Going off-grid has been one of my main goals for a while now. I'm halfway there in that I generate enough kWhs over a year to cover my usage, though at night I'm still putting electrons moved by a combo of coal and nat. gas in my car's battery.
While outages are less common now than they were just a year ago in my area, we still get nuisance flickers that last long enough to take out my internet connection and reset the clocks. Plus, PA has started to nibble away at net-metering, reducing the allowed solar capacity from 50 kW to 125% of the annual usage. We don't have any TOU rates here, but solar payback for end-of-year excess is nil, maybe 2 cents/kWh while they charge 10 cents for usage.
Eventually, net-metering is going to go away and end up costing solar owners big. This is already happening in Nevada. Having batteries gives one the ability to ditch the grid (or at least being a consumer-generator) and escape the penalties. Net metering really only gives you access to rent virtual-battery storage. You pay a monthly access fee (meter fee). Any energy you put in you get back out at no cost, but pulling more than you generate costs. Being similar to a cloud-type service, rates change, fees increase, and what is preventing the service from disappearing in the future? At least with your own batteries, your only financial obligations are initial costs and ongoing maintenance, and the only thing to stop you from using batteries are when they fail. The utility and government can't realistically deny you access to energy you create and store in your own battery bank. I think that is the motive behind wanting a battery-based system, the sense of energy independence. Of course, as WK found out last month, that sense of independence comes with some real world benefits (like not knowing that the grid was out because it isn't necessary 100% of the time).
Even with micro-inverters, adding battery capability is doable with an AC-coupled system. While not as efficient or graceful as a DC system, a bi-directional inverter and battery bank is added between your breaker panels and the meter so that the inverter can keep the micros running without grid power. The problem with these systems are that solar production is either on or off. Either the micro inverters are outputting, powering the loads and sinking any extra in the batteries, or they are off. The main inverter shifts the frequency of the AC legs up if the battery is near full to shut down the solar production. This puts a lot of un-necessary cycles on the battery pack. But this is all when operating without the grid. When the grid is present and in use, it acts just like a standard grid-tied system.
AC-Coupled is good for solar owners that already have micro-inverters and don't want to change them out. However, for a new system where you think you might want batteries in the future, either a central inverter or a setup like SolarEdge's might be better. My system is at least ground-mounted, so I only need a ladder to reach some of them from underneath, making future changes easy.
Batteryless gives you the quickest ROI. Battery-based gives you the most flexibility. But, if batteries save you multiple fridgefuls of groceries from going bad during extended outages, your ROI increases quickly.
