Backup Electrical Power for a Passivhaus Project?

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Backup Electrical Power for a Passivhaus Project?

If your house has a tight, well insulated envelope, the plumbing pipes aren’t likely to freeze — even if the power is out for days

Posted on Dec 25 2012 by Roger Normand

[Editor's note: Roger and Lynn Normand are building a Passivhaus in Maine. This is the 19th article in a series that will follow their project from planning through construction.]

Should we include a backup electrical power supply for Edgewaterhaus, since it will be an all-electric home? If so, we should include provisions for a backup electrical source as part of the overall plan, whether we decide to implement it now or later.

Backup power is like insurance: if you don’t collect a benefit while the insurance policy is in effect, all of your insurance payments were a complete waste of money. Conversely, if you collect a substantial benefit, you can pat yourself on the back for having made such a prescient investment.

There are two questions to consider in deciding whether to buy a backup electrical power capability: What is the likelihood that we lose power, whether for a matter of hours or days? And what is the impact in terms of financial loss or lifestyle accommodations?

Power outages are rare

Neighbors tell us that it is rare to lose power for more than a few hours. Most such situations are due to trees falling on power lines during storms. Users can notify Central Maine Power of a power failure by email or phone. CMP has recently implemented a slick web-based electrical power outage map. You can zoom to multiple levels of details, from the total area of service down to local street level, with summary information on the number of affected customers. That’s reassuring.


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Though there have been few brief power interruptions in this area, there is always a risk of widespread and extended power loss due winter ice storms. Many Maine communities have been without electrical power for days going on weeks when this occurs.

Pipes might freeze

Even if a loss of power seldom happens, what’s the impact if it does? There can be a substantial effect if it occurs in winter. Unless you heat with a wood stove, you need electricity to run the electrical motors on an oil- or gas-fired boiler, or the fan on a hot air system, or even the impeller on a pellet stove. So it could be a bone-chillin’ stay in the house while you await electrical power to be restored.

More critical is preventing water pipes in the house from freezing. When warmer temperatures return, a burst water pipe can cause enormous damage to furnishings and even the structure of the house, particularly if it occurs in a concealed wall. That is a special concern for us as we will likely become “snowbirds” traveling to warmer climates during the deep throes of winter.

Fortunately, a frozen pipe in Edgewaterhaus is a very unlikely event, thanks to the robust building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. of a Passivhaus building. We’ve heard one Passihaus owner in upstate Massachusetts say he left for a week in winter when temperatures dropped below freezing most of the time, but the home’s interior stayed in the high 50s, according to the data-logger he installed to monitor building performance.

We have no critical business or medical electrical usage needs. So our risk is really a lifestyle impact, and potential food loss in the summer. That suggests a no- or low-cost electrical backup approach.

Using a PV array in a power outage

What are our choices for low-cost backup systems?

We toured a nearby home with a roof-mounted photovoltaic system and a sophisticated – read expensive – series of deep-cycle batteries along with controllers to keep the batteries fully charged and a means to safely disconnect the house from the grid when you resort to battery power. We will have a photovoltaic system, but I don’t like the high cost of such a backup system, nor the idea of storing and maintaining batteries in the house. You’d also have to hope for sunny days during the outage to replenish the batteries.

Gasoline- or propane-powered generators

We have considered a small portable or stationary propane generator. But such generators are obnoxiously loud for occupants and neighbors alike, require a fuel storage tank, have to be tested monthly, require periodic maintenance, and get pricey for larger capacities.

That all seems an excessive one-time cost, plus a monthly chore we don’t need.

Your Prius can provide emergency electricity

So I was intrigued with an innovative approach I saw while attending the Building Energy 2012 symposium in Boston in March. Converdant makes a “plug-out” kit that allows a Toyota Prius to serve as a home backup electrical generator! What an interesting concept.

Why buy a separate generator when we could temporarily tap into our 2010 Prius’s efficient electrical generator, sophisticated electronic management system, battery reservoir, and ultra-quiet operation when the engine is running to recharge the batteries? No separate monthly testing required; system testing occurs each time you drive the car. No need for a separate propane storage tank; the Prius includes an 11-gallon gasoline tank easily refilled at any service station. The Converdant plug-out cost: $800 for a 2-kW kit, $1,300 for a 3-kW kit, and $1,800 for a 4-kW kit, plus an estimated $100 to install an electrical connection on the Prius, and a means to connect to the circuit(s) you want to energize.

I can only think of a few downsides and one concern. The downsides: you must manually connect the Prius to the Converdant device each time you need backup electrical power; you must either park the Prius outside, or channel the exhaust fumes outside the garage. There is no instant-on switching capability as some dedicated backup generators provide. There will be periods without electrical power. That’s not a big deal for us.

The concern regards the effect on the Toyota warranty. Toyota warrants the Prius hybrid system for 96 months or 100,000 miles. Would including an aftermarket product like the Converdant plug-out device void the Toyota warranty? No, according to the FAQ page on the Converdant website, which state that the Magnusson-Moss Warranty Act would require Toyota to prove that the Converdant device caused the hybrid system failure. I wonder whether there has been a real life test of this situation.

Specifying a panel with a transfer switch

For now, we will hedge our bet. For less than an additional $100 above the cost of a typical electrical panel, we will specify a generator-ready electrical panel with an integrated transfer switch: the bus on the upper portion of the panel powers only grid energized circuits; the bus on the lower level powers circuits either from the grid or a standby generator.

Under normal conditions, all circuits are powered via the grid. But manually throw the transfer switch in the middle of the panel, and all upper circuits are cut off and all lower circuits are powered via the backup generator.

Using the Prius as a backup electrical generator sounds very interesting. We also plan to include a circuit to the garage to allow recharging a future plug-in hybrid or all-electric vehicle.

The first article in this series was Kicking the Tires on a Passivhaus Project. Roger Normand's construction blog is called EdgewaterHaus.


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