USB-C can be spicy
USB Power Delivery goes up to 48 V at 5 A (240 W). That’s enough to run a gaming laptop—over the same port that once carried 2.5 W.
Tiny connector, big watts
DC Power Calculator — P = V × I
Inputs
Use DC values.
Amperes or milliamps.
Ohms, kiloohms, or megaohms.
Watts, milliwatts, or kilowatts.
Results
Results will appear here.
Tip: Enter exactly two values; leave the other two blank.
Understanding DC Power
In a direct current (DC) circuit, electric charge flows in one constant direction. This is different from alternating current (AC), where the flow reverses direction periodically. Common sources of DC include batteries, solar panels, and USB power supplies.
Basic Power Formula
The relationship between voltage, current, and power is straightforward in DC:
$$ P = V \times I $$
- P = Power (watts, W)
- V = Voltage (volts, V)
- I = Current (amperes, A)
For example, a 12-volt battery powering a 2-amp device consumes:
$$ P = 12 \times 2 = 24 \,\text{W} $$
Ohm’s Law Connection
Ohm’s Law links voltage, current, and resistance:
$$ V = I \times R $$
By combining this with the power formula, we get additional forms:
- $$ P = I^2 \times R $$
- $$ P = \frac{V^2}{R} $$
This makes it possible to calculate power even if you only know voltage and resistance, or current and resistance.
Units and Practical Use
Power is measured in watts (W). Larger devices may use kilowatts (kW), while small electronics often use milliwatts (mW). For students and hobbyists, knowing how to calculate DC power is essential for sizing batteries, resistors, and power supplies.
Tip: Always double-check your units! Mixing volts, milliamps, and ohms without converting to standard units can lead to wrong answers.
Frequently asked questions
Can I enter milli/mega units?
Yes — use the unit dropdowns (mV, mA, kΩ, MΩ, mW, kW). The calculator converts to SI internally.
Does this work for AC?
No — this page assumes steady DC. For alternating current, see the AC Power Calculator which handles power factor.
Is everything private?
Yes. All calculations run in your browser; no data is uploaded.
5 Fun Facts about DC Power
Phones sip in pulses
Many phone chargers use rapid DC pulses to push charge, then taper off. Your “5 V” label hides a fast dance between voltage, current, and heat.
Charging choreography
Wiring losses are I²R
Double the current and you quadruple resistive heating. That’s why higher-voltage DC (like 24 V instead of 12 V) keeps cables cooler at the same power.
Heat math
LEDs set their own volts
LEDs drop a nearly fixed voltage (e.g., ~2 V red, ~3 V blue). What saves them is current limiting—the reason resistors matter in “simple” DC circuits.
Current is king
Solar panels are current sources
PV panels happily sit near a fixed current until voltage rises to their max power point. Shade one cell, and the panel’s current collapses unless bypass diodes step in.
Sun math