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I'm little enough of a hardware person that I'm just interested for the reasons the OP mentioned; constantly switching between AC and DC and AC and DC is obviously inefficient, and batteries are expensive enough that it's probably cheaper to add a DC circuit than to double or triple the battery capacity.

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Tesla's powerwall claims 90% round-trip efficiency (AC in vs. AC out) and most SMPS power supplies are also 90%+efficient as well so you are losing <20% capacity (not 100-200%).

Numerically, I'm worried about ~50% power loss. If you are converting your solar from DC to AC there's another loss.

There still might not be enough power loss to matter, but even with 70-80% efficiency you're still talking an extra 25-50% batteries over the ideal.

90% is going to be in ideal conditions; probably not quite at full load, but not much below.

If you are using it to power a very light load, the efficiency will be much worse, as it takes a minimum amount of energy to run the inverter.

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