In the comments on my posts about backup power, several people suggested looking into natural gas generators. After learning more about them, though, and about our local infrastructure, I don't think they make sense for us.
Unlike propane, which will stay liquid at room temperature with reasonable pressure (~150psi), liquifying natural gas for efficient storage requires extremely low temperatures. This means you want to pipe it in, and our house has a pipe to feed our furnace. This is pretty common here in Boston, the main alternative being oil, delivered by truck.
Gas delivery is extremely reliable: I've never experienced an outage. It has to be this reliable, because appliances have been designed with the assumption that gas won't ever go out. Older furnaces and stoves have pilot lights, which constantly release a small amount of gas and burn it. If the gas went out the pilot would too, and then when the gas came back on the house would slowly fill with gas. This means that if there were to be an outage the utility wouldn't be able to turn the gas back on without technicians going door to door to every affected house.
(This dynamic, where natural gas utilities are trying very hard to always provide sufficient flow, was a contributor to the Lawrence explosions. In switching from an old distribution paper to a new one they missed a sensor. As they turned down the old pipe the sensor started to read low pressure. This triggered the system to increase pressure in the new, unmeasured, pipe, trying to avoid an outage. As it continued to read low pressure the system opened the valve further and further, allowing high-pressure gas to flood the system.)
Because natural gas is so much more reliable than electricity, it is a common choice for permanently installed standby generators. These are typically sized to power an entire home, and do make sense in places with frequent power outages. If you wanted a cheaper option, it is straightforward to convert a portable generator to run on natural gas—many companies sell conversion kits—and you could install an external hookup, the same as you would for a natural gas barbecue.
In thinking about whether a natural gas generator would be useful for us, the main question is what might cause an extended outage. I think there are roughly four causes:
Delivery: massive storm, poles are down, huge amount of work required for technicians to get everything working again.
Consumption: there is far more demand for power than was expected, and generation cannot keep up.
Generation: we aren't able to produce enough power to keep up with demand, likely because some important input is missing.
System: something like the 2003 Northeast blackout. Perhaps multiple failures combine in an unexpected way, equipment that is difficult to replace is damaged or untrustworthy, and even though most of the system is still in place and potentially functional it can't be used.
Thinking through these, it seems like the space of "prolonged blackout" that does not include "natural gas outage" isn't that big? We are in a dense area, the kind of place that utilities prioritize for service restoration, so I think local delivery failure is not a major issue for us.
Power in here in MA is 72% natural gas. Something that disrupted gas delivery would likely cause power outages through both (a) generation issues and (b) consumption increases as people switched to resistive heat. A disruption could look like the recent gasoline supply cyberattack, a pipeline accident, or a storm. The latter could also cause issues via consumption; this is what we saw in Texas (while wind turbine failure was heavily blamed at the time, natural gas production issues were a much bigger component).
It seems to me like residential solar, configured in a way that lets you get power when the grid is down, is a better option for us and other people in places where (a) power outages tend to be short and (b) natural gas is a large portion of local utility generation.