Last Updated: July 2026 • Reviewed for NEC 2023 Compliance
Power consumption calculation is the foundation of every electrical system design. Whether you are sizing a generator, specifying a solar array, or planning a service upgrade, the math starts with the same fundamental relationship: Watts = Volts × Amps. This guide walks through the formula, the distinction between continuous and non-continuous loads, a device-by-device consumption reference table, daily and monthly kWh calculation methodology, and three worked examples that show how to translate consumption data into system sizing decisions.
The Core Formula
Watts (W) = Volts (V) × Amps (A)
Kilowatt-hours (kWh) = (Watts × Hours) ÷ 1,000
Monthly kWh = Daily kWh × 30
Watts = Volts × Amps
The fundamental equation of electrical power is straightforward: the power consumed by a device equals the voltage applied multiplied by the current drawn. For resistive loads (heaters, incandescent lights), this is the complete picture. For inductive loads (motors, compressors, transformers), you must also consider power factor (PF), where Watts = Volts × Amps × Power Factor. A typical residential power factor ranges from 0.85 to 0.95.
For three-phase systems, the formula becomes: Watts = Volts × Amps × √3 (1.732) × Power Factor. The square root of 3 accounts for the three-phase relationship.
Key Distinction: Running vs. Starting Watts
Motor-driven loads (AC compressors, well pumps, refrigerators) require 2-3 times their running wattage to start. This inrush current lasts only seconds but must be accounted for when sizing generators and inverters. Always check the LRA (Locked Rotor Amps) or starting current on the equipment nameplate.
Continuous vs. Non-Continuous Loads
The NEC (NEC 210.19(A)(1), 210.20(A)) defines a continuous load as one expected to operate at maximum current for three hours or more. This distinction matters for two reasons:
- Breaker and conductor sizing: Continuous loads require 125% sizing of OCPDs and conductors.
- Consumption calculation: A continuous load's daily kWh is simply: (Watts × hours) ÷ 1,000. A non-continuous load's consumption depends on duty cycle — the percentage of time it actually runs at full power.
| Load Type | Examples | NEC Treatment |
|---|---|---|
| Continuous | Water heaters, EV chargers, commercial lighting, data center loads, solar inverters | 125% sizing required |
| Non-Continuous | Refrigerators, microwave ovens, hair dryers, receptacle circuits | 100% sizing (nameplate) |
Device-by-Device Consumption Table
The following table lists common residential and commercial loads with their typical running wattage, voltage, and estimated daily kWh. Use these as reference values — always verify against the actual equipment nameplate for precise calculations.
| Appliance / Load | Volts | Running Watts | Typical Daily Use | Est. Daily kWh |
|---|---|---|---|---|
| Central AC (3 ton) | 240 | 3,500 | 8 hrs | 28.0 |
| Central AC (4 ton) | 240 | 4,800 | 8 hrs | 38.4 |
| Electric Water Heater | 240 | 4,500 | 3 hrs | 13.5 |
| EV Charger (Level 2, 40A) | 240 | 9,600 | 4 hrs | 38.4 |
| Electric Dryer | 240 | 5,000 | 1 hr | 5.0 |
| Electric Range/Oven | 240 | 8,000 | 1.5 hrs | 12.0 |
| Refrigerator | 120 | 700 | 8 hrs (duty cycle) | 5.6 |
| Well Pump (1 HP) | 240 | 1,000 | 1.5 hrs | 1.5 |
| LED Lighting (whole home) | 120 | 300 | 5 hrs | 1.5 |
| Microwave | 120 | 1,200 | 0.25 hrs | 0.3 |
| Solar Inverter (7.6kW) | 240 | 7,600 (output) | 5 hrs (peak sun) | 38.0 (production) |
| Pool Pump (1.5 HP) | 240 | 1,500 | 6 hrs | 9.0 |
Values shown are typical reference figures. Actual consumption varies by equipment efficiency, age, usage patterns, and environmental conditions. Always use nameplate data for final calculations.
Daily and Monthly kWh Calculation
To calculate the daily energy consumption of a device:
Daily kWh = (Running Watts × Hours of Operation) ÷ 1,000
For loads with a duty cycle (like a refrigerator that cycles on and off), use the effective running hours. A refrigerator that draws 700W but only runs 33% of the time has an effective daily run time of 8 hours (24 hours × 0.33 = 8 hours).
Monthly consumption is the sum of all daily loads multiplied by 30:
Monthly kWh = Sum of Daily kWh × 30
For the reference table above, the total estimated daily consumption for a home with one 3-ton AC, water heater, EV charger, dryer, range, refrigerator, well pump, lighting, and microwave is approximately 106 kWh/day, or about 3,180 kWh/month. This is a high-consumption home; the U.S. average residential consumption is approximately 877 kWh/month per the U.S. Energy Information Administration.
Three Worked Calculation Examples
Example 1: Sizing a Generator for Essential Loads
Given: A homeowner wants a standby generator for essential loads during outages: well pump (1,000W running, 4,000W starting), refrigerator (700W running, 2,100W starting), lighting (300W), and a 3-ton AC (3,500W running, 10,500W starting).
Step 1: Sum running watts: 1,000 + 700 + 300 + 3,500 = 5,500W running.
Step 2: Sum starting watts (largest motor starts last): AC starts first at 10,500W while others run (1,700W) = 12,200W peak. Then well pump starts: 4,000W + 4,500W (AC + fridge + lights running) = 8,500W peak. The AC start is the worst case at 12,200W.
Step 3: Size the generator: minimum 12,200W starting, 5,500W running. A 14kW generator covers both with margin. A 26kW generator (like the Generac Guardian) covers this plus additional loads.
Daily consumption during outage: 5,500W × estimated 6 hrs effective run = 33 kWh/day.
Example 2: Sizing a Solar Array from Consumption Data
Given: A home uses 900 kWh/month. The location receives 5 peak sun hours per day.
Step 1: Calculate daily kWh: 900 ÷ 30 = 30 kWh/day.
Step 2: Calculate required array size: 30,000 Wh ÷ 5 hrs = 6,000W (6 kW) of panels at 100% efficiency.
Step 3: Apply system losses (inverter efficiency ~96%, temperature derating ~90%, wiring losses ~98%, soiling ~95%): 6,000W ÷ (0.96 × 0.90 × 0.98 × 0.95) = 6,000 ÷ 0.805 = 7,453W.
Result: Approximately 7.5 kW of panels needed. With 450W panels, that is 17 panels (7,650W actual array).
Example 3: Calculating EV Charger Monthly Cost
Given: A Level 2 EV charger drawing 40A at 240V (9,600W), charging 4 hours per day, 20 days per month. Electricity rate: $0.14/kWh.
Step 1: Daily kWh: (9,600W × 4 hrs) ÷ 1,000 = 38.4 kWh/day.
Step 2: Monthly kWh: 38.4 × 20 = 768 kWh/month.
Step 3: Monthly cost: 768 × $0.14 = $107.52/month.
Result: Charging an EV at home costs approximately $107/month for 20 days of 4-hour charges, or about $5.38 per full charge. This is significantly less than equivalent gasoline costs for most vehicles.
Need Help Sizing Your System?
Call PES Supply at (502) 790-0600 or (888) 876-0007 for expert assistance with generator, solar, and electrical system sizing.
Understanding Power Factor
Power factor (PF) is the ratio of real power (watts) to apparent power (volt-amps). For resistive loads like electric heaters and incandescent lighting, the power factor is 1.0, meaning all the power drawn is converted to useful work. For inductive loads like motors, transformers, and HID lighting, the power factor is typically 0.80-0.95, meaning some current is drawn that does not produce real work (reactive power).
This matters because breakers, conductors, and generators are rated in amps (apparent power), not watts. A motor drawing 10A at 240V with a power factor of 0.85 consumes 2,040W of real power (10 × 240 × 0.85), but the circuit must be sized for the full 2,400 VA (10 × 240) of apparent power. When sizing generators and inverters, always account for power factor by using the apparent power (VA) rather than just watts.
Practical Rule of Thumb
For residential load calculations, a power factor of 0.95 is commonly assumed for mixed loads. For motor-heavy circuits (well pumps, compressors, HVAC), use 0.85. When in doubt, check the equipment nameplate or datasheet for the rated power factor.
Frequently Asked Questions
How do I calculate watts from volts and amps?
Multiply voltage by amperage: Watts = Volts × Amps. For example, a 240V device drawing 30A consumes 7,200W (7.2 kW). For inductive loads with a power factor, multiply by the PF: Watts = Volts × Amps × Power Factor.
How do I convert watts to kilowatt-hours?
First convert watts to kilowatts by dividing by 1,000. Then multiply by the hours of operation: kWh = (Watts ÷ 1,000) × Hours. For example, a 4,500W water heater running for 3 hours uses (4.5 kW × 3 hrs) = 13.5 kWh.
What is the difference between running watts and starting watts?
Running watts (also called continuous or rated watts) is the power a device consumes during normal operation. Starting watts (or surge watts) is the additional power needed to start motor-driven loads, typically 2-3 times the running watts. Generators and inverters must be sized to handle the highest starting surge.
How do I size a solar array from my monthly kWh?
Divide your monthly kWh by 30 to get daily kWh. Convert to watt-hours, then divide by your location's peak sun hours to get the required array wattage. Apply a system loss factor of approximately 0.80-0.85 (to account for inverter efficiency, temperature, wiring, and soiling) by dividing the array wattage by that factor.
What is a continuous load and why does it matter?
A continuous load is one that operates at maximum current for three hours or more, such as EV chargers, water heaters, and commercial lighting. The NEC requires that breakers and conductors for continuous loads be sized at 125% of the load current (NEC 210.19(A)(1) and 210.20(A)). This prevents thermal overload of the OCPD during sustained operation.
Disclaimer: This guide is for educational and informational purposes. Consumption values are typical reference figures; always use actual equipment nameplate data for final calculations. Always verify system sizing with a licensed electrician or solar designer and confirm compliance with the applicable NEC edition and local amendments. PES Supply is located in Louisville, KY. Call (502) 790-0600 or (888) 876-0007 for assistance.
