UPS as an inverter / charger for off grid system

Re: UPS as an inverter / charger for off grid system

I remembered another issue, UPSs are designed to charge sealed lead-acid batteries, which charge and float at a lower voltage than flooded batteries. I compensate for this by charging each battery with a good 3-stage charger a couple of times a year. Not perfect, but I live with it. Another argument for getting a good external charger.

Re: UPS as an inverter / charger for off grid system

Mine is an online (sometimes called "dual-conversion") UPS like yours. We get frequent brownouts as well as blackouts so while I hate the hit I take by running it all the time its the price I pay to keep most of my stuff on a clean supply. Keep in mind that yours will peak at 92% but probably is lower through most of its power curve. If you can get a copy of the curve from the manufacturer it will help you decide to keep it or not.

Also your UPS is awfully large to run 24/7 off-grid. The internal useage by the system will be far higher than keeping something like a 500 or 1000 watt inverter powered up. Bottom line, you'll be spending a lot more on PV just to keep your inverter running (not even powering anything) when you would do better spending that money on a smaller inverter for day-to-day use. Most of the time a house needs less than 500 watts, the readout on mine usually says I'm using 200-300 watts 75% of the time. Keep the big guy for occasional big loads. One problem with this is you'll have to reconfigure your battery bank to run each one... 192 volts vs. 24 or 48 volts.

Another possible problem - many of those mid-level server room UPSs can not start from DC alone. They need the AC source to power up and do system checks. Often its a BIG load during that startup as it tests the inverter and power factor correction. You might luck out and be able to start it up using your smaller inverter, the guy on http://www.priups.com was able to do that with his. But yours is big enough you may need to power up your genset temporarily just to power up the UPS.

I guarantee when you price out new UPS batteries vs. flooded lead-acid you won't have to think twice. My bank cost me around $1300 and more than trippled my capacity compared with the relatively large bank that came with it (128 "bricks"). Buying those same 128 sealed batts again would have cost 3 times as much.

I added a subpanel. My UPS came with a wall-mounted bypass switch in a small cabinet, with all the big 50-amp twist-locks needed. I added a 50 amp twist-lock outlet below my genset panel which the bypass plugs into, then the subpanel and UPS plug in to the bypass. If I had to I could take the UPS and bypass away and plug the subpanel directly into the outlet coming from the genset panel, not that I will need to since I have the bypass. I moved some of the loads from the genset panel and some of the loads that were still in my service panels to the subpanel. I've ended up putting about 90% of my 120 volt loads on the subpanel and could go 100%, I just don't have a need to power non-essential things like my attic lights or garage opener through the UPS (or the genset) so they are still on the service panel. There are a few 120 volt loads only on the genset, like my microwave and some of my outside lighting. Stuff I'll need at some point when the utility is down but not all the time so no reason to put it on the UPS even though I could. My well pump is on the genset panel, and I have yet another subpanel which is really a small manual transfer switch + panel which holds the breakers for my electric water heater and one of my two A/C units. That gets fed either from the utility panel or the genset panel. This does 2 things. First, if I'm away and the power goes out, I'm not wasting fuel keeping water warm and air cool. When I get home I can manually switch one or the other over to the genset (it can't support both at the same time along with the well pump). Second reason is this keeps these big loads off of the circuit that powers most of the rest of my stuff. So they don't cause a voltage sag when they come on, causing problems. Obviously during an outage when I move them over to the genset I just live with that possibility.

Re: UPS as an inverter / charger for off grid system

We were considering the same thing with a FERRUPS FE10 for this application.

There are other UPS units that will do far better, if you can find one. We sadly missed out on getting one that was being disposed of ("if you can cart it off, you can have it") - no place at the time to store an inverter that large. Someplace I have the model... they were designed for a slew of battery cabinets for large telco/NOC/building backups, but they are NOT cost effective unless you can get them for "cart it away and it's yours". They are easily high 5 figures or well into the 6 figure range.

The advantage is they will do all the line filtering, handle power surges, dips, frequency stabilization and so on - and many (like the high end APC or FERRUPS lines) will do so without a hit on the battery except for massive drops in current. Comes in handy when powered off a generator that might not produce the best voltage/frequency output when loads change.

Re: UPS as an inverter / charger for off grid system

RobertMfromLI wrote: »

...are NOT cost effective unless you can get them for "cart it away and it's yours". They are easily high 5 figures or well into the 6 figure range.

The advantage is they will do all the line filtering, handle power surges, dips, frequency stabilization and so on - and many (like the high end APC or FERRUPS lines) will do so without a hit on the battery except for massive drops in current. Comes in handy when powered off a generator that might not produce the best voltage/frequency output when loads change.

I got ours for $200, one of those rare true Ebay deals. Not bad for a unit that cost around $10,000 new. Its 11 years old now and has around 65000 hours on its timer so I don't expect it to last forever, but when it dies I can resell the bypass and the 50 amp twist-locks built into the unit and break even. If I tear out the multi-tap transformer I might even get ahead. 8)

Ours does very well on the dirty output from my Generac. I had to adjust the slew rate on the UPS so it could keep up with the frequency swings w/o throwing a constant alarm. In the future when I switch to a mainstream inverter I'll re-create a similar "online" system using an AC charger that will accept a wide voltage range plus has power factor correction, and run the inverter 24/7. The only thing it won't do that my current setup will is supplement from battery in very low voltage situations, but I've never seen voltage that low so I'm not going to worry about it.