"Whole house UPS" and standby genset

[Ira]

We may build a new home in the next year or so, and as a happy owner of a standby genset that can power all of my existing home, I've been dreaming about ways to do it better if we do build. I know a lot of people will look at this as a luxury that isn't worth anywhere near the cost, but that's okay with me. I've been known to splurge.

First, the new home's genset will be large enough to power everything, probably 27kw-35kw.

One of the things that I find annoying is the thirty second delay after an outage begins before the genset cuts over. UPS's (and there are plenty of them) beeping, lights go off, clocks have to be reset, etc. See -- I told you it was a luxury.

So I'm wondering if I can use a large UPS (say 5kw-10kw) in conjunction with the genset that will provide uninterrupted power to some circuits while POCO power is off and the genset hasn't kicked in. When the genset cuts over, it would power everything, including the circuits that were on the "whole house" UPS (which isn't really "whole house").

The UPS wouldn't have to power anything that goes thru the 30 secound outage unnoticed. It wouldn't have to power any appliances (most of which will be gas). It wouldn't power anything that is 240V. It just needs to power most general purpose circuits that power lights, computers, HA/security stuff, small plug-in stuff, home entertainment center, etc. Note that unless something "big" is running, my current house uses about 2kw, and that's with most of the things running that I would put on the "whole house" UPS.

Unless something big was running, the only way I would know that I'm on UPS or genset is from a HA system notification.

So is this possible? How big a UPS would I need to run a 3kw load for one minute? Are there the necessary automatic transfer switches that can handle the potential sequences of events?

One last question...if I decide there are some circuits that don't need to be powered during any outage, can that be accomplished? I guess there would be one load center for the UPS-backed circuits (which would also be backed up by the genset), one load center for the circuits that only need to be backed up by the genset (30 second outage is okay), and one load center for the circuits that can stay off during an entire outage.

I would still have point-of-use UPSs for computers, LAN stuff, DVR's, TV, etc., to give me more time if the genset failed to start.

I said I was dreaming.

Comments?

Ira

 

[dBeau]

While I'm far from an expert on the topic, I've been dreaming about the same sort of system for a while now....

As for the size of the "UPS" to run 3kw, you'd need a 3kw UPS (well plus a bit to account for start up current for large inductive loads). The size you might be thinking of though is the size of the battery bank. In that case, 3kw for 10min is only about 0.5kwh. A battery about the size of a typical car battery is good for about 1kwh. Now if you want to be kind to your battery, you really dont want to run it down more than half way.

If you are talking about 27kw-35kw genset for such a small system (you're over-sized by a factor of 10) you'll be wasting a whole lot of fuel. Overall, I think you'd be better off with a much larger battery bank and much smaller generator. Going from your 3kw usage estimate let's round up a bit and get you a 5kw inverter. Then size the battery bank for 12 hours of use without needing the generator. If you average a 2kw load, that's about 24kwh but to go easy on the batteries we go for 48kwh. That would be about the size of a refrigerator. With 12 hours of run time available, you might even decide to skip the generator all together. If you expect outages to last longer, your generator can recharge the batteries. Size the generator to charge the batteries as quickly as possible. This rate will depend a bit on the type of batteries. Sealed AGM batteries can be charged to 80% pretty quickly. So a 10kw generator would do the job in just a couple of hours and then shut down for another 12 until needed again.

You'll spend a lot less on fuel for the generator, not have to worry about changing the oil nearly as often during an extended outage, and best of all, not have to listen to the generator. Of course the batteries are not cheap. But you could cut my suggestion in half and run the generator for an hour after every 6 hours on the batteries. With an inverter sized to handle the entire load of the house, the generator only needs to be big enough to charge the batteries. My guess is that a system like this would run around $20k... but that's just a guess. If you have room on your roof for a couple of PV panels, you might be able to get a nice tax break on the system too.

...then I woke up.

 

[apostolakisl]

You can find units that might work for you in the medical field. I have a battery powered unit in my office that isn't quite a ups in that it does take about a second to flip over. I don't know the specs off of the top of my head but we test it once per month by running 1000 watt load for thirty minutes. It has what are basically marine batteries in it, something like 5 or 6 of them.

Your biggest problem is not going to be your storage capacity but rather your amp capacity since it is only required to work for 30 seconds. A DC to AC power inverter that can kick out 10k watts is not going to be cheap. One battery that can do 700amps at 12 volts would be about 70 amps at 120 volts. While 70 amps is probably enough I think you might want to split that up on two batteries.

 

[Ira]

While I'm far from an expert on the topic, I've been dreaming about the same sort of system for a while now....

As for the size of the "UPS" to run 3kw, you'd need a 3kw UPS (well plus a bit to account for start up current for large inductive loads). The size you might be thinking of though is the size of the battery bank. In that case, 3kw for 10min is only about 0.5kwh. A battery about the size of a typical car battery is good for about 1kwh. Now if you want to be kind to your battery, you really dont want to run it down more than half way.

If you are talking about 27kw-35kw genset for such a small system (you're over-sized by a factor of 10) you'll be wasting a whole lot of fuel. Overall, I think you'd be better off with a much larger battery bank and much smaller generator. Going from your 3kw usage estimate let's round up a bit and get you a 5kw inverter. Then size the battery bank for 12 hours of use without needing the generator. If you average a 2kw load, that's about 24kwh but to go easy on the batteries we go for 48kwh. That would be about the size of a refrigerator. With 12 hours of run time available, you might even decide to skip the generator all together. If you expect outages to last longer, your generator can recharge the batteries. Size the generator to charge the batteries as quickly as possible. This rate will depend a bit on the type of batteries. Sealed AGM batteries can be charged to 80% pretty quickly. So a 10kw generator would do the job in just a couple of hours and then shut down for another 12 until needed again.

You'll spend a lot less on fuel for the generator, not have to worry about changing the oil nearly as often during an extended outage, and best of all, not have to listen to the generator. Of course the batteries are not cheap. But you could cut my suggestion in half and run the generator for an hour after every 6 hours on the batteries. With an inverter sized to handle the entire load of the house, the generator only needs to be big enough to charge the batteries. My guess is that a system like this would run around $20k... but that's just a guess. If you have room on your roof for a couple of PV panels, you might be able to get a nice tax break on the system too.

...then I woke up.

I hadn't considered running off batteries the entire time and using the genset to charge the batteries.

The system needs to handle multi-day outages. I live in hurricane country. When Ike hit, my genset ran for four days straight. Others fairly close by were without power for several weeks. Also, the 2kw load is just the load I wanted the UPS to power between losing POCO power and genset transfer. After transfer, the load could easily increase to over 10kw, with A/C compressors, water well pump, oven, microwave, and other high wattage appliances.

 

[Neurorad]

I think you need to expand your search to explore commercial/industrial generators that don't have a 30 second delay.

Read all you can, then find a local or regional Generac Industrial Power rep or dealer, and ask for info. I'm sure there are many other companies besides Generac you could consider too.

http://www.generac.com/Industrial/DealerLocator/

 

[apostolakisl]

Hospitals and other medical facilities have what you are looking for. . .true UPS that is generator, not battery driven. They must use battery for at least a couple seconds becuase it is impossible to start an engine that fast.

You could definitely build a battery UPS with the right size DC to AC converter and a very fast relay. The commercial units are very very expensive becuase they guarantee UPS. In your home, you could tolerate a little flicker and save 10 or 20 thousand bucks. In my office I looked at some of those systems years ago but determined that I had to use a total battery system since my office complex wouldn't allow the outdoor generator. The battery system was waaaaay cheaper and has been perfect, but is only intended to allow me to complete a surgery, at most 2 hours. The generators I looked at did have about a 2 second delay, my battery has much less.

I suppose you could do the battery driven system with engine charging. Just buy a Chevy Volt and plug it into your house. That is exactly how they work and I am sure they can kick out thousands of watts. Plus, you can drive it around when the weather is good. (no hurricanes)

Also, keep in mind, that a true UPS system will switch on if the power company has even a slight stutter. This may create undue switching and cause premature failure of the parts.

 

[wuench]

You might also consider commercial UPS (Liebert/APC). Most datacenters use these as a temporary power buffer before the generators need to kick in. I think your best bet is to consult with a commercial electrician used to working in these environments.

 

[Ira]

Also, keep in mind, that a true UPS system will switch on if the power company has even a slight stutter. This may create undue switching and cause premature failure of the parts.

Good point. I wonder if the commercial UPS's have a variable sensitivity setting (not sure if that's the right term). I work from my home, and I have two 1kVA+ UPS's within arm's reach (one APC and one GE Digital) and several more spread around the house. I seldom hear them switch over, so maybe my power (at least at this home) is fairly stable within the range that the UPS's are checking.

 

[apostolakisl]

I was wondering also about the power inverters and their ability to handle both 120 and 240. You might need 3 inverters, one for each leg of your 120 and one for your 240 appliances. And then you would need to have a subpanel separating out your 240 stuff.

I don't think the power inverters that are 240 can handle splitting the two legs up with different draws on each leg. Maybe I'm wrong, but a quick google of 240 power inverters looked like they either do 120 or 240, not both.

A call to this co. might be worth your time. It looks like it has built in switching as well.

http://www.topsalesdepot.com/bwaphppoin122.html

 

[Ira]

Looking at Tripp-lite "data center" UPS's (I assume the other majors have similar products)...you can get a 6kW 240Vac double conversion UPS for about $3500. It has hardwired input and output, which I guess would be good. According to the runtime chart, it will handle a 4kW load for four minutes, which is plenty of time for a genset to kick in if it is going to. 4kW is a pretty good cushion if I only have 2.5kW running thru it.

So I'm thinking that with something like this, I would hardwire its input to a properly sized double breaker in the main load center. The output would go to a subpanel that had all the circuits I want the UPS to "cover". The rest of the setup would be the typical whole-house standby genset with ATS setup. POCO power and genset both feed ATS. Output of ATS goes to main load center (but not UPS-protected sub-panel).

When POCO power is lost, the UPS-protected circuits don't see the loss at all. After 15 seconds of power outage, the genset starts. After 15 more seconds, the ATS switches over to genset power. At that point, the genset powers all "non-protected" circuits in the main load center, including the circuit feeding the UPS, so the UPS batteries would begin charging again.

My guess is that I would run into some problems with the above design (if it can even be done) unless I kept the number of actual circuits powered thru the UPS down to maybe three 15A 120Vac circuits.

 

[drozwood90]

take a look at the UTS from Apcc.
kind of like an ats, but can do load shedding, can power circuits from an UPS and kick on the generator to then supplement.