Inverter bypass switch

Sq. D




Total Initial Hardware Cost Estimate $6,390.00

Total Initial Hardware Cost Estimate $6,390.00

makes it possible to electrically isolate the inverter from the batteries if inverter servicing is ever required.

Interfacing with the Grid— The Subpanel

The maximum continuous output current from a Trace SW4024 is 33 amps at 120 VAC. Your home's mains panel is probably a 100 amp, 200 amp, or even larger service. So it's quickly apparent that a single inverter can't handle all the loads in a typical, on-grid household.

Usually, only certain loads are deemed important enough to require a backup power source. You need to survey the appliances in your home and decide which ones you want dedicated to your UPS system. The total appliance load must be within the inverter's output rating. In this case, that limit is 33 amps at 120 VAC (4,000 watts) continuous.

Three different brands of amp-hour meter are available: Bogart Engineering's TriMetric, Xantrex's Link 10, and the Trace TM500. All three of these meters display battery SOC and other system information.

In this intermittent, grid backup application, an 80 percent depth of discharge (DOD) is acceptable. We never recommend discharging your lead-acid batteries by more than 80 percent (20% SOC). Doing so will greatly decrease your battery bank's longevity. Without a battery amp-hour meter, you won't know how deeply you are discharging your batteries.

In an off-grid system, batteries are cycled on a daily basis. In that case, a 25 percent daily depth of discharge is good threshold to shoot for. Remember, batteries like to be at a full state of charge. The less deeply you cycle them, the longer they'll last.

DC Safety Gear

Your battery bank has the potential to deliver tens of thousands of amps if a direct short ever occurred. This is a fire hazard and a potentially life-threatening situation. So just like the circuits in your house, you want to make sure to have a fused disconnect or breaker between the battery bank and the inverter. Since this breaker is on the low voltage DC side of the system, a high current DC-rated breaker is required.

Trace manufacturers a DC250 enclosure that houses a 250 amp DC-rated breaker. This provides overcurrent protection between the inverter and batteries. It also

The next step is to identify the circuits that power the appliances requiring backup power. Circuit locations are usually listed on the inside of the cover of your mains panel. Once these circuits have been identified, they are pulled from the AC mains panel and rerouted to a new AC subpanel. Now you have a dedicated breaker panel for loads requiring an uninterruptible power supply.

In our case, the output of the inverter is run through a 60 amp breaker and on to the subpanel. In addition, grid power for battery charging must be run via a 60 amp breaker from your mains panel to the inverter. Both of these 60 amp breakers are included in the inverter bypass switch, which is housed in a separate enclosure, typically mounted above the inverter.

Inverter Bypass Switch

You always want to include an inverter bypass switch in a battery based, utility intertied system. You recall that in this UPS system, when grid power is available, it is routed directly to the dedicated AC loads by way of the inverter's internal transfer switch. Very occasionally, an inverter needs to be removed for service. When the inverter is removed for repair, so is the onboard transfer switch. This isolates the dedicated subpanel from the mains panel. That blacks out all the loads powered via the subpanel, which is definitely not the desired effect.

Without the inverter bypass switch, you would have to temporarily rewire the subpanel directly to the mains panel. Then you would have to switch the wiring back again when the inverter was re-installed. If you're not doing the work yourself, this would mean two trips by a local electrician. A bypass switch eliminates this potentially expensive headache.

A three circuit Square D load center (model number Q0403L60NS, with butterfly switch), one single pole 60 amp QO breaker, and one double pole 60 amp QO breaker are the parts that make up the bypass switch. At retail, this setup costs about US$80. The bypass switch allows you to throw one switch and route AC power from the mains panel directly to the subpanel. The breakers in the bypass switch also serve as overcurrent protection between the mains panel and the inverter, and between the inverter and the subpanel.

Non-Grid Power Sources

The local utility isn't the only source of electricity. As many off-grid HP readers can testify, there are many ways of making electricity that don't involve the utility. The Midnight Special is designed to handle them all.

AC Generators

If a utility blackout begins to stretch out for days, you need to come up with another power source to recharge the batteries and energize the system. The most common option is an engine-powered generator. Trace SW series inverters are designed to simultaneously manage both utility grid and engine-generator inputs. User programmable setpoints allow the use of most common generators.

Trace SW series inverters can also be programmed to automatically start generators based on either time of day or battery voltage. This sample system does not include a generator, since it's designed to power the specified loads for 24 hours. The majority of grid failures can be weathered by an appropriately sized battery bank.

Off-grid homes powered solely by an engine generator will also benefit greatly from the addition of an inverter/battery system. Generator-to-loads direct systems are extremely inefficient to run. Electricity produced by an engine generator costs about US$0.60 KWH.

More often than not, the generator will be operating at a fraction of its full output. Running a 10 KW generator to power a couple of lights and a TV is a tremendous waste of fuel. And you have to listen to the blasted thing whenever you want to turn on a light! Adding an inverter and batteries to the system will reduce generator run time, noise, and fuel costs by 80 to 90 percent.

Photovoltaics, Wind, & Microhydro

Since the gear used in this system is standard stuff, it can switch from a steady diet of grid power to a


Sometimes backup power demands are higher than the rated output of a single inverter. With the addition of an interface cable, two Trace SW series inverters can be stacked in either a series or parallel configuration for increased output.

For example, two series-stacked SW4024 inverters have a 120/240 VAC split phase output. The waveforms of the two inverters are 180 degrees out of phase—like the utility lines coming into your house. This configuration gives you two 4,000 VA legs, and allows you to run 240 VAC appliances. Two SW inverters can also be stacked in parallel for 8,000 VAof single phase, 120 VAC output.

Larger systems will require a larger battery bank. Always start with a survey of the loads you need to back up, and the number of minutes or hours you need to run them. In a grid backup application, your battery bank should be sized for a maximum discharge of 80 percent.

Just want to back up your computer? Off the shelf computer UPS systems may be your best bet. Want to back up your computer, a couple of lights, and your stereo? Calculate the total load you want to power and how long you want to power it. Then choose an appropriate battery bank capacity based on an 80 percent depth of discharge. Exeltech manufacturers small true sine-wave inverters. Several models are available with AC outputs from 125 watts to 1,100 watts. These inverters supply higher quality power than the utility grid. And you're the one to decide when they get shut down!

pollution-free diet of renewable energy. You can add PVs, wind power, microhydro, or a combination. This will make the system (and your home) energy self sufficient. Of course, RE resources are site specific. Most suburban homeowners don't have a stream cascading down their back yard, and wind turbines are tough to site in town.

If you have any surplus RE generated at your home, it can be placed on the grid for others to use. We

Midnight Special Inverter Settings

Menu Heading Menu Item

For 8 Concorde Sealed AGM Batteries, 440 AH at 24 VDC Inverter Mode - 1 Set Inverter Time of Day - 6

Inverter Setup - 9 Set Grid Usage

Set High Battery Cut-Out VDC Set Search Watts Battery Charging - 10 Set Bulk VDC

Set Absorption Time

Set Float VDC

Set Max Charge Amps AC

Set Temperature Compensation

For 8 Flooded Lead-Acid Batteries, 700 AH at 24 VDC Inverter Mode - 1 Set Inverter Time of Day - 6

Inverter Setup - 9 Set Grid Usage

Set High Battery Cut-Out VDC Set Search Watts Battery Charging - 10 Set Bulk VDC

Set Absorption Time

Set Float VDC

Set Max Charge Amps AC

Set Temperature Compensation estimate that about US$8,000 worth of PV and a larger battery will make this system capable of delivering about 6 KWH per day. This will run the loads listed in the table, on a site with 5.3 average sun hours per day.

A System with a Future

Considering the increasingly unreliable nature of utility grids, this system offers you a future where the lights are always on. If a storm brings down the power lines and puts out the lights, this system can bring them back. If the local utility gets irresponsible and shuts off the power because they can't make a profit or supply the demand, your lights will still be on. And then there is the tantalizing possibility of adding solar, wind, or microhydro energy to this system. The Midnight Special is a blackout buster with a clean and green future.


Joe Schwartz, Home Power, PO Box 520, Ashland, OR 97520 • 541-512-0201 • [email protected]

Richard Perez, Home Power, PO Box 520, Ashland, OR 97520 530-475-3179 • Fax: 530-475-0836 [email protected]

Bogart Engineering, 19020 Two Bar Rd., Boulder Creek, CA 95006 831-338-0616

[email protected] TriMetric AH meter

The Energy Master Spreadsheet is available for free from the Download section at or on the HP Solar CD-ROMs

Concorde Battery Corp., 2009 San Bernardino Rd., West Covina, CA 91790 • 800-757-0303 or 626-813-1234 • Fax: 626-813-1235 [email protected] Batteries

Exeltech, 2225 East Loop 820 N, Ft. Worth, TX 76118 800-886-4683 or 817-595-4969 Fax: 817-595-1290 [email protected] • Inverters

Surrette Battery Company Limited, PO Box 2020, Springhill, NS B0M 1X0 Canada • 800-681-9914 or 902-597-3767 • Fax: 902-597-8447 [email protected] • Flooded lead-acid batteries

U.S. Battery,1675 Sampson Ave., Corona CA 92879 800-695-0945 or 909-371-8090 • Fax: 909-371-4671 [email protected]

Trojan Battery Company, 12380 Clark St., Santa Fe Springs, CA 90670 • 800-423-6569 or 562-946-8381 Fax: 562-906-4033 • [email protected] • Flooded lead-acid batteries

Xantrex Technology Inc., Distributed Residential and Commercial Markets, 5916 195th St. NE, Arlington, WA 98223 • 360-435-8826 • Fax: 360-435-2229 [email protected] Trace inverters, E-Meter, Link 10 & TM500 AH meters, and DC250

Recommended Setting

SRCH (search) Set Hr/Min/Sec FLT (float) 29 VDC 16 watts 28.8 VDC 1:00 Hr. 26.8 VDC 20 Amps AC Lead-Acid

SRCH (search) Set Hr/Min/Sec FLT (float) 32 VDC 16 watts 29.6 VDC 1:00 Hr. 26.8 VDC 20 Amps AC Lead-Acid

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