Alternator Size Calculator
Enter the car's base electrical load, your amplifier's RMS power and class, other accessories and the stock alternator. The calculator adds them with headroom and picks a standard alternator size.
- Alternator size
- Total demand
- Hot idle output
Base load is often 50 to 80 A on a modern car.
Amps = (base + RMS ÷ 13.8 ÷ efficiency + accessories ÷ 13.8) × 1.2. A car with a 70 A base load and a 1,500 W class D amplifier needs about 247 A with headroom, so a 250 A alternator.
Alternator size by audio
Class D, 70 A base. Search for a power.
| Amplifier | Demand | Alternator |
|---|---|---|
| 0 W | 84 | 90 |
| 500 W | 138 | 140 |
| 1000 W | 193 | 200 |
| 1500 W | 247 | 250 |
| 2000 W | 301 | 320 |
| 3000 W | 410 | Dual |
| 4000 W | 519 | Dual |
Amps.
Audio power and alternator
Class D, 70 A base. Slide the amplifier power.
The alternator is drawn with its rating.
How to size an alternator
Loads to amps. Step through it.
Base
Vehicle load.
Upgrade the Big 3 cables with a high output alternator.
Stock, high output or dual
Alternator options. Compare them.
Factory.
Upgrade.
Extreme.
Hot idle output is what keeps the lights steady.
Before you upgrade
Alternator checks. Tick each item.
Tick each item as you go.
Make sure the battery can handle the new charge rate.
What you get
Alternator sizing. Each tile explains one result.
Amps.
Total.
Amps.
Amps.
Output.
Dim headlights on bass hits mean the alternator is too small.
Frequently Asked Questions
No question matches that search.
What size alternator do I need for my sound system?
Add about ninety amps for each thousand watts of class D audio to the base load.
How many amps does a car use?
About fifty to eighty amps for a modern car with lights on.
Do I need a high output alternator?
Usually above about one thousand watts of audio.
What is the Big Three upgrade?
Thicker charge, engine ground and chassis ground cables.
Does a capacitor replace a bigger alternator?
No. It only smooths short peaks.
What is hot idle output?
What the alternator makes at idle when hot, often about sixty percent of its rating.
Power for every beat
Size the alternator to your loads and the lights stay bright when the bass hits.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.