How to Calculate Watts (Electrical Power)
The watt is the SI unit of power — the rate at which energy is transferred or converted, equal to one joule per second. In an electrical circuit, power in watts is simply the product of voltage and current: P = V × I. This calculator uses that direct definition to turn a voltage and current reading into a wattage figure, then extends it into practical terms: how much energy a device consumes over a day (in kilowatt-hours) and roughly what that costs on a typical electricity bill.
The power formula: P = V × I
Multiply voltage (in volts) by current (in amps) to get power (in watts): P = V × I. For example, a household circuit at 120 V drawing 5 A delivers 120 × 5 = 600 W. This is algebraically equivalent to the other two forms of the power law, P = I²R and P = V²/R, which are used instead when resistance is known but voltage or current is not — all three describe the same physical quantity of energy transfer per second.
From watts to kilowatt-hours and cost
Watts measure an instantaneous rate, but electricity bills are based on energy consumed over time, measured in kilowatt-hours (kWh). To convert, multiply power in watts by the number of hours used, then divide by 1000: E (kWh) = P (W) × t (h) ÷ 1000. Multiplying that energy figure by your utility's rate (dollars per kWh) gives the cost to run the device for that period — this calculator reports both the daily energy use and the daily and monthly cost so you can compare appliances or estimate a bill before you buy or run something.
Frequently Asked Questions
What is a watt?
How do I calculate watts from volts and amps?
What is the difference between watts and kilowatt-hours?
How much does it cost to run a device rated in watts?
Common Tip Percentages
| Service | Typical Tip Range | Notes |
|---|---|---|
| Restaurant (sit-down) | 15–20% | 20%+ for exceptional service |
| Food delivery | 10–20% | Consider extra for long distances |
| Taxi / rideshare | 10–20% | More for assistance with luggage |
| Hotel housekeeping | $2–5/night | Leave daily, not just at checkout |
| Barber / hair salon | 15–20% | On pre-discount total |
| Pizza delivery | 10–15% or $3–5 minimum | Higher in bad weather |
General Household Budget Ratios (50/30/20 Rule)
| Category | % of Net Income | Examples |
|---|---|---|
| Needs | 50% | Rent, groceries, utilities, insurance |
| Wants | 30% | Dining out, entertainment, hobbies |
| Savings / Debt | 20% | Emergency fund, retirement, extra payments |
Common Measurement Conversions for Everyday Use
| Quantity | US / Imperial | Metric Equivalent |
|---|---|---|
| 1 cup | 8 fl oz | 237 mL |
| 1 pint | 16 fl oz | 473 mL |
| 1 quart | 32 fl oz | 946 mL |
| 1 gallon | 128 fl oz | 3.785 L |
| 1 lb | 16 oz | 453.6 g |
| 1 ft | 12 in | 30.48 cm |