My experience taught me the price of a natural gas-powered automatic genset might not have been as outrageous as I first thought (now down to under $2K).
If you go the gas route, you’ll need 3 to 5 KW capacity, which is in the range of $500. Remember, you get what you pay for, and there’s some real junk out there.
You’ll need a manual transfer switch to protect linemen from accidental backfeed. Don’t even THINK of running a generator without one, anyone who says they “always” remember to pull their main breaker is playing with soeone elses life. They’ll run another $250 or so if you install it yourself. Probably 2-3X if you have to hire an electrician. There are some cheap things that add a double-breaker and a metal slider plate for about $100 or so.
You don’t want to run your genset in an enclosed area, not even a garage. Definitely NOT in your house! I eneded up building a separate detached generator shed. Cost will depend on your DIY skills.
You’ll need at a minimum to run a feeder cable or buy a generator cable and wire it in (or hire an electrician to do it).
You’ll also need to mess with gasoline, I change mine out ever year and still treat it with StaBil. It’s a hassle.
Good luck. For those who need electricity for a medical device or other life-safety reason, I’d strongly recommend the packaged backup power unit, running from LP or natural gas.
In addition to the advice above, try to get one where the engine is not made by China, but you need to be careful as even Japanese manufacturers may sub out manufacture of the engine
For someone in California, I wouldn’t consider natural gas. It won’t be there in the aftermath of an earthquake. Big propane tank, or diesel, or gasoline, stored on site.