Even once solar makes sense for the house I live in, its a tougher sell in a rental: If I were to do install solar on a rental, there's the added complexity of "how do I charge the tenant for electricity?". I could not charge them, but have higher rent. But it takes a pretty sophisticated tenant to properly account for the value there, so that would make the units appear less price competitive than they actually are. I could install a meter and charge them the same way as a power company does. But does that make me a power company? Are there a bunch of new rules I would have to follow? What do I do if someone doesn't pay the bill? I'm sure I could figure it all out, but it makes the investment less attractive, which means the price of solar relative to buying from the grid has to be even better before I would go down that route.
(edited for typos)
Just buy another electricity meter and install it after the inverter.
It seems like there are two justifications for solar if you’re on the grid. Either you want to do right by the environment (totally good, though very pricey, reason to do it!) or your electricity costs are very high. If there are others I’d love to understand them. Maybe if you’re a prepper (not making fun) then that’s another reason.
Financially, most likely he could have an even higher "ROI" with a much smaller solar battery (sized for the daily use of time shifting), plus a generator for long term outages. Unless your grid rates are such that you can buy electricity off peak, charge the battery, and then profitably sell it back to the grid (effectively operating your own small time distributed grid storage), sizing a battery to provide for multiple day outages is cost prohibitive.
If you don't have net metering, then you get bitten by needing to massively oversize your solar array to still fill the batteries on the worst case dim winter days, so that you can make it through the next night (likely powering necessary heating loads). That overproduction is then wasted on peak days unless you can find something clever to do with the excess electricity.
I'm sure there are other scenarios with time-varying electricity rates, but I'd think that calculation would be much closer to general time shifting with the backup power as an added benefit, rather than revolving around the backup power.
You'd need some pre-planning to segregate different circuits to do partial transfers and/or some smart appliances which can operate in reduced power modes. In the survival mode, maintain a critical baseload of lighting and food storage, perhaps even with some freeze-proofing heating mode that does not hit regular comfort targets. Use high-load equipment only when there is excess solar input beyond what is needed for battery maintenance etc.
You could also provide the right kind of exterior input to allow a portable generator to be added to the system in an emergency situation. This could support higher peak loads or recharge batteries when solar input is insufficient.
It's not exactly the same as just adding battery backup, because the marginal cost of adding battery backup to a solar inverter is far lower than getting an inverter for just backup purposes, and solar allows a smaller battery.
It's way easier to capacity plan with a fuel based generator though. My 35kW generator can go at least several days on a 500 gal propane tank, regardless of weather. Potentially indefinitely if propane delivery is available, but I lose phone service when the power is out beyond 4 hours, so I'd have to venture out to get signal to call, and if power is out for more than a few days, the weather is probably pretty bad and delivery may not be available.
With a solar + battery system, handling potential multiday outages, I'd be concerned about my winter where there's less than 9 hours of daylight and it's often mostly overcast, and I'm running my heatpumps to warm the house. But if I have solar/battery capacity to make that work, I'm going to have way too much capacity for most of the year.
Solar + battery for grid backup makes more sense in places with shorter duration outages and were peak electricity use is in the summer rather than the winter.
In the past year or two we’ve had an eight day power outage, five day power outage, multiple 1-2 day power outages, and who knows how many hour-ish interruptions and gasoline or propane for the generator get expensive and inconvenient running it for long periods of time.
Our main limit to how long we can comfortably survive without power is that we’re on well and septic. Without power the pumps for both aren’t running so we have no running water and no sewer. We have bottled water on hand to at least cover drinking, but if we were to just shit in a hole outside we would basically just be dumping raw sewage into our water.
Local power generation and storage is very interesting to me.