$3500 doesn't include the inverter or transfer switch. Transfer switch is maybe $200, and it's not clear what the inverter might cost. It's a nontrivial part, since it would be responsible for daily cycle estimation and cost optimization.
For $3300 you can get a 17 kW generator with inverter and transfer switch [0], which will run for days on a house-sized propane tank. At my house in the woods, we do sometimes get multi-day power outages after storms. Though, to be fair, you need a concrete pad and 30 square feet to install the generator.
It doesn't seem like a competitive backup system, but load shifting is still appealing.
For $3300 you can get a 17 kW generator with inverter and transfer switch [0], which will run for days on a house-sized propane tank. At my house in the woods, we do sometimes get multi-day power outages after storms. Though, to be fair, you need a concrete pad and 30 square feet to install the generator.
It doesn't seem like a competitive backup system, but load shifting is still appealing.
[0] http://www.costco.com/Honeywell-17-kW-Automatic-Standby-Gene...