A Simple Solar Electric Backup System

A Simple Solar Electric Backup System

By Ben Hoffman

If the electrical grid goes down, I need a backup to keep my freezers and wood furnace going. The furnace, a wood gasifier connected to a 500-gallon water storage tank, will burn for six hours when loaded with wood. It is essential to maintain water flow from boiler to water tank to prevent overheating and damage to the boiler. Since winter power outages are fairly common, often lasting for three days or more, I needed long-lasting backup and didn’t want a gasoline-powered generator subject to the problems of fuel polluted with 10 percent ethanol. Grain should be used to feed people, not infernal combustion engines.

My solution was a Xantrex (now Schneider Electric) 1,800-watt solar power package from Northern Tool. With two 150-watt solar panels and two 100-amp sealed batteries, it had the staying power to keep everything running at least until the fire burned out. In addition to keeping the freezers and furnace going, it provides emergency lighting and telephone service.

The Xantrex manuals on installation and operation are complete, though not reader friendly. The answers to some questions are clothed in engineering double talk. The installation is well explained, and I installed the power unit in a corner of my sunroom within eight feet of the solar panels. The short wiring minimized both cost and power loss. By minor modification of some wiring in the cellar, I hard wired two circuits into the power package, one for the furnace, one for the freezers.

The power unit produces modified sine wave AC that is fine for emergency lights, freezers and circulators in my heating system. Modified sine wave is not recommended for variable speed fans such as the one controlling air to my furnace (which causes some minor inconvenience in monitoring and shutting down the fan motor). The breaker box (left) feeds two circuits, one for the freezer and one for the furnace. To work on the system, the switch at upper left disconnects grid power from the power unit. When the solar panels are charging, the Xantrex unit charges the batteries and feeds power to the freezer and furnace loads; at night, it passes grid power to the batteries and loads. above: The battery pack and power unit (above) are enclosed in a cabinet on my sun porch to minimize the lead-in from the solar panels to an inverter that passes 12 volts DC to the power unit. The power unit receives juice from the solar panels to charge the batteries and deliver 120 volts AC to the freezers and furnace. It also has provision for wiring in a wind generator.
The power unit produces modified sine wave AC that is fine for emergency lights, freezers and circulators in my heating system. Modified sine wave is not recommended for variable speed fans such as the one controlling air to my furnace (which causes some minor inconvenience in monitoring and shutting down the fan motor). The breaker box (left) feeds two circuits, one for the freezer and one for the furnace. To work on the system, the switch at upper left disconnects grid power from the power unit. When the solar panels are charging, the Xantrex unit charges the batteries and feeds power to the freezer and furnace loads; at night, it passes grid power to the batteries and loads. above: The battery pack and power unit (above) are enclosed in a cabinet on my sun porch to minimize the lead-in from the solar panels to an inverter that passes 12 volts DC to the power unit. The power unit receives juice from the solar panels to charge the batteries and deliver 120 volts AC to the freezers and furnace. It also has provision for wiring in a wind generator.

The freezers draw the most power, and since they will hold for two or three days without power, as soon as we get an outage, I throw the circuit breaker that turns off the freezers. This leaves max power for the furnace and emergency lighting. Thirteen-watt spiral light bulbs draw negligible power. As soon as the furnace burns out, I switch the freezers back on and use the kitchen cookstove to heat the house. The batteries are largely maintained by grid power, but when outages occur, even a minimal amount of daylight will recharge them.

The solar panels are mounted on the side of the sunroom, close to the ground, rather than on the roof. Roof mounting would gain more sunlight, but there would be more power loss from the longer wiring; plus, in winter, the panels are easily cleaned of snow. The panels produce electricity with indirect sunlight and will even generate with a layer of snow. They are mounted on a hinged frame so the angle to the sun can be easily changed to maximize solar collection. Rigid panels would be set at 45 degrees at my latitude, but I can vary the angle from 15 in winter to 90 degrees in midsummer.

In addition to meeting emergency needs, the system cuts my electric bill by $12 to $15 each month in winter. Summer should be even better. Best of all? No noise, no fouled spark plugs and no gasoline storage.

Leave a Reply

Your email address will not be published. Required fields are marked *