Reverse UPS?

MikeK6
MikeK6 Registered Users Posts: 1

I've two UPSs in my home already, - to safeguard my computers and what not.

But I'd take a dip into the solar panel kiddie pool would there something like a UPS that works in reverse. Where the unit would run off the DC battery(s) and then switch over to the 110 volt home A/C when the battery is drained.

Any sources I should take a look at first?

Thanks,

Comments

  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited February 2019 #2

    Welcome to the forum @MikeK6

    Supposing someone proposed the notion of burning cash in a furnace but once all the cash is burned they wanted to switch over to gas, you'd think that would be crazy and of course you'd be right.

    Using batteries to power loads then switching over to grid power when they are depleted is much the same, the cost of the battery supplied energy far exceeds the cost of grid power, so it would would naturally be better to use grid predominantly.

    The way to use solar effectively would be to charge the batteries, then use excess energy to support loads or supplement them, leaving the batteries to support loads in the case of a grid outage. Standard UPS systems do not work well in a situation they're not designed for, the batteries are not designed for cyclical use, but rather for float applications, meaning they would have a short life expectancy.

    There are ways to use solar in conjunction with grid however it usually requires equipment designed for the purpose, or some creative solutions using other devices, for very small scale though it's usually impractical. No intention of discouragement just pointing some potential problems that would be encountered.

    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin

    As McGivor says, while it is possible to do this--It does not save any money. Typically, UPS systems use "float batteries" that are designed to sit around for a couple of years with a few discharge cycles (5-100 or so). So, if you try to cycle the batteries in the UPS, you will find that they will not last very long (and you would need a solar charge controller to properly charge the batteries).

    You could also get a very "confused" UPS charging system too if the batteries are charged from an external power source (or they may work fine--You have to research/test a few products to figure out).

    In general, I would suggest that you build a standalone off grid power system that is large enough to give you some use during a power failure (or if you want to try running some loads off grid).

    For example, 2x 6 volt @ 200 AH "golf cart" batteries in series (12 volt @ 200 AH battery bank). An ~300 Watt AC TSW inverter. ~375 Watt solar array (10% rate of charge) and a ~20-30 Amp solar charge controller.

    Mount the Solar panels outside on a south facing structure, that is relatively wind resistant.

    Add a hydrometer to monitor the flooded cell lead acid (FLA) battery cells, and a DC Current Clamp DMM (measure DC current and AC/DC voltages in your system), plus a Kill-a-Watt type power meter--And you would have a nice/capable off grid power system and the tools to understand how it works, and debug it if something goes wrong. And it would run a laptop or two pretty nicely (very roughly, around 600 WH per day).

    I can go through the math (tell us what major city you are near, so we can figure out your hours of sun per day), and give you some links to the items above.

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭

    would there something like a UPS that works in reverse. 

    Yes, there are plenty of hybrid inverters out there that will do that. The Outback Radian is one such inverter. There are smaller/cheaper ones as well.

    But as others have pointed out that is by far the most expensive way to do it. You need the inverter plus a battery plus solar, and batteries wear out. You'll not only spend upwards of $6K on the system, you'll have to buy new batteries every 2-10 years depending on how hard you cycle them.

    If you want to "dip your toe" in with solar, find a local company willing to install a small grid tie system. You'll pay about the same after incentives, but you'll generate more power every day - and won't have to replace batteries in 2 years.