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Alternator Load Calculator

Enter your alternator rating and total accessory load to see how much of its capacity is used.

How it is calculated

The tool turns the total power of everything switched on into a current and compares it with the alternator's rated current. Load current is I = P ÷ V, where P is the total load in watts and V is the system voltage in volts. Utilisation is I ÷ A × 100, where A is the alternator's rated output in amps. Headroom is A − I in amps and turns negative when the load exceeds the rating. Alternator rating, load and voltage must all be above zero, and voltage must lie between 5 and 60 V; anything outside that shows no result. A running engine's charging voltage of about 14 V gives a smaller current than 12 V for the same watts. The comparison uses the rating you type, not the lower output an alternator gives at idle.

Worked example

An alternator rated at 100 A, with accessories drawing 900 W at 14 V. Load current: 900 ÷ 14 = 64.3 A. Utilisation: 64.3 ÷ 100 × 100 = 64.29%. Headroom: 100 − 64.3 = 35.7 A. Result: 64.29% of the alternator's capacity is used, with a load of 64.3 A and 35.7 A to spare. A 60 A unit carrying 1200 W at 12 V would show 166.67% and −40.0 A of headroom, meaning it is overloaded.

Try this example →

Common mistakes

  • Entering the load in amps. The field is total watts, so add the wattage of each accessory instead of its current.
  • Using 12 V for a running engine. With the engine on, the system sits near 14 V, and 12 V overstates the current for the same watts.
  • Typing a cell or battery voltage such as 3.7 V, or a value in millivolts. Voltage outside 5–60 V gives no result at all.
  • Adding up only the extra accessories. The total must include the car's own demand, such as ignition, fuel pump, lights and blower, or the percentage is too low.
  • Ignoring peak versus continuous use. Winch pulls or a cooling fan starting are short peaks, while lights and a fridge are continuous, so work out the sustained figure first.

About this calculator

An alternator load calculation compares a vehicle's total electrical draw — headlights, climate control blower, infotainment, heated seats, auxiliary lighting, winches, and any aftermarket accessories added together — against the alternator's rated output current, usually expressed in amps, to find what percentage of its capacity is actually being used at any given time. Alternators aren't rated to sustain their peak output continuously at idle RPM, so the practical safety margin most installers target is keeping sustained load under roughly 70-80% of rated capacity.

This calculation matters most to anyone adding significant aftermarket electrical accessories — off-road lighting, winches, sound systems, or overlanding setups with fridges and battery chargers — because an alternator running consistently near or above its rated output overheats, wears out its brushes and bearings faster, and in the worst case can't keep the battery charged at idle, leading to a dead battery even while the engine runs.

Your accessories' total watts are divided by system voltage to get the load in amps, which is set against the alternator's rated output; you see the percentage of capacity in use, the load in amps and the headroom left, enough to decide whether a higher-output alternator is needed.

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