There are some inverters on the market that can directly power a load with no battery or grid connection.

I guess this was one of the first: http://www.smainverted.com/how-to-explain-secure-power-suppl...

At $500 additional cost, calling it a design issue is probably the correct thing.

Of course an auxiliary plug is way less convenient than whole house power, but a couple thousand watts is way more convenient than no electricity.