12 V battery at 2 A
Battery sizing: $$ P = V \times I = 12 \times 2 = 24\ \text{W} $$ A 12 V accessory drawing 2 A needs about 24 W while it is running.
Watts / Volts / Amps / Ohms DC Power Calculator
Inputs
What do you want to find?
Advanced mode: enter exactly any two known values and leave the other two blank.
Use DC values.
Amperes, microamps, milliamps, kiloamps, or megaamps.
Ohms, kiloohms, megaohms, or gigaohms.
Watts, microwatts, milliwatts, kilowatts, or megawatts.
Results
Results will appear here.
Tip: Guided modes highlight the recommended two inputs; advanced mode accepts any valid pair.
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DC watts, volts, amps, and ohms calculator
This DC power calculator solves the relationship between voltage (V), current (I), resistance (R), and power (P). Use it to calculate watts from volts and amps, amps from watts and volts, ohms from volts and amps, or any other two-value Ohm’s law combination in a direct current circuit.
DC power in plain language
Direct current (DC) means electricity flows in one steady direction, like in batteries, solar panels, and USB chargers. Voltage is the electrical “push,” current is how much charge is moving, and resistance is how much the circuit resists that flow. Power is the rate of energy use and is measured in watts. In simple DC circuits, these values are tied together by two core relationships: the power equation and Ohm’s law.
Formula table by value to find
The basic DC power formula is $$ P = V \times I $$ and Ohm’s law is $$ V = I \times R $$. Combining them gives each common output formula:
| Find | Use these formulas | When you know |
|---|---|---|
| Resistance (R) |
|
Voltage/current, voltage/power, or power/current |
| Current (I) |
|
Voltage/resistance, power/voltage, or power/resistance |
| Voltage (V) |
|
Current/resistance, power/current, or power/resistance |
| Power (P) |
|
Voltage/current, voltage/resistance, or current/resistance |
How to use the calculator
- Choose what to solve: power, voltage, current, resistance, or advanced any-two mode.
- Enter two known values, such as volts and amps or watts and volts.
- Pick the correct units, including micro, milli, base, kilo, mega, or giga choices where available.
- Click Calculate to solve the missing DC values.
- Review the substituted formula steps and confirm the assumptions match your real-world setup.
- Use Copy result or Copy link when you need to save or share the calculation.
Worked examples
5 V USB at 3 A
USB devices: $$ P = 5 \times 3 = 15\ \text{W} $$ A 5 V, 3 A USB supply can deliver up to 15 W before conversion losses.
9 V across 1 kΩ
LED and resistor checks: $$ P = \frac{V^2}{R} = \frac{9^2}{1000} = 0.081\ \text{W} $$ The resistor dissipates 81 mW, so a 0.25 W resistor has comfortable margin in many open-air builds.
100 W load on 24 V
Solar and control systems: $$ I = \frac{P}{V} = \frac{100}{24} = 4.17\ \text{A} $$ A 100 W DC load on a 24 V bus draws about 4.17 A.
Common DC reference charts
These quick charts cover common 5 V USB, 12 V battery/automotive, 24 V control, and 48 V solar/telecom systems.
Amps for common wattages
| DC system | 10 W | 25 W | 50 W | 100 W |
|---|---|---|---|---|
| 5 V USB | 2 A | 5 A | 10 A | 20 A |
| 12 V battery | 0.83 A | 2.08 A | 4.17 A | 8.33 A |
| 24 V control | 0.42 A | 1.04 A | 2.08 A | 4.17 A |
| 48 V solar/telecom | 0.21 A | 0.52 A | 1.04 A | 2.08 A |
Watts for common currents
| DC system | 0.5 A | 1 A | 2 A | 5 A |
|---|---|---|---|---|
| 5 V USB | 2.5 W | 5 W | 10 W | 25 W |
| 12 V battery | 6 W | 12 W | 24 W | 60 W |
| 24 V control | 12 W | 24 W | 48 W | 120 W |
| 48 V solar/telecom | 24 W | 48 W | 96 W | 240 W |
Units and practical tips
Power is measured in watts (W), while small electronics may use microwatts (µW) or milliwatts (mW), and larger DC systems may use kilowatts (kW) or megawatts (MW). Current is in amps (A), voltage in volts (V), and resistance in ohms (Ω). If you input microamps, milliamps, kilohms, megaohms, or other supported units, the calculator converts everything to standard units internally.
Tip: Always double-check units. Mixing volts with milliamps or ohms without converting can lead to incorrect results.
Assumptions and limits
This calculator assumes steady-state DC and ideal Ohm’s-law behavior. AC real power calculations need RMS voltage/current and power factor. Non-ohmic loads such as LEDs, motors, batteries, and switching supplies can change behavior with temperature, speed, state of charge, control electronics, or startup surge, so simple resistance math may not describe every operating condition.
Safety note: High-current DC systems can overheat wires, connectors, fuses, and batteries. Mains-related or high-energy electrical work should be designed and checked by a qualified person using appropriate safety standards.
Frequently asked questions
How do I calculate watts from volts and amps?
For steady DC, multiply voltage by current: $$ P = V \times I $$ For example, 12 V × 2 A = 24 W.
How do I calculate amps from watts and volts?
Divide power by voltage: $$ I = \frac{P}{V} $$ For example, a 100 W DC load on 24 V draws about 4.17 A.
Can watts be converted to volts without amps or resistance?
No. Watts alone do not determine voltage. You also need current, resistance, or another circuit constraint to solve voltage.
Does DC power use power factor?
Simple steady-state DC power does not use AC power factor. For alternating current, see the AC Power Calculator which handles RMS values and power factor.
What is the difference between watts and watt-hours?
Watts measure power, the rate of energy use at an instant. Watt-hours measure energy over time: watt-hours = watts × hours. A 24 W device running for 3 hours uses 72 Wh.
How much resistor wattage margin should I use?
A common practical choice is to use a resistor rated at least two times the calculated dissipation. Use more margin for high temperatures, enclosed spaces, pulsed loads, or reliability-critical circuits.
Which two values do I need?
Any two of V, I, R, P are sufficient for an ideal Ohm’s-law DC circuit. The calculator uses V = IR and P = VI = I²R = V²/R to compute the other two.
Is everything private?
Yes. All calculations run in your browser; no data is uploaded.
Quick DC sizing notes
Higher voltage lowers current
For the same wattage, a 24 V system draws half the current of a 12 V system. Lower current reduces I²R wiring losses.
Cable and fuse sizing
Resistance checks need conditions
Measured resistance may change as parts heat up. Motors, lamps, batteries, and LEDs can draw very different startup or operating currents.
Real loads vary
Watts are not capacity
Watts describe how fast energy is used. For runtime, multiply by time and compare against battery energy in watt-hours.
Runtime estimates
Use resistor power margin
If math says a resistor dissipates 0.081 W, a 0.125 W part is close while a 0.25 W or larger part usually has better thermal margin.
Heat reliability