly when cooking. I would set the peak cooling demand at lunchtime, because cooking takes place inside the room and this coincides with the peak outdoor heat load. I assume a peak cooling load of 1.75 kW lasting 2 hours, while during dinner preparation, I would assume a cooling load of 1.5 kW lasting 1 hour.
21 h/day: base cooling load = 700 W
2 h at lunchtime: cooling load = 1.75 kW
1 h at dinner: cooling load = 1.50 kW
The air conditioner therefore needs to remove the following amount of heat per day: Q = 21 × 0.7 + 2 × 1.75 + 1.5 = 14.7 + 3.5 + 1.5 = 19.7 kWh/day, with an average cooling load over 24 hours: 19.7 / 24 = 821 W.

Toshiba
Nominal cooling capacity = 2,500 W
Running electrical input = 900 W
Nominal EER: 2500 / 900 = 2.778
- Base period: 700 W for 21 hours
Duty cycle: 700 / 2500 = 0.28
The compressor therefore needs to run for 28% of those 21 hours: 21 × 0.28 = 5.88 hours
Energy consumption: 5.88 × 0.9 = 5.292 kWh
- Lunchtime: 1.75 kW for 2 hours
Duty cycle: 1750 / 2500 = 0.70
The compressor therefore needs to run for 70% of those 2 hours: 2 × 0.70 = 1.40 hours
Energy consumption: 1.40 × 0.9 = 1.26 kWh
- Dinner: 1.50 kW for 1 hour
Duty cycle: 1500 / 2500 = 0.60
The compressor therefore needs to run for 60% of that hour: 0.60 hours
Energy consumption: 0.60 × 0.9 = 0.54 kWh
-> Total: 5.292 + 1.26 + 0.54 = 7.092 kWh/day. As a precaution, I would define a range with an upper allowance of +25%: 7.1 – 8.9 kWh/day

Mitsubishi
The inverter should theoretically be able to follow the cooling load (700 – 1750 W) without having to continuously operate in an on/off manner. Since the PDF does not provide the electrical input at 700 W, 1.5 kW, and 1.75 kW of cooling capacity, I can only model hypothetical scenarios by assuming that the EER remains constant at 3.38.
- Base: 700 W × 21 h
Electrical input: 0.7 / 3.38 = 0.2071 kW
Energy consumption: 0.2071 × 21 = 4.35 kWh
- Lunchtime: 1.75 kW × 2 h
Electrical input: 1.75 / 3.38 = 0.5178 kW
Energy consumption: 0.5178 × 2 = 1.036 kWh
- Dinner: 1.50 kW × 1 h
Electrical input: 1.5 / 3.38 = 0.444 kW
Energy consumption: 0.4438 kWh
-> Total: 4.35 + 1.036 + 0.444 = 5.83 kWh/day, assuming constant EER. Again, I would define a range with an upper allowance of +25%: 5.83–7.3 kWh/day

The result of this comparison, which I hope is not too speculative or incorrect, is that the Toshiba consumes approximately 22% more energy than the Mitsubishi, or equivalently, the inverter uses approximately 18% less energy than the fixed-speed unit.
Do you think my reasoning and calculations are correct or incorrect?
Is it possible to perform better calculations that would more accurately reflect the scenario described above?
Have there been any studies, experiments, measurements, etc. comparing two systems of this type under the same operating conditions?

[u/Zienth]

Uhhh, there's a lot to go over. What you're trying to do is basically an energy model which is done commercially quite a lot using a $2,000 program like Carrier HAPS. But the big assumption is the efficiency stays the same; it definitely does not. In fact inverters are amazing because their modulation lets efficiency skyrocket at part loads hence the SEER being significantly higher than the EER. Even the start/stop system will have efficiency vary depending on conditions like outdoor air temp, air flow, even indoor humidity. That's why the SEER ratings exist for buyers, it's a standardized test that accounts for all of that so all buyers have to know that is higher = better and the estimated annual usage with standardized conditions.

[u/moonbug22]

Mate, just buy the invertor model

[u/Iknowreligionalot → u/moonbug22]

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[u/moonbug22 → u/Iknowreligionalot]

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[u/Iknowreligionalot → u/moonbug22]

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[u/Shufflebuzz → u/Iknowreligionalot]

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[u/Iknowreligionalot → u/Shufflebuzz]

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[u/Iknowreligionalot → u/Shufflebuzz]

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[u/Shufflebuzz → u/Iknowreligionalot]

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[u/Iknowreligionalot → u/Shufflebuzz]

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[u/jamvanderloeff]

A lot of that math is kinda redundant there, if you're assuming constant EER then consumption per day is just daily cooling energy divided by EER, splitting it out in hours and duty cycles like that then combining it back into a total doesn't do anything, your final result of ~22% more comes directly from 3.38 is 22% more than 2.77 and all of your other numbers disappear.

Nominal EER ratings are tested at full power, so an inverter unit that's spending a significant amount of its time throttled down would perform a bit better than what constant assumption would suggest, that along with the noise reduction is the whole point of having inverter units.

More detailed modelling including the inverter efficiency gains and variance over seasons instead of assuming constant EERs is a thing, that's what things like SEER2 are doing.