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Thread: Using an Inverter/Battery system as whole-house backup solution

  1. #1

    Default Using an Inverter/Battery system as whole-house backup solution

    I have a generator with a manual transfer switch that selects between the utility mains or the generator. The switch only allows one or the other (break before make). I would like to replace the generator with an inverter/battery solution. A good 1800RPM diesel generator is expensive and is fuel inefficient (burns fuel during partial load). A better backup solution is a battery/inverter system that can be charged with a much cheaper generator running full load but only occasionally. This would also be much quieter, running the generator during the least intrusive parts of the day. The cost of the 1800RPM diesel is comparable to a 6kW inverter/battery solution (assuming rather small 300Ah/48V, but a good start).

    I read the installation and operation manual for the Schneider XW6048. The installation manual suggests leaving the AC mains and breaker box untouched and connecting the XW6048 to an "Inverter Load Sub Panel". Does this mean I need a second breaker box? Does the XW6048 "pass through" utility AC during normal operation, or is the AC-Out strictly from the inverter?

    The logical layout would be a transfer switch that interrupts utility mains and selects the inverter (similar to how generator transfer switches work). An automatic transfer switch (also available for generators) does this automatically, and switches back the the utility mains when voltage is returned.

    Can the XW6048 work this way? Also, are there competing units that I should be considering?

  2. #2

    Default Re: Using an Inverter/Battery system as whole-house backup solution

    Welcome to the forum.

    You're going to have to define what you mean by "whole house back-up". This is because the average household is equipped with a 240 VAC 200 Amp service these days. That's 48kW of power potential. Obviously one 6kW inverter can't produce that.

    What Xantrex is talking about is the standard install for a grid-tie/back-up inverter: you select certain critical loads and connect them to a sub-panel which is fed from the main panel via the inverter. In the case of the XW, this allows it to back-feed the grid through the main while also being able to power those critical loads when the grid goes down.

    For back-up purposes only (no grid-tie) any good off-grid inverter-charger will work. The wiring direction is the same:
    Main Panel ---> Inverter ---> Critical Loads
    During normal operation the inverter does nothing but keep its batteries charged from the utility power which is also 'passed through' to run the critical loads. When the grid goes down the inverter automatically switches over to power the loads.

    So the first step is to determine the size of the loads you must keep running during an outage and how long you expect that outage to be. That will determine what size inverter to have (including whether you need 240 VAC or just 120 VAC) and how big the battery bank has to be. Pretty much the same as picking a generator in that respect, except that you can easily refuel a generator. Recharging the batteries (as from solar) is another issue. For instance you want to recharge from a small gen: that means either using the inverter-charger in 'charge' mode (meaning disconnect from the main panel via transfer switch so that other circuits aren't energized) and having a generator large enough to charge the batteries and provide power to the loads in use. Recharging from solar panels is another matter, and involves further expense and installation.

    With me so far?
    Four 175 Watt panels, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  3. #3

    Default Re: Using an Inverter/Battery system as whole-house backup solution

    Thank you for your reply. I have decades of extensive experience in the energy sector, starting in the early 1990s racing electric vehicles in the APS Electric 500 and Boston Tour de Sol. I remember running a SEAS Zinc-Bromine battery in the "T-Star" while the Johnson Control guys fudged their JCI ZnBr battery and sent James Worden to the hospital in a helicopter. The next year, I was on the Solectria team running both the red and white converted Geo Metros. We couldn't bring our own car, because the prior year's JCI fiasco resulted in outlawing of all ZnBr batteries, including our T-Star. Oh well. In more recent decades, I was the founding chairman of a national utility grid-intertie energy storage council in cooperation with FERC. That's all big commercial and telecom stuff on the multi-MW scale.

    Back to the question at hand: So the Xantrex XW6048 "passes through" the utility AC. Looking at the specs, it can only pass-through 60A. This is too small to feed my primary electrical panel. So I can't pass-through the utility AC to my electrical panel. I can run more than 60A worst-case if several appliances are operating simultaneously, such as the stove, oven, dish washer, air conditioner, washer machine, etc.

    When I walk into my garage and flip the transfer switch to "generator", the genny is connected straight to the main electrical panel. Then it's up to my discipline to not overload the genny by using appliances simultaneously. I would think I can use the XW6048 with similar discipline. It seems overkill to install a separate breaker box for "critical" circuits just for the XW6048. That would require huge tearup of my existing electrical panel and re-routing of several circuits to the new sub-panel. Yuck! The transfer-switch downside is somebody needs to be home to flip the switch and disable the power-hungry appliances to prevent generator overload.

    So my goal is to use the XW6048 without tearing up my main electrical panel, but that comes at the cost of having load discipline and somebody present to control those loads. An automatic transfer switch could be used, but runs the risk over overloading the inverter if several appliances are running. A manual transfer switch could be used, but runs the risk of nobody being present to flip the switch.

    In regards to battery charging, any genny can supply AC to the XW6048, which can use the power to supply loads and charge the batteries with whatever remains. Good consumption discipline means overall usage will be low and the majority of power will be delivered to the batteries. We could also run appliances during the generator run-time (such as clothes washer), so the batteries are used primarily for light usage when the generator isn't running. Running the generator a couple hours at a time can deliver 14kWh of energy, quite a bit! To me, it sounds much better to run the generator occasionally at full-load, instead of continuous at light-load. The specific fuel consumption curves of the generators support this hypothesis. And since the generator will only see intermittent operation in their most efficient range (full load), cheap generators are suitable! (3600RPM 7kW diesel generators can be found for $1500. 1800RPM generators start at $7000.)

    So, the question remains: it is advisable to use an inverter with a transfer switch configuration (similar to how a generator is configured). instead of tearing up my electrical panel to reroute loads to a new sub-panel?

    The other question is: is the XW6048 the best choice? I like the average rating (6kW) combined with excellent surge rating (12kW). This high surge rating allows for worry-free operation of the air conditioner.

  4. #4

    Default Re: Using an Inverter/Battery system as whole-house backup solution

    You can use an inverter as a generator in the manner you describe. You just have to remember it's a 6kW 'generator'. With the right circuit protection in place on the inverter output, it will trip before the inverter burns up. Usually an inverter will fault and shut down before that happens.

    The Xantrex would not be my first choice as it has GT capability which you would no be using. For the same reason I wouldn't pick an Outback Radian (8kW) although they are quite a bargain at about $4,500 http://www.solar-electric.com/ourags8wain.html A straight forward off-grid inverter like the Magnum http://www.solar-electric.com/maenms4444wa.html would be a better value. Yes it is only 4.4 kW, but like the Xantrex and the Outback inverters it can be "stacked". In this case up to four in parallel giving a total of 17.6 kW (if you need that much).

    Another issue: the battery bank itself. Some of the loads you list are nasty: electric stove/oven, air conditioner. Having enough inverter to meet those demands is one thing, but you also have to have enough battery bank so that when the heavy load comes on the DC Voltage doesn't suddenly drop like a rock causing everything to turn off. So having a small battery bank and an auto-start gen to replenish as needed is not necessarily going to work. Perhaps the route to go is use the gen for large loads and the inverter for light duty (that isn't worth firing up the gen for). BTW they do make auto transfer switches but ... more money of course.
    Four 175 Watt panels, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  5. #5

    Default Re: Using an Inverter/Battery system as whole-house backup solution

    We could also run appliances during the generator run-time (such as clothes washer), so the batteries are used primarily for light usage when the generator isn't running.
    Exactly. Run the big stuff with the generator, maybe mornings and evenings, then run on batteries the remainder. Central air conditioning is a huge luxury, probably the first thing to jettison.

    Choices:

    Schneider XW6048 6kW $3000
    Magnum MS4448PAE 4.4kW $2150
    Outback Radian GS8048 8kW $4100

    Everybody runs $500/kW, although cost-per-kW comparison can be an unfair comparison. But it sets the price for entry.

    Thanks for your advise. For now, I'll configure a whole-house energy monitor to datalog consumption. Use data to point my direction.

  6. #6
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    Default Re: Using an Inverter/Battery system as whole-house backup solution

    Don't feel bad about doing a $$$/kW price comparison--When I used to help spec. power supplies for computers--You bet that was a major discussion point ($500 per kW for a switching power supply in the early 1980s for a "smaller" custom production computer system was a good price back then).

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  7. #7

    Default Re: Using an Inverter/Battery system as whole-house backup solution

    It sounds like you have the technical skills to do what I do in my current backup setup. I use a 12 kw Generac for the big loads (2 ton A/C or elect. water heater, plus well pump) for about 2 hours per day, and a Prius+UPS for most 120 volt loads during the evening. The UPS was a few hundred bucks on Ebay, plus I had to make a minor modifcation to the Prius (see my signature for a link). You could use your existing generator input & transfer switch.
    12 kw Generac, Yamaha EF2400is, 4.2 kw APC UPS powered either by battery or a Prius. Really.

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