In many ways providing backup power to a home resembles the problem confronted by those using recreational vehicles. Most of the time, backup power isn't needed. The RV isn't occupied, while the home has utility power. A small fraction of the time a great deal of power is needed, perhaps to run air conditioning or a cooking appliance. Nearly all the time at least a small amount of power is needed, to run communications gear, food storage refrigerators or lights. The solution developed usually consists of a generator large enough to run the intermittent loads while charging batteries and then uses batteries via an inverter to power the small continuous loads when the generater isn't running. The RV industry has developed a wide range of hardware to satisfy this need. However, the low "usage factor", the fraction of time the system is actually in use, is very unlike that of a residential backup system. In an RV the usage factor is modest and predictable. Operating cost is insignificant compared to purchase cost, so running efficiency matters little compared to purchase cost for the case of a recreasional vehicle. The inverter is commonly packaged together with a battery charger and a transfer switch. When AC is available from mains or generator it is supplied to the loads and battery charger directly. When AC goes away the load is transferred to the inverter. When AC returns the loads and charger are transferred back to mains or generator. The transfers are effected in less than one cycle, so no service interruption is detected. It's possible to arrange a two-stage fully-automatic backup: First go to the inverter, if the batteries run low start the generator. Apart from those rich enough to own several, folks use their home all the time. Total cost of backup power is much more dependent on running costs, though initial purchase remains critically important. With an RV one usually knows about how much it'll be used and can plan accordingly, With home backup power there's no such certainty. The usage factor for backup power systems is probably lower than for most RVs. Vacation time is more abundant than blackout time, but blackouts occur with little to no warning. The motivational pressures are utterly different. Despite the differences in motivational pressure, RV power technology is closely aligned with residential backup power requirements. It makes practical sense to base home backup solutions on existing, commodity RV power technology unless that's a gross mismatch in desired features. The only obvious mismatch up front is that home backup is in standby all the time while RV applications only a few percent of the time, being completely turned off for the remainder. The operational difference between RVs and homes is the standby time. Mostly, RVs are unused and off-grid, except perhaps for things like trickle chargers. A home will be in standby mode, using grid power but ready to switch to battery-backed inverter power, essentially all the time. That makes the standby losses of the inverter/battery system considerably more important than in RVs. An inverter in UPS mode will dissipate a few percent of rated power continuously idling the inverter. Batteries require some amount of maintenance charge and the charger isn't 100% efficient. That makes it advantageous to size the UPS load near the minimum, to conserve standby energy consumption. 100 watts of standby draw over ten years will amount to thousands of dollars. The other mismatch between RV use and home standby use is running fuel consumption. On an RV the generator fuel consumption is tiny, compared to that of the RV. For home backup, fuel consumption is tiny as a matter of cost, but it can matter critically from an availability point of view. Events that lead to power shudowns tend to interrupt any fuel distribution other than piped natural gas. Service stations can't pump fuel without electricity and propane sellers usually use electric pumps as well, even though the fuel is self-pressurized. That's what makes natural gas most attractive. 20230801