The 240v AC outputs on my inverter are L1, L2, and G. Measuring L1 to L2 is ~240v. Without bonding either L1 or L2 to G (tied to 8 foot copper ground rod), L1 to G is ~90v, and L2 to G is ~150v. Apparently this is common for single phase.
Without either L1 or L2 bonded to G, the gen 2 UMC threw...