Backup Power for Utility Outages
Backup Power for Utility Outages
Many folks whose power comes from utility lines contact us hoping that off grid solar power systems will get them through utility outages. Backwoods Solar can supply back-up systems to cover utility outages. Usually solar modules are not needed in utility backup systems, because sunshine tends to be scarce when weather knocks out utility lines. Our Xantrex XW or Outback Radian systems with batteries is a complete package for back up power and it also offers you the opportunity to sell power to the grid if that fits your design!
These standby inverters will keep batteries fully charged when utility power is on, then immediately take over some of your circuits with hardly a flicker if the utility fails. This is all automatic and quiet. It can carry you through limited duration power outages. Since batteries contain less energy than a tank of generator fuel, battery and inverter systems must be designed to take over only carefully selected circuits in the home. Using specially efficient, low power appliances and lights just as we do in off the grid homes, and connecting only your most critical power needs to the standby inverter system allows longer operation on a given battery size. And you can add more batteries to increase the time period that can be covered. These standby inverter and battery systems can automatically support a few selected emergency needs during short utility outages. When the utility fails, the system can instantly deliver power to a computer, cash register, pellet stove, or lights for frequent but short utility outages. Batteries are kept at full charge by utility power until an outage, at which time the inverter automatically takes over the selected emergency needs.
Additionally, a quiet, durable propane fueled generator set with a manual or automatic transfer switch can power the whole house during hours, days or even weeks of power outage emergency. No major conservation or wiring changes in the home will be required (but it is wise to add several propane fuel direct-vent room heaters). We can suggest the best quality, quiet, long-lived generator (and non-electric direct-vent gas heaters) to back up a conventional utility connected home during power outages. A local licensed electrician should install the transfer switch between a generator and your utility served circuits.
Usually a generator is the simplest solution. Without implementing conservation measures or modifying your home at all, an electrician can install a transfer switch that disconnects the house (or just part of it) from the utility meter, and connects it instead to your own generator. This can be manually operated upon power failure, or can be automatic transfer that starts the generator for you and transfers the house circuits over to it whenever the power fails. A manual rather than automatic start and transfer means you can run the generator only during the busiest parts of the day which should be enough to keep water pumped, food frozen, and supply lights and TV and other appliances with power for living off the grid.
Propane generator fuel is clean burning and keeps longer in a large tank than other fuels, also does not have to be handled to refuel the generator. A propane fueled generator can get you through utility outages of several days or several weeks if you have enough fuel on hand. But the generator may not be large enough to handle everything in the house, particularly if you heat, cook and make hot water electrically. For a smaller generator on a larger home, it is best to have a second circuit breaker box installed, and separate the circuits the generator can handle into the second box. This makes transferring to generator and operating the system easier and foolproof.
For more advanced systems, we combine both: Generator and Inverter/battery system so the batteries are re-charged once each day by the generator. In this system the generator runs only a few hours a day while the heaviest appliances are used, but power for lights and TV/computers and other small appliances is available 24 hours a day from the battery/inverter system. This saves fuel, noise and wear on the generator by eliminating over 3/4 the required hours of generator operation.
Preparing for the next Power Outage
Many residents of Florida and other hurricane alley states have called Backwoods Solar for information about back-up systems for their homes. Whether the cause of a power outage is a hurricane in Florida, an ice storm in the Midwest, a snowstorm in the inland Northwest (home to Backwoods), California style rolling outages during high use times of year, or just a good old-fashioned brown-out like the one that affected a large portion of the Eastern US in 2003, people are always calling us looking for a way to keep critical (and some not-so-critical) loads operating.
There are several different ways that you can prepare for a power outage that lasts from hours to days. A battery-based inverter system, a solar powered battery/inverter system, or back-up generator systems are all possible solutions. As you look toward this winter, or next hurricane season, these options can help maintain your lights, refrigeration, security system and even the television. Like a UPS (un-interruptible power supply) for your computer, these systems for your home will keep the lights on, when everyone else’s are off.
1) Limited power battery/inverter system. Most of us won’t be interested in powering all the loads we might have in our home in an emergency. We’ll prioritize important loads that we always want available and isolate them from the main 200 amp service panel that typically runs an entire home’s electrical circuits. In this type of system, the grid power from your utility is connected to a DC to AC inverter and batteries. The inverter’s built-in battery charger will use grid power to keep the batteries fully charged, ready to use; and the utility power will be “passed through” the inverter to the “sub-panel” to power those dedicated loads. Once the grid fails, the inverter will continue to supply power. It will be using the DC power stored in the batteries and converting it to AC power. Correctly sizing the battery bank will be crucial to providing adequate back-up power for your loads and maintaining the battery’s health (see “SIZING” article in this newsletter). You’ll need to calculate the daily watt-hours you want to provide for once the power goes out. And you’ll need to plan for the number of days you expect to need the power. Remember, the more loads you have and the longer you power them, the more the system will cost. See the detailed example below.
2) Limited power battery/inverter system with Solar back-up. This system would be like the above, but would incorporate solar modules and a charge controller to help extend the autonomy of the system, or make those selected loads fully autonomous. The solar array may be roof mounted or pole mounted in the yard, with full sun exposure. The output would be wired to the charge controller and connected to the battery bank for battery charging when the sun was shining. This is a system similar to the one many Backwoods customers already enjoy to power their homes- with the grid acting as the back-up power source instead of a generator as in typical off-grid systems. More expensive than #1, this system would allow continued operation in the case of longer outages.
3) Limited power generator back-up. This would entail a smaller gasoline powered generator- like a Honda- that would be used to run the items you wanted to power when the grid went down. Size the generator for the maximum loads necessary, and allow room for the starting surge of appliances. The loads may run directly from the onboard outlets on the generator through an extension cord and switched outlet strip for minimal lighting, television and refrigeration. The new Honda “inverter” series of generators has a variable speed throttle that is very fuel efficient and reduces noise. As the load increases, so too does the RPM of the generator to provide the needed power. Of course, storing the gasoline can be problematic, and depending on the outages may be scarce once the outage occurs. Honda, and Yahama, should be locally available through those dealers. Consider adding batteries and inverter to allow smaller loads (lights and TV perhaps) to be on without the noise or expense of running the generator for every little load.
4) Full back-up power. Take the back-up system for a hospital as an example. That system would include an automatic or manual transfer switch that would allow the building to be connected to grid power when available, or switch to the back-up system when necessary. A large generator powered by natural gas or propane would provide all the needed power to run all loads (or selected loads) in the event of a power failure. While Backwoods doesn’t deal with this configuration specifically, there should be companies in your area with the expertise to help you should you feel this is needed in your application.
Looking at example #1 more closely, a homeowner might want several room lights, television, refrigerator, microwave and other items to run if and when the power goes out. So, removing those specific circuits from the main service panel and rewiring them into a “sub-panel” (or directly into the PS2AC in the Outback PS2 system) will allow them to always be available to the homeowner. The 120V AC input for that “sub-panel” will be grid power or inverter power depending on whether the grid is operational or not. A circuit from the main service panel will be wired to the inverter. That will allow grid power to pass through the inverter to the sub-panel to power those fulltime loads, while at the same time using the internal battery charger in the inverter to maintain the batteries at a full charge. Once the inverter breaker in the main service panel is shut off, or if the power goes out, the inverter will draw DC voltage from the batteries to convert into AC output to power the sub-panel. So the chosen circuits will not lose power. You’ll have lights and refrigeration while others in the neighborhood don’t. Just as in an off grid application, the correct sizing of the inverter and battery bank is critical to the function and performance of the back-up system you are creating. So knowing the total watt-hours per day that will need to be provided for, and the simultaneous wattage used is necessary. In my grid connected home, if I chose to run the circuits in two bedrooms and the kitchen, I could estimate my daily emergency usage fairly easily. I’d plan for the following emergency loads- four CF lights on 5 hours each day (15 watts each x 4 lights x 5 hours = 300 watt hours per day), my refrigerator (1500 watt-hours per day), television and radio (maybe 80 watts x 2 hours = 160 watt hours), microwave (1200 watts x ½ hour = 600 watt hours). I’d add another 10% to be sure I could alter my uses if needed and not adversely impact my autonomous run time. Based on those loads I’d need a battery bank that would provide for roughly 2800 watt hours each day (about 116 amp-hours used per day @ 24VDC) I expected to be without power. For two days of battery back-up I would need 4 Trojan L-16HC batteries (420 amp-hour capacity @ 24 VDC) with a depth of discharge (DOD) of around 55%. Maybe you’d want 4 days of autonomy (464 amp-hours used), that would require 8 Trojan L-16HC deep cycle batteries (840 amp-hour capacity @ 24 VDC). An Outback PS2 Power System with an FX2024T inverter, Mate control and AC & DC load centers would handle all the loads I was considering running while the grid was down. I have no 240V AC loads that I need to power, so that saves on cost and complexity of the system by not requiring dual inverters and/or a transformer, or larger battery bank as part of the system. I could decide to have less days of autonomy, use less power, or supplement with solar to extend my length of autonomy and any of those changes would affect the size and cost of the back-up system.
This Outback Quad stack is a larger version of the Outback PS2 Power System we have designed for modest backup power needs.
Components for the above system would include the following:
1 Outback PS2FX24S power system with FX2024T inverter and disconnects.
Trojan L-16HC batteries
3 2 gauge battery interconnects
So making the best of a bad situation doesn’t have to mean huddling in the dark. Call us for help in preparing your system. Integrating alternative energy products into your home system can be a great hedge against what Mother Nature throws at you. As our off-grid customers already know, there is nothing like being your own power company, especially when the big storm hits!