Air conditioningInverter vs Fixed-speed AC: How They Work and What You Really Save
In summer the electricity bill is often the biggest household expense, and air conditioning tops the list. Does an inverter unit really save money? Here is the full breakdown.
1. How the compressor drives consumption
- Fixed-speed AC: the compressor is either fully on or off. Each restart draws a large inrush current, which is the main source of waste.
- Inverter AC: once the target temperature is reached the compressor slows down instead of stopping, running continuously at low power.
2. Real cost comparison (120 summer days)
| AC type | Average power | Daily usage (kWh) | Summer cost (at $3.5/kWh) |
|---|
| Traditional fixed-speed | approx. 800 W | 6.4 kWh | NT$ 2,688 / HK$ 1,075 |
| Top-rated inverter | approx. 350 W | 2.8 kWh | NT$ 1,176 / HK$ 470 |
💡 Three expert AC saving tips
- Set 26-28°C with a circulation fan: every 1°C higher saves about 6% of AC power.
- Avoid frequent on/off cycles: if you leave for 1-2 hours, keeping an inverter running uses less energy.
- Clean the filters every two weeks to regain 10%+ heat exchange efficiency.
3. Reading efficiency labels: CSPF, APF and energy grades
Taiwan rates air conditioners with CSPF, Japan uses APF, and Hong Kong uses a 1–5 grade energy label. Higher numbers mean less electricity for the same cooling output. For a 3.6kW (about 1.5 ton) unit running 8 hours a day across a 120-day summer, a CSPF 5.0 model uses roughly 250–320 kWh less than a CSPF 3.5 model. At 0.35 per kWh that is close to 110 per summer, so the price premium of an efficient unit is typically repaid in three to four years. Efficiency is a cost you pay every year; the purchase premium you pay only once.
4. Sizing matters: both undersized and oversized units waste power
An undersized unit runs at full compressor speed for hours and never down-modulates, so it can use more energy than a correctly sized one. An oversized unit reaches setpoint too quickly, short-cycles, dehumidifies poorly and leaves the room feeling clammy. A common rule of thumb is 450–500 kcal/h per 3.3 m², plus 20–30% for west-facing rooms, top floors or large glazing.
- Small bedroom (10–16 m²): ~2.2kW, typically 500–700 W draw
- Master bedroom (20–26 m²): ~3.6kW, typically 700–1,000 W
- Living room (33–46 m²): ~5.0kW, typically 1,200–1,600 W
- High ceilings or heavy sun: add 20–30% capacity
5. Is dry mode really cheaper than cooling?
Dry mode runs the fan slowly for a long time to condense moisture. When the room is already hot it simply runs longer, so total consumption is not necessarily lower. Dry mode is a good fit for humid but mild days below 28°C; in peak summer, cooling at 26–28°C with a circulation fan remains the most efficient option.
6. Verify your own bill with the calculator
Add the air conditioner on the home page, change the wattage to the rated input on your unit's nameplate and set the real daily runtime. The calculator converts that into daily kWh and a monthly cost instantly, and the time-of-use card on the right shows how much shifting laundry or EV charging to off-peak hours would save.
Monthly cost of common air-conditioning scenarios (0.35 per kWh)| Scenario | Average draw | Hours/day | kWh/month | Monthly cost |
|---|
| Bedroom inverter AC (night) | 450 W | 8 h | 108 | ≈ 37.8 |
| Living room inverter AC | 1,100 W | 6 h | 198 | ≈ 69.3 |
| Old fixed-speed AC | 900 W | 8 h | 216 | ≈ 75.6 |
| AC + fan (1°C warmer) | 420 W | 8 h | 101 | ≈ 35.3 |
Frequently asked questions
- Should I leave an inverter AC running all night?
- For continuous use, holding a steady temperature beats frequent restarts. If you will be away for more than two hours, switching off still wins.
- Do plug-in AC energy savers work?
- Most simply throttle compressor output, which is equivalent to raising the setpoint by 1–2°C. Adjusting the thermostat costs nothing.
- Why is my real bill higher than the estimate?
- Most regions use tiered tariffs where the unit price rises with consumption. This tool uses an average rate, so heavy users may see a slightly higher bill.
Key takeaways
- Efficiency ratings drive long-term cost; the premium usually pays back in 3–4 years.
- Correct sizing matters — too big and too small both waste energy.
- 26–28°C plus a circulation fan is the cheapest saving you can make.
Three Hidden Power Hogs in Your Kitchen
Beyond air conditioning, the kitchen hides appliances that draw power 24/7. Adjusting a few habits can save hundreds a year.
1. Storage water heater
It reheats the tank around the clock even when unused. Add a timer so it only heats an hour before shower time.
2. Old single/double-door fridge
Aging compressors and worn door seals leak cold air. Keep the fridge about 70% full so air can circulate.
3. Hot water dispensers
Continuous keep-warm reheating is expensive. A kettle plus a vacuum flask can cut this by up to 70%.
4. Microwave vs air fryer vs oven
A microwave puts energy directly into the water in food and is the most efficient reheating method; an air fryer heats a small chamber and comes second; a conventional oven must preheat a large cavity and is the most expensive. Reheating a 300 g portion costs roughly 0.02 kWh in a microwave, 0.15 kWh in an air fryer and 0.35 kWh in an oven.
5. Fridge placement and settings
Leave 5–10 cm behind the fridge and at least 2 cm at the sides. Pushed against a wall, the condenser runs hotter and the compressor runs longer — easily 100 kWh extra per year. Keep the fridge at 3–5°C and the freezer at -18°C; colder settings do not preserve food better. A fridge works best about 70% full, while a freezer is more efficient when nearly full.
6. How much do vampire loads really cost?
Microwave panels, induction hobs, oven clocks and dishwasher displays each draw 1–3 W on standby. A typical household has 15–25 such devices, adding up to 150–400 kWh a year — about a third of a fridge's annual use. Put rarely used kitchen gadgets on a switched power strip.
- Timer on a storage water heater: saves 300–700 kWh/year
- Kettle + vacuum flask instead of a hot water dispenser: 400–900 kWh/year
- Replacing a 15-year-old fridge: 300–600 kWh/year
- Managing standby loads: 150–400 kWh/year
Annual kitchen appliance consumption (0.35 per kWh)| Appliance | Average draw | Equivalent hours/day | kWh/year | Cost/year |
|---|
| Storage water heater | 1,500 W | 2.0 h | 1,095 | ≈ 383 |
| Fridge, 10+ years old | 150 W | 10 h | 548 | ≈ 192 |
| Modern inverter fridge | 80 W | 10 h | 292 | ≈ 102 |
| Hot water dispenser | 120 W | 12 h | 526 | ≈ 184 |
| Dishwasher (daily) | 1,200 W | 1.0 h | 438 | ≈ 153 |
Frequently asked questions
- Do I need to defrost my fridge?
- For non-frost-free models, defrost twice a year. Frost thicker than 5 mm raises consumption by 10–20%.
- Is a dishwasher cheaper than washing by hand?
- A full load uses about a third of the water of hand washing but more electricity. Eco mode during off-peak hours is the cheapest combination.
Key takeaways
- Always-on heating and cooling appliances cost far more than high-wattage short-use ones.
- Ventilation clearance and sensible temperature settings are free savings.
- Standby loads add up to 150–400 kWh a year.
Do Off-peak Rates Really Save Money? A Time-of-Use Guide
Many utilities offer time-of-use tariffs to balance day and night demand. Is it worth switching for a typical household?
Three households that benefit most
- EV owners: overnight charging cuts a large share of the annual charging cost.
- Night owls and shift workers: most usage falls between 00:00 and 09:00.
- Households that schedule laundry: run washers and dishwashers overnight on a timer.
Off-peak windows around the world
Hong Kong utilities are mainly tiered, with off-peak options for EV and commercial plans; Taipower's residential time-of-use off-peak window is usually 22:30–07:30; most US TOU plans put off-peak from 21:00 to 15:00 the next day; the UK Economy 7 tariff offers cheap power from 00:30 to 07:30; Japanese night plans typically run 23:00–07:00. Always confirm with your utility — the advisor card on the home page applies the parameters for your detected region automatically.
How to tell whether switching pays
It comes down to how much of your consumption is shiftable. If more than 30% of monthly kWh can move to off-peak hours — EV charging, laundry, dishwasher, storage water heater, dehumidifier — a TOU tariff usually wins. If someone is home all day with flat consumption, the higher peak rate can increase your bill instead.
- Record kWh and cost from the last six bills
- Build your appliance list in the calculator
- Shiftable share above 30%: TOU is usually worth it
- Shiftable share below 15%: stay on the tiered tariff
Timers and smart plugs make it real
Savings only appear if the load actually moves. Washers and dishwashers have delay-start; water heaters accept a mechanical or Wi-Fi timer; EV chargers schedule in their app; dehumidifiers and purifiers suit smart plugs. Stagger start times about 30 minutes after off-peak begins so several appliances do not surge together.
Off-peak windows and indicative discounts| Region | Typical off-peak | Discount | Best shiftable loads |
|---|
| Hong Kong | 23:00–07:00 (select plans) | 10–20% | EV charging, water heater |
| Taiwan | 22:30–07:30 | 30–45% | Laundry, dehumidifier, EV |
| United States | 21:00–15:00 (varies) | 30–50% | EV, pre-cooling |
| United Kingdom | 00:30–07:30 (Economy 7) | 40–55% | Storage heating, hot water |
| Japan | 23:00–07:00 | 25–40% | Heat-pump water heater, laundry |
Frequently asked questions
- Do I need a new meter?
- Most regions require a smart meter, installed by the utility, usually free or for a small fee on residential plans.
- Can I switch back?
- Yes, though utilities often require 6–12 months on the plan before changing. Run the numbers first.
- How is the saving on this site calculated?
- We take the shiftable appliances in your list, multiply their monthly kWh by the regional off-peak discount factor, and report a conservative estimate.
Key takeaways
- TOU pays off when more than about 30% of your load is shiftable.
- Timers and smart plugs are what turn the tariff into real savings.
- Off-peak windows vary widely — confirm with your utility.