Core answer: Electric power P = UI (watts = volts × amps). China's 220 V mains on a 10 A socket delivers at most 2,200 W; a 16 A air-conditioner circuit delivers 3,520 W. Your electricity bill counts energy, not power: 1 kWh ("one degree") = a 1,000 W appliance running for 1 hour ≈ ¥0.55 at residential rates.
Power, energy, and the bill
- Power (W) is the *rate* of consumption — what the nameplate says.
- Energy (kWh) is power × time — what the meter counts.
- Cost = kWh × tariff. A 1,500 W heater on 8 h/day uses 12 kWh/day ≈ ¥6.6/day ≈ ¥200/month — the reason winter bills shock people.
| Appliance | Typical power | 1 hour costs (¥0.55/kWh) |
|---|---|---|
| LED bulb | 10 W | ¥0.006 |
| Laptop | 60 W | ¥0.03 |
| Refrigerator (avg) | 100 W | ¥0.06 |
| Air conditioner (1.5P) | 1,100 W | ¥0.61 |
| Electric water heater | 2,000 W | ¥1.10 |
| Instant water heater | 6,000 W | ¥3.30 |
The three power formulas
P = UI = I²R = U²/R. The last one explains the danger of undervoltage regions: a 2,200 W/220 V heater on a sagging 180 V line draws only (180/220)² × 2,200 ≈ 1,470 W — lukewarm. And it explains transmission: sending power at 500 kV instead of 220 V cuts current ~2,300×, slashing I²R line losses to nearly nothing.
Worked examples
Example 1 — Can the strip handle it? A power strip marked "2,500 W MAX" feeding a 2,000 W heater + 600 W PC + 150 W monitor = 2,750 W — overloaded. The strip warms, contacts oxidize, fire risk rises. Move the heater to a wall socket.
Example 2 — Reading a breaker. A "C16" breaker trips around 16 A: 220 × 16 = 3,520 W. Running a 3,000 W water heater plus a 1,100 W kettle (4,100 W total) trips it within seconds-to-minutes. Split across two circuits.
Example 3 — Phantom load audit. A home with 15 W of standby (router, TV standby, chargers) burns 15 × 24 × 365 = 131 kWh/year ≈ ¥72 — worth a smart strip.
Common mistakes and myths
- Confusing W and Wh — watts are instantaneous; watt-hours are cumulative. A "100 W" bulb says nothing about the bill until you multiply by hours.
- Trusting peak vs rated power — appliance ads quote peak (e.g., 2,000 W AC "max"), the meaningful number is rated/average draw.
- Ignoring starting current — motors (fridge, AC compressor, pumps) pull 3–7× rated current for a second at startup; this is why breakers use "C-curve" (brief surge tolerant) rather than instant-trip.
- Thinking thicker cables give more power — cable size limits *safe current*; the load decides how much it draws. Oversized cable wastes money, undersized cable heats up.
- Believing "energy-saving modes" are magic — savings come from lower power or shorter run time; check the actual wattage with a plug-in meter.