Three tools in one: solve any Ohm’s Law variable (P, V, I, R), build an appliance load list with daily and monthly energy costs, and convert between electrical units instantly.
Understanding electrical power is fundamental to solar design, circuit sizing, and energy auditing. Whether you're calculating the amperage draw of a circuit for wire sizing, determining the daily kWh load of a client's home for solar sizing, or simply converting between watts and kilowatts for a proposal, this three-mode calculator covers it all in one tool.
The Ohm's Law mode solves for any of the four fundamental electrical variables: Power (P in watts), Voltage (V in volts), Current (I in amperes), and Resistance (R in ohms). Enter any two known values and the calculator instantly solves for the remaining two. The Appliance Load Builder lets you build a complete load list, entering each appliance's wattage and daily usage hours to calculate total Wh/day, kWh/month, and monthly electricity cost.
For solar professionals, the tool's most valuable output is the solar array size needed to power the entire load list, calculated from daily Wh consumption, peak sun hours, and system efficiency. This makes the Wattage Calculator an essential pre-sizing tool before running detailed system design calculations.
Solves all six Ohm's Law relationships: P=VI, I=V/R, V=IR, R=V/I, P=I²R, and P=V²/R. Enter any two values and get all four variables instantly.
Build a complete home or business load list appliance by appliance. The calculator totals daily Wh, monthly kWh, monthly cost, and the solar array size needed to offset the full load.
Convert between W, kW, Wh, kWh, BTU/hr, and horsepower instantly. Eliminates manual conversion errors in proposals, datasheets, and permit applications.
Use the Ohm's Law mode to calculate current draw from known wattage and voltage before selecting wire gauge, breaker size, or conduit fill. Essential for any electrical design work from panel to outlet calculations.
Use the Appliance Load Builder to inventory a client's electrical loads before sizing a solar system. The total Wh/day output feeds directly into the System Size Calculator for an accurate solar array recommendation.
Help clients understand which appliances consume the most electricity and where energy efficiency improvements will have the highest impact on their electricity bill before or after going solar.
Select from three modes using the tab navigation: Ohm's Law (for solving P, V, I, R relationships), Appliance Load Builder (for creating a complete load list), or Unit Converter (for translating between electrical units). Each mode updates the input fields accordingly.
In Ohm's Law mode, enter any two of the four variables: Power in watts, Voltage in volts, Current in amperes, or Resistance in ohms. The calculator solves for the remaining two variables using all six Ohm's Law relationships simultaneously.
In Appliance Load Builder mode, click "Add Appliance" and enter the appliance name, wattage (running watts, not surge), quantity, and daily usage hours. Use the quick-reference table below for typical appliance wattages if you don't have the nameplate data available.
For each appliance, enter realistic daily usage hours. A refrigerator runs 24 hours per day but cycles on and off, so actual draw is approximately 30-40% of rated wattage. An air conditioner may run 8-10 hours per day in peak summer. Accurate usage hours are critical for load calculation accuracy.
The calculator displays total Wh per day, kWh per month, estimated monthly electricity cost, and the solar array size in kW needed to offset the entire load list. Use these numbers to start the conversation about solar system sizing with the client.
All calculations use the fundamental laws of electrical circuits. These relationships are exact for DC circuits and apply to AC circuits using RMS values.
P = V x I (Power = Voltage x Current)
P = I^2 x R (Power = Current squared x Resistance)
P = V^2 / R (Power = Voltage squared / Resistance)
I = V / R (Current = Voltage / Resistance)
V = I x R (Voltage = Current x Resistance)
R = V / I (Resistance = Voltage / Current)For each appliance:
Wh_per_day = Watts x Hours_per_day x Quantity
Total_Wh_day = Sum of all appliance Wh_per_day
Monthly_kWh = Total_Wh_day x 30 / 1000
Monthly_Cost = Monthly_kWh x Rate_per_kWhDaily_kWh = Total_Wh_day / 1000
Array_kW = Daily_kWh / (PSH_per_day x 0.80)
Where PSH = Peak Sun Hours for location (default 4.5 hr/day)
0.80 = System efficiency derate factorWatts to Kilowatts: kW = W / 1000
Watt-hours to kWh: kWh = Wh / 1000
BTU/hr to Watts: W = BTU/hr x 0.2931
Watts to BTU/hr: BTU/hr = W / 0.2931
Horsepower to Watts: W = HP x 745.7
Watts to Horsepower: HP = W / 745.7
Worked example: A home office: standing desk (50W), dual monitors (60W), laptop (65W), LED lamp (10W), phone charger (5W), fan (40W). Total running: 230W. At 8 hours/day: 230W × 8 = 1,840 Wh = 1.84 kWh/day. Monthly: 55 kWh × $0.16 = $8.80/month. Annual cost: $106. A single 400W solar panel generates approximately 1.56 kWh/day at 4.5 PSH — enough to power this office.
Calculations sourced from SurgePV’s Wattage Calculator — surgepv.com/tools/wattage-calculator/
| Appliance | Running Watts | Surge Watts | Typical Daily Hours |
|---|---|---|---|
| Central AC (3-ton) | 3,500 W | 5,000 W | 8-10 hr (summer) |
| Window AC (1-ton) | 1,200 W | 2,400 W | 6-8 hr (summer) |
| Refrigerator | 150 W (avg) | 1,000 W | 24 hr (cycles 30%) |
| Electric Oven | 2,000 W | 2,400 W | 1 hr |
| Microwave | 1,200 W | 1,400 W | 0.5 hr |
| Dishwasher | 1,200 W | 1,800 W | 1 hr |
| Washing Machine | 500 W | 2,000 W | 1 hr |
| Electric Dryer | 5,000 W | 6,000 W | 1 hr |
| Water Heater (electric) | 4,500 W | 4,500 W | 2-3 hr |
| Desktop PC | 200 W | 300 W | 8 hr |
| LED TV (55 in) | 80 W | 80 W | 4 hr |
| LED Bulb | 10 W | 10 W | 6 hr |
| Ceiling Fan | 75 W | 75 W | 8 hr |
| EV Charger (Level 2) | 7,200 W | 7,200 W | 4-6 hr |
Motor-driven appliances like AC units, refrigerators, and washing machines draw 3-7x their running wattage at startup (surge watts). Solar inverters must handle surge loads without tripping. When sizing an off-grid or backup solar system, always check inverter surge capacity against the highest surge load in the system.
Appliance labels show maximum rated wattage, not average consumption. A microwave rated at 1,200W input may only produce 900W of cooking power (75% efficiency). A refrigerator rated at 150W average may cycle at 600W for 20 minutes per hour. Always measure actual draw with a plug-in watt meter for critical sizing calculations.
Any load with an electric motor (AC, refrigerator, pump, washing machine, power tools) draws a starting surge current of 3-7x running current. A 1,500W (running) air conditioner may surge to 6,000-9,000W at startup. This surge must be factored into inverter sizing and breaker sizing to avoid nuisance tripping.
AC voltage and current are sinusoidal waveforms. The RMS (Root Mean Square) value is the AC equivalent that produces the same heating effect as DC. Standard 120V AC is 120V RMS, with peak voltage of approximately 170V. Always use RMS values when applying Ohm's Law to AC circuits; peak values will give incorrect power calculations.
Ohm's Law describes the relationship between voltage (V), current (I), resistance (R), and power (P) in any electrical circuit. For solar professionals, it is used constantly: to verify wire ampacity (I = P/V), to check voltage drop on DC runs (V_drop = I x R), to size breakers (I = P/V, then apply 125% NEC multiplier for continuous loads), and to verify inverter output capacity. Understanding Ohm's Law is the mathematical foundation of all electrical work.
Watts (W) is the basic unit of electrical power. Kilowatts (kW) is simply 1,000 watts. Solar panels are rated in watts (e.g., a 400W panel); solar systems are sized in kilowatts (e.g., an 8 kW system). Energy consumed over time is measured in watt-hours (Wh) or kilowatt-hours (kWh): a 100W light bulb running for 10 hours consumes 1,000 Wh or 1 kWh. Your electricity bill charges you per kWh.
The nameplate on appliances shows rated input power in watts (W) or amperes (A) at rated voltage (V). If only amperes and voltage are shown, multiply to get watts: P = V x I. A device labeled "120V, 8A" draws 960W. Note that the nameplate shows the maximum rated power; actual average consumption may be 50-80% of rated for cycling appliances like refrigerators and AC units.
Running watts (also called rated watts) is the continuous power draw of an appliance during normal operation. Surge watts (also called starting watts) is the brief peak current drawn when a motor-driven appliance starts up, typically lasting 1-3 seconds. For solar inverter sizing, the inverter must handle the surge watts of the largest motor load. For energy calculation and solar panel array sizing, use running watts multiplied by operating hours.
Single-phase power (standard residential): P = V x I x power_factor. Three-phase power (commercial/industrial): P = V x I x 1.732 x power_factor. For three-phase calculations, the 1.732 factor (square root of 3) accounts for the three-phase vector sum. Most residential solar systems are single-phase. Commercial solar installations may be three-phase, requiring the 1.732 multiplier in all power calculations.
DC power: P = V x I exactly. AC power: P = V x I x power_factor (PF). Power factor accounts for the phase angle between voltage and current in AC circuits with reactive loads (motors, transformers). Purely resistive loads (heaters, incandescent bulbs) have PF = 1.0. Motor loads typically have PF = 0.8-0.95. Solar inverters output AC power at PF close to 1.0. When sizing circuits for AC loads, always account for power factor if working with industrial equipment.
Power factor (PF) is the ratio of real power (watts, doing actual work) to apparent power (VA, total power drawn from the source). A PF of 0.85 means 85% of the power drawn from the utility actually does useful work; 15% is reactive power circulating in the circuit. Low power factor increases current draw without increasing useful power, requiring larger wires and transformers. Utility companies may charge commercial customers penalty fees for low power factor below 0.90.
Digital meters display cumulative kWh. Read the meter at the start and end of a period, then subtract: kWh = End_Reading - Start_Reading. Smart meters (AMI meters) allow you to download interval data from your utility's website, showing hourly kWh consumption. This hourly data is invaluable for solar sizing because it reveals peak demand periods, helping to optimize battery storage sizing and time-of-use rate management.
Phantom loads (also called standby power or vampire power) are the electricity consumed by devices while turned off or in standby mode. A typical U.S. household has 40+ devices drawing standby power totaling 5-10% of total electricity consumption. Common phantom load culprits: cable boxes (15-30W 24/7), gaming consoles in standby (1-8W), microwaves with digital clocks (3-4W), and smart TVs (1-3W). Over a year, phantom loads can add 500-1,000 kWh to annual consumption.
Annual cost formula: Cost = Watts x Hours_per_day x 365 days / 1000 x Rate_per_kWh. Example: A 5,000W electric dryer used 1 hour per day at $0.14/kWh: 5,000 x 1 x 365 / 1000 x 0.14 = $255.50 per year. Running the dryer on solar-generated electricity (after system payback) reduces this to near zero in marginal cost, which is a powerful selling point for including major appliances in solar offset calculations.
Calculate the required power supply capacity for your solar system components and electrical loads.
Size an uninterruptible power supply to keep critical loads running during a power outage or solar system downtime.
Calculate the power supply wattage needed for your PC build based on component specifications and usage profile.
Verify that your solar inverter can handle the running and surge wattage of all appliances simultaneously, especially motor loads.
Determine the ideal solar system size in kW based on your energy usage, location, and shading conditions.
Size a battery storage system to cover the critical loads identified in your appliance load audit for backup power or off-grid operation.
.svg)
