Adding battery backup to a grid-tied PV system with DC coupling means running some of your solar array through a switch and a new 600V charge controller, and - just like with AC coupling - adding a new battery-based hybrid inverter. Your new inverter/charger will charge your batteries from your solar power or the grid (whichever is available). When the grid goes down, the new inverter powers you critical loads using the charged batteries. To recharge your batteries, you'll flip the switch to route the portion of your PV power running through the charge controller toward your battery bank until the batteries are charged up and ready for the next grid outage.
The primary advantage of DC coupling vs AC coupling for battery backup is that with a charge controller involved, you can get away with a smaller battery bank (especially if you use lithium-ion solar batteries, which can be discharged fully) which, in turn, means the new inverter can be smaller.
The primary disadvantages of DC coupling vs AC coupling are that you'll have to buy that 600V charge controller and be ready to flip the switch to send your solar power to the batteries if they need a charge while the grid is down.
The DC coupling solutions below range from the more DIY (our Morningstar kit where you'll have to do the wiring) to the plug-and-play (like the SimpliPhi AccESS units, which include everything pre-wired together in an enclosure). If you're unsure about how to best add battery backup to your grid-tie solar system, give our techs a call at 800-919-2400.