55 W at 13.5 V is 4.07 A. After × 1.25 the estimate names a 7.5 A fuse. Is that the circuit you are checking?
Tool
Headlight bulb amperage
The estimate is I = P / V. Voltage is a field. The default is 13.5 V, a typical running figure with the alternator online. You can set 12 V for a resting battery. The fuse suggestion multiplies current by 1.25 and rounds up to a common blade value: 1, 2, 3, 4, 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, or 100 A. That rounding rule is educational. It is not a vehicle-maker spec and not an ABYC marine rule.
This figure is an educational estimate. It is not a product listing, not DOT or SAE certification, and not legal advice on aftermarket lamps. Aiming and wattage do not make a lamp street-legal. Check the vehicle manual and local law.
When this estimate fits
- You know the watts, and you want the current at 12 V or 13.5 V.
- You want a named fuse step from the 1.25 rule on this page, not a hidden OEM chart.
- You are not asking the page to size a whole vehicle electrical system.
Worked numbers from this formula
110 W of paired lamps at 13.5 V is 8.15 A. Current × 1.25 is 10.18 A, so the next listed step is 15 A.
The same 55 W at 12 V is 4.58 A. Did you pick the resting-battery voltage on purpose?
Questions
Why 13.5 V and not 12 V?
A running car with the alternator online is often near 13.5 V to 14.4 V. The default is 13.5 V. You can type 12 V if you want a resting-battery figure.
Is the fuse step a legal requirement?
No. The page multiplies current by 1.25 and rounds up to a common blade value. That is an educational rule, not DOT advice and not a shop work order.
What if the current is above 100 A?
The listed steps stop at 100 A. The estimate still names the amperes and says it will not invent a larger fuse.
Open the wattage calculator for the inverse formula. Add several circuits if you have more than one lamp.