You have a watt figure and a bus voltage—now you need amps for wire, fuse, or shunt ratings. This guide walks through calculating current from power and voltage with the assumptions electricians and installers document on every job.
Benefits
- Single-step math: amps = watts ÷ volts when power factor is effectively unity.
- Clarifies which voltage to use (12 V battery, 24 V RV, 120 V branch) so results match the protective device.
- Connects the result to NEC-style continuous-load margins and inverter surge specs.
How it works
- Confirm load power in watts (continuous, not brief surge unless that is what you are sizing for).
- Use the circuit voltage that actually drives current through the conductor or device.
- Divide watts by volts; round up for fuse/breaker steps and compare to equipment maximum continuous current.
FAQ
How do I calculate amps from watts and voltage?
Amps = Watts ÷ Volts. A 240 W heater on 120 V draws 2 A. A 600 W inverter load on 12 V is 50 A at the battery—before efficiency loss, which may require dividing by inverter efficiency separately.
Which voltage should I enter?
Use the voltage at the point where you are rating current: battery terminals for DC inverter feeds, line voltage for branch circuits, or nameplate voltage for appliances. Mixing system voltage with cell voltage produces wrong amps.
What about three-phase or power factor?
This guide targets single-value P ÷ V for DC and unity-PF AC. Three-phase and reactive loads need √3, power-factor, or measured current—do not apply a simple watts ÷ line voltage without the correct formula for that topology.
Technical specifications
- Formula: I (A) = P (W) ÷ V (V).
- DC: exact for resistive loads at stated bus voltage.
- AC unity PF: I ≈ P ÷ V for single-phase resistive loads at nominal V.
- Next steps: dc-cable-size, battery C-rate, and breaker sizing per local code.
Document your assumptions
Write watts source (meter, nameplate, estimate), voltage point (battery, panel, device), and whether the load is continuous or intermittent. Amp calculations are only as trustworthy as those three inputs.
Inverter and battery path
AC watts at the outlet are not the same as DC amps at the battery. Divide by inverter efficiency if you are back-calculating from AC load to battery current: I_battery ≈ P_AC ÷ (V_dc × η).