One of the biggest problems with energy production from renewable sources is that two of the biggies, solar and wind, are inconsistent producers .. the sun only shines during the day, and the wind blows whenever it damn well pleases.
The solution for those who wish to stay off the grid entirely is batteries, but batteries have serious problem: they're expensive to buy, expensive to maintain, don't last very long, and turn out to have multiple inefficiencies which I will detail below the squiggly thing ..
So you have a solar system that generates 20KWh/day. Five percent is lost in the charging system (1KWh), another five percent in the inverter. Charge/discharge of lead acid batteries carries a 30% chemical conversion loss (you get 30% less than you put in), for another 6 KWh lost. So of your original 20KWh generated, you actually see something like 12KWh available for your use.
Battery technology continues to improve, and I have no doubt that eventually we'll have ways of storing enough power in a small package that the entire electricity infrastructure will change to a battery based system. Imagine going to Wally World, picking up a car-battery-sized object which would power your entire home energy needs for a year!
But we don't really have to wait for a better battery .. at least not those of us who have a current connection to the electrical grid and live in a state with net-metering regulations. Here's how it works:
You purchase and install photovoltaic panels and connect them to a special piece of equipment called a "grid-tie inverter". This type of inverter is designed to be fed a source of DC current (PV or wind) which it converts to household AC synchronized to the power grid. Generation which is tied to the grid must all be synchronized (AC is a sine wave, you can't have generators all making sine waves willy-nilly, they all need to have the peaks and troughs match almost exactly), once synchronized that power is indistinguishable from that which is entering your house from the grid.
So you have your PV array(s) and grid-tie inverter(s) connected up, and the sun comes out. The inverter generates power which it feeds into your house distribution. If you are generating more than your house currently needs, the excess flows out of your house and back into the grid. If you are generating less than your house currently needs then extra flows in to your house from the grid. Current flowing in to your house makes the power meter spin in a positive direction, current flowing out of your house makes it spin backwards!
Here's the cool part .. if you had to store that excess power in a battery, you'd be paying that hefty 40% surcharge in electrical and chemical inefficiencies. By feeding that extra power to the grid, you avoid it all! The power grid is a perfect battery! Whatever you feed into it, you can get back out of it with no storage cost at all!
Once you realize that the grid is your friend, you can start thinking about solar in a different way. If you're not maintaining batteries, then it becomes much less important to have a perfect site. If you just look at your total annual power budget it turns out that all of the excess generation you do in the summer during long hot sunny days can be used in the winter when you're not generating much at all, courtesy of the grid.
The key is the net-metering concept .. the power company (your meter) keeps a tally of what goes in and what goes out. If, on a given month, you generate more than you consume, then they bank the excess for your use in future months. If you consume more than you generate, they draw from your bank before selling you 'new' energy. So a grid-tie system actually lets you sell your power back to the utility at retail rates most of the time. Once a year they total everything up, if you have annual excess they pay you cash at the wholesale rate. So if you properly size your system so it doesn't generate more than your annual usage, everything happens at retail and you save a bundle!
Complete 5KW grid-tie packages run around $20K today, they'll only come down in the future as PV manufacturing continues to get cheaper. You can even do it on the cheap .. ebay has 300-500W micro grid-tie inverters for a couple hundred bucks that would support a couple of 100W panels and just plugs into an outlet, no electrician needed. You can distribute your generation capability around as needed.
I'm a firm believer in battery technology, and I think it will change our energy future in positive ways, but while waiting for the perfect battery don't overlook the one the power companies have provided for us!