Can you actually take your home off the grid? This seems to be the holy grail of solar fans everywhere. The answer is yes, but it’s easier said than done. (See my previous post about off-grid systems for some background). If you have utility power and want to have off-grid capacity in case of a blackout, you need a battery backup system for your solar array.
I recently designed a 5.6kw pv system with battery backup capacity for all the critical loads of the house. A well pump, 2 refrigerators, 4 ceiling fans, and lights for the whole house were backed up on a 700 Ah battery bank. SMA has a new product called the Sunny Island that works very well for grid tied and off-grid operation and seamlessly makes the transition between the two. The Sunny Island is a battery inverter that changes energy from AC to DC and back. One thing I really like about this product is its flexibility. You may have noticed I said AC to DC. Most inverters, at least the older kinds, only convert from DC to AC. In these older systems the DC solar energy would go to a charge controller that would charge the battery, and on-demand energy would get converted to AC at the inverter and go out to the loads. A simple line diagram would look like this:
In the Sunny Island system energy from the panels is converted at the inverter to AC and then sent to the load. If the load is satisfied the extra energy can be used to feed the battery or feed the grid. If the grid goes down the Sunny Island senses this and allows the load to be fed by the inverter or from the Sunny Island, which is pulling from the batteries.
So the line diagram would look like this:
PV panels–>Inverter–>Load<–>Sunny Island<–> Batteries or Grid
The beauty of this system is that in off-grid mode the load can be fed by the panels by way of the inverter if it’s sunny out. Or it can be fed by the batteries by way of the Sunny Island if the sun is not out. Of course in the real world your energy demand and solar production is in constant flux and the Sunny Island works in real time to pull from the best source to meet your needs.
The Sunny Island is currently more expensive than the older model inverters but in my opinion it’s completely worth it if you are doing a large load like a house because of its capabilities. If you were doing a small load there are other less expensive options.
There are a few things to be careful of, becaue the product isn’t perfect! Older Sunny Island models did not accept a 2/0 wire which is the minimum as specified by the NEC, so we had to use 2 -1/0 wires in parallel. This made for a very tight squeeze in conduit and 1/0 wires do not bend easily. I spoke with the techs at SMA and they were aware of this problem. Since SMA was designed by Germans there were a few things that did not go so well in the transition to the North American market. They were aware of the problem and said they had a new line of SI’s coming out that would accept 3/0 wires max. I would highly recommend you get the right model and have less wiring and less hassle. Also, be sure to use proper fine-stranded battery grade wires for good flexibilty.
Batteries can be dangerous. Since many complications arise in the design process it’s very important you choose an installer who has experience designing and installing battery systems. NABCEP is a good place to start looking, but make sure to ask if they have experience in off-grid systems because not all do. Listen to your PV designer’s advice and have realistic goals. The battery system will not be able to feed a home’s central hvac system or an electric cooking range and the costs would quickly spiral out of control if you tried to design a system to for those heavy loads.