The HF genny might be a better/cheaper option than a battery. Quick google search shows that the typical electric trimmer is 5 or 6 amps. You would probably want to use a 1000watt inverter for headroom and 120VAC/5amps pulling close to 60 amps from the 12v battery including inverter losses. A group 31 size deep cycle would only get you an hour run time on that load. Then you have to wait 8+ hours to recharge it.