Khái niệm cơ bản về điện: Watts, Ampe, kWh và giới hạn mạch

Mọi hướng dẫn thiết bị trên WattSizing giả định bạn chuyển đổi giữa watt, amp và kWh mà không nhầm công suất tức thời với năng lượng theo thời gian.

Hướng dẫn điện cơ bản này là lớp toán sau máy tính: quan hệ kiểu Ohm ở 120 V và 230 V, giới hạn mạch, và công thức kWh cấp hóa đơn.

Sai sẽ nhảy aptomat (W quá lớn trên dây 15 A) hoặc pin quá nhỏ (coi W là kWh không có giờ).

Để thiết kế off-grid cho cả nhà, xem Hướng dẫn hệ solar off-grid 2026. Lập danh sách tải tại Cách Lập Danh Sách Tải Để Định Cỡ Hệ Mặt Trời Off-Grid, rồi mô hình hóa đỉnh tải tại WattSizing Calculator.

Watts vs watt-hours vs kilowatt-hours

W = instant power. Wh = W × h. kWh = Wh ÷ 1,000. Utility bills use kWh. Inverters list W continuous; batteries store Wh/kWh.

Battery analogy: A 1,000 Wh battery driving a 500 W load lasts 2 hours in ideal math—in practice inverter loss and Peukert effects shrink that.

Demand vs energy: Utilities may bill kWh plus demand kW on commercial rates—residential usually kWh only, but TOU changes when kWh is expensive.

Hose analogy extended: W = flow rate (gal/min); kWh = total gallons in the bucket. A low-flow faucet left on overnight fills a big bucket—high flow briefly may not.

Appliance labels: Light bulb lists W (rate); utility bill lists kWh (total). Inverter lists W max; battery lists Wh storage.

Generator shopping trap: “3500 W generator” is a power cap; it does not mean “3500 kWh of fuel”—fuel is energy stored in gasoline.

Battery: “2 kWh battery” at 500 W load → 4 h ideal, ~3.4 h after inverter loss—always separate storage kWh from load W.

Capacity market vs home: Industrial demand charges use kW peak—homes rarely; still useful if you add large EV and utility offers demand rate.

Carbon: kWh × grid factor = emissions—power factor does not change kWh on residential meter.

Generator shopping trap: “3500 W generator” is a power cap; it does not mean “3500 kWh of fuel”—fuel is energy stored in gasoline.

Battery: “2 kWh battery” at 500 W load → 4 h ideal, ~3.4 h after inverter loss—always separate storage kWh from load W.

Capacity market vs home: Industrial demand charges use kW peak—homes rarely; still useful if you add large EV and utility offers demand rate.

Carbon: kWh × grid factor = emissions—power factor does not change kWh on residential meter.

Tariff time-of-use: kWh cost changes by hour; W does not—schedule EV and dishwasher on off-peak if your rate rewards it.

Carbon accounting: kWh × grid intensity—power factor irrelevant on residential bill.

2,000 W at 120 V → amps

I = P ÷ V = 2000 ÷ 120 ≈ 16.7 A—exceeds 15 A circuit continuous practice. Needs 20 A breaker and wiring.

Wire check: 16.7 A on 14 AWG 15 A circuit is over continuous practice—use 12 AWG 20 A for dedicated 2,000 W loads (kettle, space heater).

Breaker trip: Magnetic trip may hold brief overage; thermal trip accumulates heat over minutes.

Cord length: 16.7 A at 120 V end of 50 ft 14 AWG extension may see 112 V at load—device pulls higher amps to deliver same W heat in kettle.

Dual appliances: Two 1,000 W devices on one 20 A circuit: 16.7 A combined—legal continuous if under 16 A 80% rule on 20 A.

Cord length: 16.7 A at 120 V end of 50 ft 14 AWG extension may see 112 V at load—device pulls higher amps to deliver same W heat in kettle.

Dual appliances: Two 1,000 W devices on one 20 A circuit: 16.7 A combined—legal continuous if under 16 A 80% rule on 20 A.

Space heater cord: 14 AWG 25 ft on 1,500 W is marginal—voltage drop feels like "weak heat" but draws more amps at plug.

3,000 W at 230 V → amps

3000 ÷ 230 ≈ 13.0 A—fits many 16 A EU breakers with derating rules; still verify local code.

EU socket limit: Many circuits 16 A × 230 V = 3,680 W theoretical—3,000 W kettle + margin fits with correct breaker; UK ring circuits different—verify local code.

Three-phase: Not covered here—single-phase P = V × I only.

UK ring final: Ring circuits share load—do not apply simple I=P/V without local installer guidance.

Camping EU: 3 kW site limit common—13 A fuse at 230 V ≈ 2,990 W hard cap.

UK ring final: Ring circuits share load—do not apply simple I=P/V without local installer guidance.

Camping EU: 3 kW site limit common—13 A fuse at 230 V ≈ 2,990 W hard cap.

UK kettle: 3 kW 230 V 13 A fused plug common—13×230=2990 W design point.

15 A circuit watt limit

15 × 120 = 1,800 W theoretical; 80% continuous ≈ 1,440 W planning figure for resistive loads.

NEC 80% rule: Continuous loads over 3 hours should not exceed 80% of breaker rating—15 A × 0.8 × 120 V = 1,440 W planning figure.

Voltage drop: Long 14 AWG runs reduce voltage at the load—motors draw more amps for the same work.

Two-kettle mistake: 1,500 W + 1,500 W = 3,000 W → 25 A on 15 A breaker—instant trip even if each kettle alone is "legal" alone on separate circuits.

Motor exception: 15 A circuit may start LRA 80 A briefly if breaker curve allows—still not continuous.

Christmas lights: 20 strings × 40 W = 800 W plus 1,200 W space heater on same 15 A room circuit—trips.

Refrigerator exception: NEC dedicated circuit rules exist because motor + heat loads are common kitchen mistakes.

Plug strip fire: Six-outlet strip with space heater + printer + monitor can exceed strip rating even if 15 A wall breaker has not tripped yet.

AFCI nuisance: Arc-fault breakers on bedroom circuits trip on damaged lamp cords—unrelated to watt math but common confusion.

Christmas lights: 20 strings × 40 W = 800 W plus 1,200 W space heater on same 15 A room circuit—trips.

Refrigerator exception: NEC dedicated circuit rules exist because motor + heat loads are common kitchen mistakes.

Plug strip fire: Six-outlet strip with space heater + printer + monitor can exceed strip rating even if 15 A wall breaker has not tripped yet.

AFCI nuisance: Arc-fault breakers on bedroom circuits trip on damaged lamp cords—unrelated to watt math but common confusion.

20 A circuit watt limit

20 × 120 = 2,400 W; 80% ≈ 1,920 W continuous planning.

Kitchen dual appliance: 1,920 W continuous budget allows one 1,500 W heater plus 400 W margin—not heater plus microwave together.

Dedicated 20 A: Bathroom, laundry, and kitchen counter often 20 A in modern NEC homes.

Workshop: 1,800 W table saw on 20 A dedicated is correct; adding shop vac on same circuit while cutting trips thermal breaker.

Kitchen islands: Often 20 A with two small appliance circuits—still cannot run two 1,500 W heat devices on one receptacle chain simultaneously.

Workshop: 1,800 W table saw on 20 A dedicated is correct; adding shop vac on same circuit while cutting trips thermal breaker.

Kitchen islands: Often 20 A with two small appliance circuits—still cannot run two 1,500 W heat devices on one receptacle chain simultaneously.

Two-tool garage: 1,800 W saw plus 1,200 W shop vac on 20 A circuit trips if both start together—stagger or separate circuits.

Watts from amps and volts

P = V × I (resistive DC approximation). AC with power factor: P = V × I × PF. Motors may draw amps without delivering full real W—use meter when PF < 1.

Clamp meter: Read amps on one conductor × voltage × PF = watts real. PF 0.7 motor at 10 A 120 V → 840 W real, not 1,200 W.

HP conversion: 1 HP motor ≈ 746 W output; 1 HP nameplate draw often 900–1,200 W input at load.

RV 30 A service: 30 A × 120 V = 3,600 W on single-leg RV panels—not 7,200 W—read RV wiring before loading.

Inverter chargers: May show amps DC and amps AC on different screens—do not mix without conversion.

RV 30 A service: 30 A × 120 V = 3,600 W on single-leg RV panels—not 7,200 W—read RV wiring before loading.

Inverter chargers: May show amps DC and amps AC on different screens—do not mix without conversion.

Motor example: 10 A at 120 V with PF 0.65 → 780 W real, not 1,200 W—clamp meters with PF display help.

kWh from watts and hours

kWh = (W × hours) ÷ 1,000. Example: 1,500 W × 2 h = 3 kWh. Minutes: convert to hours first (15 min = 0.25 h).

Partial hour: 900 W microwave 4 min → 900 × (4/60) ÷ 1000 = **0.06 kWh**.

Monthly stack: Ten devices—sum each (W × h/day) ÷ 1000 for whole-home kWh/day estimate before solar sizing.

Blended rate: If utility is $0.12 off-peak and $0.28 on-peak, kWh math is still W×h—cost weighting is separate spreadsheet.

Standby phantom: 15 devices × 5 W × 24 h = 1.8 kWh/day—5 × 24 ÷ 1000 per device adds up.

Weekly habit: (W × hours/week) ÷ 1000 for laundry, (W × hours/day × 7) ÷ 1000 for fridge—mixing units causes 10× errors.

Power meter: Kill A Watt style devices integrate kWh directly—skip hand math for unknown appliances.

Practice worksheet: (1) List appliance W from nameplate. (2) Estimate hours/day. (3) Multiply and sum to kWh/day. (4) Separately list largest single W and largest motor surge—battery sizing uses step 3; inverter uses steps 4.

Common exam mistake: “2000 W heater for 24 h = 48 kWh” confuses running W with energy—correct is 48 kWh only if the heater is fully on all day; thermostats cycle.

Blended rate: If utility is $0.12 off-peak and $0.28 on-peak, kWh math is still W×h—cost weighting is separate spreadsheet.

Standby phantom: 15 devices × 5 W × 24 h = 1.8 kWh/day—5 × 24 ÷ 1000 per device adds up.

Weekly habit: (W × hours/week) ÷ 1000 for laundry, (W × hours/day × 7) ÷ 1000 for fridge—mixing units causes 10× errors.

Power meter: Kill A Watt style devices integrate kWh directly—skip hand math for unknown appliances.

Fridge example: 150 W running 35% duty → 150×24×0.35÷1000 = **1.26 kWh/day**—never multiply 150×24 without duty.

Quick reference (resistive, PF≈1):

W	Ampe @ 120 V	Ampe @ 230 V
1.500	12,5	6,5
1.800	15.0	7,8
2.400	20.0	10.4

If two loads match rows that sum above your breaker’s 80% continuous column, stagger them—math beats hope.

Generator vs breaker: A 3,500 W generator on a 30 A 120 V duplex may offer 3,600 W theoretical per leg—but motor surge on that leg still trips the breaker or inverter even when “average” amps look fine.

Battery vocabulary: A 2 kWh battery is energy storage; a 2,000 W inverter is power cap—you need both numbers for off-grid laundry or fridge planning.

HP reminder: 1 mechanical HP ≈ 746 W output; electric motors draw more than HP at the plug due to efficiency—use a meter on well pumps and table saws, not HP alone.

Three checks before buying hardware: (1) Largest continuous W. (2) Largest surge W with overlap allowed. (3) kWh/day for battery—different answers, all required.

Split-phase homes: 240 V across two 120 V legs—4,800 W on one appliance does not mean the other leg is unused; balance 120 V loads when planning portable generator cord splits.

Minutes to hours: Always convert minutes to decimal hours before kWh math—15 min = 0.25 h, 4 min ≈ 0.067 h—rounding errors matter on short-duty appliances like kettles and microwaves.

Worked habit: For each appliance, write W, hours/day, and kWh/day on one line—if kWh/day is missing, you have not finished the row. Enter the table in the WattSizing Calculator to test whole-home overlap.

RV note: 30 A at 120 V service is 3,600 W max on the pedestal—different math than a 200 A house, same formulas for P = V × I and kWh = W × h ÷ 1000.

Inverter labels: A 2,000 W inverter can feed 2,000 W of load until it trips—there is no hidden kWh capacity in the inverter number; energy comes from batteries or the grid behind it.

Cost tie-in: At $0.16/kWh, a 3 kWh laundry day costs $0.48 in energy—cheap on the bill, but still needs correct amps and surge planning on backup hardware.

Safety: This guide is planning math only—aluminum wiring, damaged outlets, and panel upgrades require a licensed electrician, not a watt formula. When in doubt, measure with a meter and photograph your panel labels before storm season. Then model stacks in the load list guide.

Worked example: kettle + circuit limit

Kettle: 1,500 W at 120 V → 12.5 A.

Same circuit: microwave 1,400 W → 11.7 A.

Combined if both on: ~24 A → 15 A breaker trips.

Energy: kettle 4 min → 1,500 × 0.067 ÷ 1000 ≈ 0.10 kWh—cheap bill, loud peaks.

Câu hỏi thường gặp

Điện áp có thay đổi watt cho cùng một thiết bị không?

Resistive yes roughly; motors differ.

Tại sao 80% máy cắt?

NEC continuous load practice—heat builds over time.

kWh trên hóa đơn so với W trên nhãn?

kWh integrates W over time.

Tôi có thể chạy 2.000 W trên 15 A không?

No for continuous—needs 20 A.

230 V so với 120 V cho cùng một W?

Lower amps at higher V—same W heat output.

Hệ số công suất của máy phát điện?

Low PF loads need more VA headroom.

Làm thế nào để chuyển đổi HP sang W?

≈ 746 W per HP mechanical—motors not 100% efficient.

Pin Wh và kWh?

1 kWh = 1,000 Wh.

Ổ cắm 20 A có luôn là 2.400 W không?

Plan ~1,920 W continuous, not nameplate max 24/7.

Thực hành ở đâu?

Use the WattSizing Calculator with your measured hours.