My approach is simple; I use as much free energy from the sun as my system can supply, and switch to grid-supplied electricity only when necessary. This strategy may seem odd to those who are conditioned to think of a battery-based system as a backup to the grid. It’s counterintuitive. The first step is to get comfortable with the concept of using the power grid as a backup system.
When both sources of electricity are available, make sure that no one in the house uses grid power. Design the system to switch automatically, with battery power as the default. Switch to grid power ONLY when batteries are depleted. Many inverters have this capability built-in, but you can purchase an Automatic Transfer Switch if yours does not. By transferring all lights and electrical outlets in your home to battery power, no one has access to grid power, and therefore no one will be running up your electric bill by accident or without your knowledge. If your PV system is small, high-power items will have to be excluded. Your central air conditioner, for example, will probably not be switched to battery power. Once your batteries become discharged, late at night perhaps, lights and outlets in your home will be switched to grid power. Fortunately, this is the time when electricity use will be at its lowest level for most households.
Inverters draw energy from the batteries until battery voltage drops to a point where the inverter can no longer function. Typically, the cut-off point is about 10.5 volts (for a 12 volt inverter). Unfortunately, allowing batteries to discharge that much can be harmful to them. You’ll need another way to switch your inverter off. And, once the batteries are drained to that level, they should be recharged fully before reconnecting loads. Reconnecting batteries to the load too soon could result in chronic undercharging, which would shorten the life of the batteries. Some charge controllers allow you to configure disconnect and reconnect set points, or they have that functionality available as an option. Look for that feature in the equipment you’ll purchase. With these things in mind, let’s review system functionality:
The sun rises in the morning, and batteries become fully charged.
Selected AC loads are switched to battery power via the inverter.
The sun sets in the evening and battery voltage declines.
When battery voltage falls to a preset level, AC loads are switched to grid power.
The cycle repeats each day.
The concept I’ve described here is simple, but effective. It eliminates the waste that would occur if you were to switch manually. After all, you’re not setting at the controls 24/7 watching for the ideal time to switch, are you? By switching automatically you might save hundreds of dollars each year on your electric bill. If your PV system is small, start with one or two rooms and add rooms as you add solar panels and batteries to your system. An electrician can easily wire in additional circuits as your system grows.
I use this strategy in my home, with a chest freezer and refrigerator as the only loads. Instead of tying into my existing house wiring, I’ve added separate wiring to those appliances. Because I didn’t tie in to the existing house wiring, I saved myself the cost of having an electrician do the wiring. I’ve observed that my system switches to battery power on sunny days at about 10:30am. It switches back to grid-supplied AC at about midnight. It’s easy to see that I could use more batteries and solar panels. I plan to expand, and I hope that by the end of next year I’ll be able to add to the existing loads. By making sure that the size of the load exceeds the capacity of the PV system I know that I’m getting as much power as the system is capable of producing.
The system as described should cut your electric bill considerably, but you can cut it even more if you watch for opportunities. A properly designed system should include enough PV capacity to fully charge your battery bank each day, and excess energy is often wasted. Watch for opportunities to use that otherwise wasted energy. For maximum efficiency, use appliances mid-day with power coming directly from the solar panels.
The way to get the most from any Off-Grid PV system is not to let any solar-generated electricity go to waste.
An automatic transfer switch should be installed by a properly trained and licensed electrician.
John
