Hitachi 18-volt battery-powered 12" string trimmer with TWO battery packs and charger.
(From Lowes or Amazon, it only has one battery pack.)
Does the job with no oil and gas to mix, no toxic exhaust fumes to breathe, no loud noise, and no yearly maintenance hassles and expense. You will never switch back to a gas trimmer after using a battery trimmer.
This trimmer works very well on normal grass trimming, and a battery pack lasts up to an estimated 20 minutes, maybe less with heavier cutting. Used a little, I think it's still on its original string.
Large yard? Add two more battery packs and another charger for $100 for longer continuous daily use times. Buying battery outdoor power equipment is like paying for all the gas and oil you'd ever buy (plus maintenance costs) up front with nothing else to add.
Also, compatible 18V Hitachi hammerdrill/driver, impact driver, recip saw, and circular saw available if you're building up your cordless tool collection.