It is, for large enough values of “day”.
You’re in New Jersey, I gather 
A friend of mine got solar and at the time, there was some kind of state program that paid something like 2/3 of the cost. Is that still in place? That would dramatically shorten the payback time. Does your 8-10 year figure use that as a basis?
I don’t know how many states have anything like that in place. We’re mid-Atlantic (DC area) and until about 5 years ago, our HOA explicitly forbade solar panels; a state law was passed that outlawed such rules, so we could put them in - but we don’t have a spare 50K (or whatever it is now) to do so.
Friends of mine in Phoenix had done the math 10 or so years back - and even in Phoenix (similar climate to LV) she said the cost tradeoff was not worth it at that time.
For NJ whatever is offered for energy conservation is generally found here: Board of Public Utilities | Residential<br />Customers.
The only remaining help for Solar is the SREC program I mentioned in another post. I got a 70% rebate for my first set of panels and was one of the earliest adopters. My second set of panels only got a Federal Tax rebate, I’m not even sure that is still available.
The first set of panels were more expensive but between the rebate and at the time higher SRECs those paid for themselves in 6 years. That was pretty cool. At the time I did it as a wide eyed optimists green more than as an investment. The second set of panels were cheaper and have pretty much paid for themselves and it took 9 years. I went from 6700 watts to 10000 watts with the expansion.
Battery systems are expensive, regulations are often tough but quite doable and there is never any incentives.
The Aus batteries are not for daylight / night-time energy storage. They are for minute-to-minute energy storage. They make money by providing energy when it is really expensive – not by providing energy at night.
Battery technology is improving. The batteries aren’t in place just because solar sucks (the trigger incident was problems with wind and gas-fired stations). The Aus grid apparently used to have flywheel storage at critical places: that is being replaced by batteries.
The problem in Phoenix is that the energy companies really don’t want to support home generation and the commission in charge of regulating the power companies aren’t pushing them. If you want net metering with my power company you can get it, but you get put onto a peak demand pricing program where a big chunk of your bill is determined by the highest power usage (I think over a 1 hour period) during the month.
They just announced a new metering system with one meter for incoming power and one for outgoing power. Supposedly you will be allowed to keep a normal rate plan. Different rates for incoming and outgoing power. They aren’t advertising the rate that they will buy power for, but I bet a small fraction of what they sell it for.
Just realized I should have mentioned this in an earlier post. Las Vegas does get a small percentage of power from hydro.
The more solar panels you hook up to the grid there, the less water used to generate power at the times the panels are generating. That water can then be used at other times. So, like the Columbia River situation, you have what is effectively a solar power storage system for trivial additional cost.
Now, since Las Vegas’s hydro usage isn’t all that great, once solar really takes off things will change. The extra solar power can be sold to the bigger users of Hoover and other dams’ hydro power who then can manage draw down better.
Factor in the need to have hydro power or other power generation sources as a backup to solar. Unlike solar, which is affected by weather conditions, such as a tornado, the water doesn’t stop flowing. Don’t put all your eggs in one basket. Solar is great, but it’s a fine balance for the utility companies to ensure when some eggs break, there’s a source of new ones to replace them.
But keep in mind in many areas of the country there are already hydroelectric systems built and running. And in some cases, like around the Colorado River, the water is in high demand for multiple uses, including hydro. So if you avoid using some of the water some of the time that leaves more water available at other times.
Storing renewable energy isn’t a serious problem in many cases now and will be less of a problem in the future as things adapt.