- Have you measured your actual overnight use?
- Have you measured the actual use you have when solar is not providing your electricity, which is when the battery will discharge (whether or not it is “overnight”)?
In my experience, a small home with barely enough space on HALF the roof for 5.6kWp solar (so if we get some more solar in morning direction on the OTHER half, it would be a 10kWp system too), we use up the full capacity of 27kWh EVERY DAY during the winter. Part of the problem is that our 5.6kWp isn’t enough to fully recharge the batteries, but the other part is that we actually use a lot. Much of our use is electric space heating; do you have any high draws overnight, such as A/C, electric heater, etc.?
- With the batteries I have, there is no concern of the existing inverter, because they already have all their own inverters built in. I have the Tesla PowerWalls. In the case of the Tesla PowerWalls, once you’ve paid for their standard system, in USA you get backup for free, whether or not you want it, although it COULD be engineered out of the system at extra cost for no gain, and you’d have equipment sitting there capable of backing up just not working.
So, I have no idea what you’re talking about when you mention “do I need some sort of inverter/charger for the battery system too?” Yes, of course you do, is my answer, BUT, you MIGHT be able to use the solar inverter IF THE SOLAR INVERTER has unused battery ports ALREADY BUILT in to it THAT WILL BE FULLY COMPATIBLE with the new batteries you purchase, almost none of which is true of off-the-shelf inverters actually installed anywhere at the time I got solar two years ago (maybe some of that has changed). Typically the few inverters that DO have battery capability built in are (a) proprietary, meaning they lock you in to one battery type, and that battery type is usualy bankrupt, out of business, no longer made, or unavailable by the time you want to install, and (b) that battery capability is rarely installed in solar installations when a battery isn’t ALREADY ordered and being installed at the same time.
So, my recommendation is to go spec your existing inverter, what its full capabilities are, how much of its full capabilities are available to you to add use of, and what those specifications are, find out what batteries meet those specifications (likely NONE), AND THEN start to add to your possible battery setups all the other battery configurations you could get that each require whatever they would require given what you have and don’t have.
I.e., you’ll likely need the charger, inverter, and whatever controllers necessary, just for the batteries, depending on what inverter capabilities are not used on your existing inverter(s).
The easiest is to figure out how many 13.5kWh Tesla PowerWalls you need (keeping in mind a buffer at the bottom) to get you whenever the sun isn’t shining, order a Tesla PowerWall or 2 or 3, but the lead time for that is long.
Almost every inverter company has their own locked in battery type of the week; it’s best to specify them as a pair via one of their engineers. Get quotes from each one and see what you can do!