Inverter/charger sizing and split-phase basics

An inverter/charger (Victron Multiplus, Magnum, Xantrex Freedom) does three things: inverts DC to AC when off shore power, charges the house bank when on shore power, and passes shore AC through with an automatic transfer switch. Sizing rules: (1) Continuous output should be 60-70 percent of your worst-case AC load, a 3000W inverter comfortably runs a 2000W load continuously with room for microwave surges. (2) Charger amps should be 20-30 percent of house bank Ah for AGM, 40-50 percent for lithium (lithium accepts fast charging). (3) DC input: 400A of continuous input at 12V for a 5000W inverter is not fantasy, you need 4/0 cables to the bank and a MEGA fuse sized above the inverter's peak DC draw. Split-phase (US 240V shore power): most inverters are 120V, so 50A 240V shore inputs are wired as two 120V legs going through the transfer switch on each leg independently. Never bridge legs in the inverter or you'll create dangerous voltage differences. Common failures: (1) Whine or buzz on AC output, poor DC connection, tighten every terminal. (2) Transfer switch chatter, worn contacts, replace the whole unit. (3) Charger stuck in absorption forever, misconfigured battery type in the profile, or a defective battery. (4) Overheat shutdown at rated load, check ventilation, most inverter/chargers need 3-4 inches of clearance and can't be buried in a locker.