How Many Watts Does a String Trimmer Need to Run?

String trimmers are among the lightest-duty power tools in the yard, and their electrical demands reflect that. A corded trimmer uses less than half the power of a circular saw, and a battery trimmer only draws from the wall while its pack charges. Still, the numbers vary enough between models, and between thin grass and thick weeds, that it is worth understanding what your trimmer actually uses before planning cords, circuits or a charging setup.

Corded string trimmers

Corded electric trimmers are typically rated between 3.5 and 6.5 amps at 120 volts. Light-duty models with 12 to 13 inch cutting swaths and 0.065 inch line sit at the low end, about 400 to 550 watts. Heavier 14 to 15 inch trimmers using 0.080 inch line and marketed for thicker growth draw 650 to 800 watts. Some corded models convert to edgers, which does not change the draw much, though edging along concrete loads the motor more than open grass.

Load changes more than you might think

Spinning freely, a trimmer might draw only 50 to 60 percent of its rating. Cutting tall wet grass or woody weeds pushes it to the full rating. Tangling the line around stems briefly increases current as the motor strains, and running a thicker line than the head was designed for adds drag on every revolution.

Battery string trimmers

Battery trimmers dominate new sales, running on 18 to 20 volt, 40 volt, 56 volt and 60 to 80 volt platforms. While cutting they draw from the battery, not the wall; the motor in a 40 to 60 volt trimmer might pull 300 to 900 watts from the pack depending on speed setting and load. The electrical question then becomes charger wattage and pack energy.

Ranges reflect common models; check the current spec sheet for your trimmer’s amp rating or battery watt-hours.

Runtime and energy per job

A typical quarter-acre suburban lot with fences, beds and a driveway edge needs 15 to 30 minutes of trimming. A 6-amp corded trimmer running at an average of 600 watts for 25 minutes uses about 0.25 kWh, roughly 3 to 6 cents of electricity. A 40-volt 4 Ah pack holds 160 watt-hours, which at an average draw of 400 watts gives about 20 to 25 minutes, usually just enough for that lot. Variable-speed triggers and eco modes on battery trimmers can extend runtime by 30 to 50 percent in light grass.

Charging overhead

Chargers lose 10 to 20 percent as heat, so refilling a 160 Wh pack draws about 180 to 200 Wh from the wall. Rapid chargers finish in 30 to 60 minutes but draw more power while doing it; standard chargers may take two hours at a lower wattage. The total energy is similar either way.

Cord gauge and length for corded trimmers

Because trimmers wander around the yard, cords tend to be long. For a 6-amp trimmer, a 16-gauge cord works up to about 100 feet with modest voltage drop; beyond that, step up to 14 gauge. Many people use lightweight 18-gauge cords from indoor use, which can overheat and starve the motor at 100 feet. Use outdoor-rated cords and a GFCI outlet.

Line and head choices affect power

Thicker line cuts tough weeds better but increases aerodynamic drag and motor load. Twisted and serrated line profiles cut more aggressively and can be quieter, while square line bites harder into woody stems. Bump-feed heads let out new line quickly; fixed-line heads using pre-cut pieces can reduce tangling. Matching line diameter to the head’s specification keeps current draw near the rated figure.

FAQ

Can a string trimmer run on an outdoor outlet shared with patio lights?

Yes. At 400 to 800 watts, a corded trimmer uses well under half of a 15-amp circuit, so lights and small loads alongside it are fine.

Does a brushless battery trimmer use less energy?

Brushless motors are typically 10 to 20 percent more efficient than brushed motors, which translates to longer runtime per charge and less heat, though the difference in wall energy per year is small.

Bottom line

Corded string trimmers draw about 400 to 800 watts, and battery trimmers draw 40 to 550 watts from the wall only while charging. A full trim of a typical suburban lot uses a quarter kilowatt-hour or less. Use the right line size and a properly rated cord to keep the motor near its efficient range.