When we began upgrading the off-grid PV system on our new mountain home, our first priority was to add more energy storage, and for the new batteries to be a more advanced technology than our existing lead acid battery bank.
Lead acid batteries have been a mainstay of the off-grid PV world for its whole history, so many PV guys will assume that is the best approach for storage, but it's no longer true. The newer Li ion batteries are smaller, lighter, cheaper over the lifetime of the system, and can be drained to near zero without damaging them. All of these factors made choosing a Li ion battery for our new energy storage system a no-brainer.
We installed about 20 kWh of Li-FePO4 batteries along with our new 8 kW 240 VAC inverter. As I've described in another article, the 240V inverter feeds the house panel, including new 240 V loads like the electric stove, and when the Li ion batteries are nearly depleted, automatically switches back to the old 120 VAC inverter and lead acid batteries, for another 10 kWh of backup for one 120V leg of the house panel. Once the period of cloudy weather is over, the main system recharges the backup batteries as well as the main storage bank. In the long run, we will probably dedicate one of the small arrays of older solar panels to the job of maintaining the energy stored in the lead acid battery bank, via one of the dormant charge controllers still connected to that system.
Our new Li-FePO4 batteries have been an absolute game-changer! Not only do we have about 3 times the energy storage capacity, but we are using a smaller fraction of the energy we generate just to maintain the batteries. We didn't realize how much of our generation was going to "floating" the lead acid batteries to keep them full until we moved them to backup status, where they could stay full and happy until we needed them.
You see, lead acid batteries are really lazy workers. They like to lie around and eat all the time, whether they are working or not. They really don't like to put out any energy, and they will only expend about half the energy they have before they are damaged and their working life reduced. And they are high-maintenance, unless you spend more money for the sealed gel lead acid batteries (which we did, because we didn't fancy adding water to the cheaper kind, in a cold, rather dark cabinet in a cold, rather dark garage, nor did we want any more toxic leaks, such as we had to clean up around the battery cabinet we inherited with the house.)
Li ion batteries, by contrast, are amazing workers. They are happy to put out all of their energy on a daily basis and only eat when they are working. Working until they are exhausted does not damage them or reduce their lifetime. And they are smaller and lighter for the same energy capacity -- by a factor of about 4! Whereas our lead acid battery bank took a beefy man with a large box truck and a lift gate to deliver and mechanical assistance to move and install, and occupies a large custom-built cabinet at the rear of our garage, twice the capacity of Li ion batteries hangs on the wall tucked away under our new inverter, and was installed by hand by one ordinary-sized human who brought them in his car. Like I said, total game changer!
With our new Li ion batteries on the job, we found that even though we had the same small generation capacity we started with (2.5 kW), we had much more ability to run our loads. We were able to install a refrigerator, radon fan, and heat recovery ventilator fans, and run them most of the time. (We also run our other heavy loads -- sauna, electric car charger, power tools, big screen TV -- but we still have to be careful about the timing of those, so that we don't go into a period of several days of cloudy weather with less than a full battery bank and regret it.)
It also took a stressor out of our life not to have to hover over the lead acid batteries, worrying that we would be gone or sleep through a deep-cycle event (literally), damage them, ruin our investment, and have to go through the major effort and time delay of buying new ones and having them delivered and installed. No more crawling into the dark, cold garage to puzzle over an LED readout of the battery voltage and cross-correlate it with a chart of voltage vs energy stored to figure out where we were. That definitely wasn't part of the pleasant, carefree climate-friendly lifestyle we were intending to create here! The new batteries and inverter came with an online app that allows us to see exactly what the energy stored in the Li ion batteries is, from the comfort of our chairs or from anywhere. And there is a beep that alerts us when we go to backup power and another that signals that we are switching back to primary power.
In the end, our improved PV system will include a new 5 kW ground mount array of PV modules to triple our generation, which will be another huge advance for us, but we have gone another year with just the new batteries and inverter and have done just fine. I would recommend Li ion batteries over lead acid batteries for PV backup without hesitation. I expect that they will continue to become cheaper, more efficient, longer lasting, and be made with more environmentally friendly materials as the new battery technologies develop. But already they are far superior to lead acid batteries in our experience.