We have a natural gas 15kw generator purchased in 03 for our residental home. It runs about 75% of the house. It cost $4,400 back then which included the Standby Gen. a 2nd electerical box with transfer switch and all the wiring. The unit runs a self test once a week on the date and time you set. Best thing we have done for our house. We live in Mich. and just lost power for 24hrs. and we didnt skip a beat, although felt bad for our neighbors. We pay $180 for the yearly maintenance, which includes the oil, filter change and a load test.