PriUPS Update 30 Oct 2005

Success!  The PriUPS project, mentally started before I ordered my Prius in Jan 2004, physically begun in early 2005, is finally "complete" in October 2005.  (Of course "complete" is a relative term.  No project is ever complete.) 

A house without emergency power provisions is wired simply:  The incoming electric service is wired to a panel with a number of circuit breakers.  These breakers, ranging from 15 or 20A (Amperes) for room outlets, to 50A or more (air conditioning, electric stoves, etc.,) are in turn connected to the circuits they power.  That's how this house started out. 

A house with a large emergency generator capable of powering the full house has a "transfer switch" added.  This simply means that the incoming electric service is routed to one input of the transfer switch, and the output of the generator to the other input of the switch.  When the generator comes on due to an electric service failure, the transfer switch connects the generator to the house.  When the electric service is restored, the switch transfers the house back to the utility line.

Because the Prius can supply a limited amount of power and my house is relatively large, I had to use a different strategy.  A number of appliances take a disproportionate amount of power, and yet are not truly necessary in an emergency, or at least do not require continuous power.  Accordingly, I got a licensed electrician to create a new "subpanel" and move the circuit breakers I wished to associate with the UPS to this new panel.  Here are some of the decisions I made:

Breakers Moved to UPS subpanel	Breakers NOT Moved
Refrigerator	Air conditioner
Freezer	Central vacuum cleaner
Fish tank	Pool pump (I have a choice; the little fiddies don't)
Sump pump	Washer/dryer
Well pump	Electric Oven
Furnace motor	Big Teevee
Computers	Some ceiling lights

The obvious point, I guess, is that it's better to be warm and dry than clean and well fed.  Of course, these are just some of the selections, but clearly the items necessary to prevent damage or spoilage get the power nod.

I've connected a networked computer to the UPS so I can monitor the system anywhere there's an internet connection, and operation has been nominal. 

The computer shows the UPS battery voltage, AC line voltage, and a rough indication of how much power the UPS is being called on to deliver to all the breakers in the emergency panel.  This example is typical.  Normally the sustained load on the UPS is well under 10% of capacity, and the peak load is between 20 and 30 per cent for a few seconds per hour.  It should be noted that the UPS defines 100% load as normal operation, and the UPS can actually deliver quite a bit more for brief periods.  It seems I was somewhat pessimistic when selecting circuits to add to the emergency panel since I felt the load would be higher than the UPS is actually declaring it to be.  If I revisit the wiring I may add more circuits to the emergency panel.

The program, in addition to the logging shown above, computes a histogram (hourly and daily) of the amount of power used at each percentage, and a total of all power used during the day (the number "5267" above).  This number is relative since the UPS output is in "ASCII art" which I have to reinterpret, with ineluctable rounding and interpretation errors.  Even so, it provides trend info.  After a few seasons I may go back and see what I can learn from it.  In addition to creating a history, the program provides an audio alert ("mains power failure, Richard"), for the few minutes per day that I'm not glued to the computer screen, and sends an email alert as well.  These latter two features are critical:  If you're running on a UPS that's in a different part of the house, its alarm is inaudible and you won't know there's a power failure!  And, email fanatic that I am, I can learn about a problem that way even if I'm not monitoring the UPS at my local (or remote) computer.

Living with The PriUPS House

One of the boons of using a Prius instead of a generator is that "maintenance" is pretty much automatic.  The car is being driven most days, there is always gas in the tank, and it's subject to the routine maintenance that all cars should get.  With a generator, there should be routine maintenance but it is an extra effort and is frequently neglected, occasionally with dire results.  The Prius has a different set of negatives, the two major ones being a somewhat limited power capability, and the fact that it may not be present when a utility failure occurs.  Also, the UPS batteries (without a Prius connection) have a limited useful runtime, and it is difficult to restart the UPS if it shuts down.  (This issue is discussed here.)  Finally, this is my project:  I am somewhat eccentric and other members of the household don't follow it avidly or necessarily memorize how it all works.  Accordingly, I decided to prepare a looseleaf notebook with aircraft-style checklists to handle utility power outages. 

Here it is

What remains to be done?

I chose, I think wisely, to do all the house electrical work in an "official" manner.  I got a town permit, had licensed electricians do the work, and then had the inspector come to bless the installation.  On the first pass the inspector failed the job.  This had nothing to do with the UPS/Prius connection, but rather was due to some arcana of the electrical code that dictates alternate-wiring of the circuit breakers, and an additional charge of Misdemeanor Failure to Spackle.  This has all been remedied and I'm awaiting the Return of the Inspector.  There are some things I want to do after the inspector is done, e.g., make a proper installation of the computer monitor, but I have to leave the area clear for him.