Inverter Standby Losses

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electronic devices such as computers and TVs have built-in internal power supplies. So just like your cordless phone, your TV is likely using electricity unless it's unplugged from the wall outlet. Plus, most TVs and many other appliances have remote control receivers that are powered all the time, ready for you to hit their power buttons to turn the appliances on.

phantom loads

Standby Losses & Renewable Electricity Systems

Minimizing standby losses is an important aspect of optimizing a home renewable electricity system. Every watt-hour you save is a watt-hour your RE system doesn't need to produce, and money you save in up-front system costs.

The majority of solar-electric (photovoltaic; PV) systems installed are grid-tied, and rely on batteryless inverters to convert the DC electricity generated by your PV array to AC electricity used in your home or sent to the grid. Most batteryless inverters are designed with low standby loss in mind, and will shut down overnight, drawing only a fraction of a watt after the sun goes down. In grid-tied systems with battery backup, standby losses can be more significant compared to batteryless systems, since battery charging and management is required. Correct programming will minimize inverter energy consumption overnight.

Keeping standby losses to a minimum is extremely important in off-grid RE systems. Any electricity demand beyond what the RE system can generate is typically provided by an engine generator, which is noisy, polluting, and expensive to run and maintain. One design approach for large off-grid PV systems (3 KW array and greater) is to simply add additional modules to offset standby energy use by appliances and inverters in idle mode. This approach is most convenient, but requires a larger cash outlay up front. For small off-grid systems (1 KW array or less), unchecked standby losses can quickly become one of the largest loads on the system.

Home-scale, battery-based inverters typically have two modes that affect standby energy usage—idle, and a standby mode commonly referred to as "sleep" or "search" mode. Modern, battery-based inverters can draw as much as 22 W per inverter without loads (at idle). Multiply this by 24 hours in the day and your inverter can quickly become a significant load on the

Modern inverters are designed to minimize standby energy use.

system, consuming more than 500 WH per day. The same inverter, programmed to go to into an energy saving sleep mode if AC loads are not present, will draw about 4 W, or 96 WH per day, consuming only about 18 percent of the energy of idle mode.

Eliminating all phantom loads will allow your inverter to go into sleep mode, saving energy when you're not actively using AC electricity. Then, if an AC load is turned on, it takes a moment for the inverter to "wake up," ramp up to full voltage, and power the AC appliance. This momentary inconvenience is well worth it for many small system owners focused on maximal system performance and minimum generator run time.

Comparing PV output and initial costs to energy consumption helps drive home the benefits of minimizing standby energy use. Let's assume that you have installed an array with 100-watt PV modules at a location with 5 daily peak sun-hours, and appliance and inverter standby losses amount to 1,200 WH per day. For a batteryless, on-grid system, an overall annual system efficiency of about 80 percent is realistic in many locations. To offset the daily standby energy use only, the output of three 100 W modules would be required (1,200 WH/day - 5 sun-hrs./day = 240 W; 240 W - 0.8 (efficiency factor) = 300 W; 300 W - 100 W/PV module = 3 PV modules). Off-grid systems have a conversion efficiency of about 70 percent, so four 100 W modules would be required to offset the standby energy use in that case.

You can expect to pay about $600 for a 100 W module in today's market. That means your standby energy use would cost you $1,800 in up-front PV module costs in the batteryless grid-tied system described above, and $2,400 in the off-grid system example. Add this to the cost and nuisance factors related to generator use when a few days of cloudy weather appear, and additional installation and balance of system equipment costs, and minimizing standby energy use can amount to big savings.

Finding the Phantoms

While new appliances are getting more energy efficient, most of our homes have all manner—and all ages— of appliances. You may have \ a new high-definition TV, but an old cordless phone. As a result, the standby losses of appliances can vary radically between one home and another. To complicate things, standby loss figures are rarely presented on an appliance's labeling. In the end, actual hands-on measurement is the only way to accurately quantify standby losses. Luckily, inexpensive, handheld digital power meters are available to help you determine your appliances' standby energy use (see Access). Other than a piece of paper, a pencil, and a calculator, a power meter is all you need to ferret out phantoms around your home.

In general, identify appliances with external power supplies, remote controls, continuous digital displays, or phantom loads

rechargeable batteries. Chances are that these products will have standby losses. Plug the power meter into the electrical outlet, and the appliance being measured into the meter. Set the display to watts and jot down the power draw when the appliance is turned off, powered up, and operational. Multiplying the wattage you measure by 24 hours (or the amount of time the appliance is not in use each day) will give you a daily energy loss (WH) figure for the appliance. An hour or so spent roaming around your house with paper and meter in hand will ___ __ probably be all the time it takes to determine W'flll how many phantom loads are present, and how much energy they're using each day. Once you've identified the worst offenders, the next step is to truly shut them down.

Defeating Phantoms

You'll likely need to live with some phantom loads, such as cordless phones that need to be ready to go at any time. Answering machines, on the other hand, can easily be replaced by voice mail provided by your phone company. Once you've identified unnecessary phantom loads around your house, the next step is to figure out how to conveniently shut them down when they're not in use or required.

One option is to unplug any appliance that has a standby loss, but this can be inconvenient. Instead, using plug strips can be a simple and effective strategy to defeat phantom loads. They cost about $5 each, and are available at hardware and appliance stores everywhere. Plug strips can be used to group appliances that you regularly use in conjunction with one another—your modem, computer, and monitor can be on one strip, while hardware you use less frequently, like your printer, scanner, and external hard drive, can be on another. If you're rewiring circuits in your home, or building from scratch, well-planned electrical circuits can include switched receptacles that will shut down phantom loads at the flip of a switch.

Serious About Standby Losses

The subsidized, low cost of grid electricity in the United States has led many of us into bad habits when it comes to energy use, and it has also served as a disincentive for appliance manufacturers to design highly efficient products with low standby losses. As electricity costs continue to escalate, and as federal programs like Energy Star continue to reward manufacturers with efficient designs, we can expect the trend of improved appliance energy efficiency to continue.

Some of the super energy-efficient homes featured in Home Power can run all their household loads using the same amount of electricity that a typical American household wastes on phantom loads. So see how low you ■ iM i can go—get your hands on a power meter, jWlWlIlI' and spend an hour or two surveying your

Electronic Advances

In rerent yerrs, mrnufrrturers hrve berome more rwrre of the need to minimize strndby losses in rpplirnres they design rnd manufacture. Much of the progress here hrs been spurred by federal efforts—including the well-known Energy Star program—that are setting guidelines to push manufacturers to design more efficient appliances with low standby losses.

Forward-thinking appliance manufacturers are using four major design approaches to reduce standby losses:

• Improved designs for more efficient transformers

• Moving the power switch to the input side of the power supply, to completely power it down when it's not in use

• Advanced designs that limit the number of electronic components required for standby service

Intelligent charging rechargeable batteries circuits in products with

As electricity prices continue to escalate, and energy consumers continue to become more aware of the impact inefficient appliances have on both their pocketbooks and the environment, appliance and electronics manufacturers will continue to refine their product designs with energy use in mind. The TV set that had 30 W of standby loss several years ago likely has less than 10 W today. Thanks to programs like Energy Star, appliance design is headed in the right direction, and appliances are getting more efficient all the time.

home for phantom loads. You'll be rewarded by shaving a bit off your next electrical bill, and you'll sleep better at night knowing that your efforts are helping to reduce our overall energy use, and the pollution associated with fossil fuel and nuclear electricity generation. And when you do install an RE system to power your home, there will be significant savings in up-front equipment costs (see sidebar). Remember, energy saved is energy that doesn't need to be generated in the first place.

Access

Joe Schwartz, Home Power, PO Box 520, Ashland, OR 97520 • [email protected]www.homepower.com

Energy Star • www.energystar.gov • Information on energy efficient, low standby power appliances

Watt-Hour Meters:

Brand Electronics • 888-433-6600 • www.brandelectronics.com • Brand meter

Electronic Educational Devices • 877-928-8701^ www.doubleed.com • Watts Up? meter

P3 International Corp. • 888-895-6282 • www.p3international.com • Kill A Watt meter

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