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Is there a commercial AC inverter that will use grid power if the DC input is too low (not grid-tie)?

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Question by Eric_C: Is there a commercial AC inverter that will use grid power if the DC input is too low (not grid-tie)?
The application I am interested in is utilizing a solar array to supplement an on-grid house without the high-cost of a grid tie inverter. The device I have in mind is something that would take both AC input from the grid and DC input from a solar panel, but would not use them both at once, so it would not need expensive sine-wave matching or anti-islanding electronics. I have had no luck searching online, but I might be looking for the wrong thing.

For example: If you had a refrigerator that averages 500 watts, and a 700 solar watt array, the device would power the refrigerator for 4 or 5 hours during the day and switch back over to grid power at night. It would not have to be any more electrically complicated than a 1000 watt UPS. The inverter would power the fridge when it is on and store extra power in the battery when the fridge is off. Once the output from the solar panels drops and the battery voltage drops, the device would switch to grid power. I believe this device would be cheaper because a 1000 watt UPS (with a bigger battery than would be required for this application) costs about $ 500, while a grid-tie inverter from a reputable company costs well over $ 1,000 + installation by an electrician.

Thank you

Best answer:

Answer by Ed
Maybe it sounds too simple, but it seems you want a solar array and inverter to supply 120 VAC at or above the load demand and charge a battery. Then when there is little or no solar power to the inverter, the battery would take over. When the battery could no longer supply the load, you would switch over to grid power.

Why would you not want just a voltage sensing relay on the battery, aromatically transferring the load from the inverter to the grid?

It would be tricky to find a way to keep the load attached to the grid, even after the array begins producing again. This would involve ignoring upcoming array out until the array could support the load.

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Posted August 31st, 2012 in Cheap 1000 Watt Online. Tagged: , , , , , , .


  1. tinkertailorcandlestickmaker:

    Most UPSs are meant for very intermittent use. You say “well over $ 1000” how much over is that really? twice as much as an intermittent duty UPS for a device designed to do the job sounds pretty reasonable to me, but if “well over” is, say, $ 1800 then I can see where you might not be so interested.
    You might consider an automatic generator transfer switch, presumably a non-grid-tie inverter will have a low voltage shutdown feature, if you wire the inverter to the “utility” side of the transfer switch and grid power to the “generator” side it should do pretty much what you want, for a refrigerator anyway, if you are running a computer on it it will probably crash during switching (unless you use a UPS there) On the other hand an automatic transfer switch might cost close to $ 400, and you’d still need some inverter even if not a grid-tie one, so the grid-tie inverter is sounding better and better.

  2. roderick_young:

    A UPS type inverter is made to power computer equipment, and thus, the output does not have to be very sinusoidal, and it’s cheaper to make it that way. If you plan to power a motor like the compressor in a refrigerator, you really want a pure sine wave. Furthermore, the UPS inverter isn’t designed for prolonged service. In normal operation, the UPS passes line voltage to the output, and only in a complete power failure is the inverter ever used. Pure sine, continuous-duty inverters are available, but at a higher price.

    You’re right about needing a different battery, too. A UPS battery only has to last one time really, not continually deep-discharge. And if your replacement battery was lead-acid, then you would have to maintain it. You would also need a charger for the battery. The existing charger in a UPS wouldn’t do so well, because that was designed to have a steady supply, not a varying one.

    A final drawback is that something like a refrigerator doesn’t run all the time. If the refrigerator isn’t running, and the battery is charged, you’re just throwing away any excess power generated.

    If you’re talking about a very small system like 1000 watts, then a conventional inverter is going to be expensive, as you say. But you could put up a few microinverters like the ones made by Enphase and still achieve a grid-tied result. As for the electrician, I don’t think that would be too much to just hook it up to the service panel, if you’ve already run all the wires down there. In the case of your UPS with a transfer switch, I’m assuming you would run similar wires yourself.

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