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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

Backup generators for emergency power can be fueled by gasoline or propane. It's often hard to know whether it's worth investing in a generator that is very rarely used.

[Editor’s note: Roger and Lynn Normand are building a [no-glossary]Passivhaus[/no-glossary] 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.

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 envelope 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.


  1. dankolbert | | #1

    I too was thrilled when I heard of this arrangement - I believe Alex Wilson is also setting his house up this way. I think it's a great idea and am talking with some current clients about including this arrangement - please keep us informed on anything else you learn on the subject, Roger.

  2. watercop | | #2

    I dunno...
    I think the Prius stunt has potential, but I'm not all that jazzed about risking the warranty on a $30k car to accomplish something a $1k genny might do just as well.

    I've had a Honda EU2000 portable genny for years. It is lightweight, quiet, starts easily and sips gas. In fact, I often load it into a wheelbarrow and roll it to quick projects on my property more than 50-100' from a power outlet instead of stringing multiple extension is that easy to use.

    Off the top of my head, I wonder if the Prius cooling system is designed to provide enough cooling for the gas engine to provide enough power to run the Converdant power takeoff while the car is not other words no "ram" air is being pushed through the radiator by the motion of the car. That may not be a problem during a Maine ice storm, but I'm not sure I'd risk it in warm weather.

    As to Magnuson Moss, Toyota may not be able to prove that the Converdant mod caused a failure, but if they denied coverage, it would be an uphill battle to pursue a claim in court. The Prius is not intended for use as a stationary generator, and using it so would likely constitute misuse or be considered outside the realm of "merchantability" or "fitness for a particular purpose"

  3. DEnd2000 | | #3

    Magnusson-Moss Warranty Act
    In this case the Magnusson-Moss Warranty Act, would probably not apply as the vehicle is used outside of it's intended purpose. But I'm not a lawyer nor am I familiar with Toyota's warranty. I would not recommend the Coverdant device as for $100, it can not include the necessary modifications to the Prius to allow it to function well as a generator. These would include things like a larger radiator, an oil cooler, and heat sheilding. Passenger vehicles are not designed to sit still for long periods of time, and can easily overheat during periods of long idling even in below freezing temperatures. All of that said I would not hesitate to use the Coverdant system if you were looking at a use case of less than an hour at a time, I'd just keep a close eye on the engine temp.

    As electrical generators cars are very inefficient, well actually cars are very inefficient anyway no matter what the use case is... Also in extended power outages, the power outage will affect a very large area, during these times fuel will not be readily available. What I would recommend is getting a generator that is able to power your fridge and freezer, power a few lights and charge your cell phones. Cooking and heating should use an alternate fuel source. A 20 lb tank of propane will last quite a while for just cooking duty (with a burner not a grill). Looking at the main floor plan for the house it appears you also will have a wood burning stove which will work for heating the house, cooking and warming water for sponge baths...

  4. ecdunn | | #4

    Inside temps
    A tight, well insulated home will not drop below about 54 or 55 degrees with the heat off for several days. I have built many energy efficient homes and none have ever gone below this temperature. My own house, a 1977 spec ranch house that I have upgraded the insulation on and made much tighter, i turn off the heat when I leave town, no matter what the forecast. Have never had a temp below 54 recorded. Pipes in the envelope will not freeze.

  5. GBA Editor
    Martin Holladay | | #5

    Response to Ed Dunn
    Your approach won't work in Fairbanks, Alaska. It's all about location.

    It's easy to do in San Diego.

    In between Fairbanks and San Diego -- it depends.

  6. lutro | | #6

    How much PV will you have?
    The brief mention of photo-voltaics says, "We will have a photovoltaic system", but it says nothing about the size of the system. Later, the author says, "Under normal conditions, all circuits are powered via the grid." How will the PV system be integrated into the house?

    While I can appreciate the downsides of installing a battery bank just to handle a rare power outage, I wonder if the PV system provides any useful support for this house during those outages. During a summer's day, a PV system might provide all the power needed until the grid connection returns. A multi-day outage in the cloudy days of winter might reduce the PV output to below a useful threshold, but perhaps having some power for part of the day could make a valuable difference, depending on the house and the lifestyle.

  7. GBA Editor
    Martin Holladay | | #7

    Response to Derek Roff
    The vast majority of residential PV systems in the U.S. provide no usable electricity to homeowners during grid power outages -- even at noon on a sunny day.

    Here's why: PV modules produce low-voltage DC electricity. Home appliances use 120 volts AC or 240 volts AC, so you need an inverter to change the DC power to AC power of the appropriate voltage.

    The inverters used for almost all grid-connected PV systems don't work when there is a grid-power outage. Without an inverter, there's no way to use the DC electricity produced by your PV array.

    So, if you want to have power during a grid power outage, you need two pieces of equipment: (1) a big battery bank, and (2) a special inverter appropriate for off-grid PV systems. These two pieces of equipment are expensive.

  8. Expert Member
    Dana Dorsett | | #8

    Maine weather is closer to San Diego than Fairbanks AK
    Martin, seriously- the average mid-winter daily temps in Fairbanks is lower than 99% outside design temps for most of Maine, and this is a PassiveHouse, not padded out 1970s rancher. With all plumbing entering from underground (standard PassiveHouse methodology) the odds of a freeze-up without auxilliary power for a few days is nil, even during a cold snap.

    In Japan the kilowatt Honda grid-tied cogenerators can be had with a change-over switch to isolated mode, with a lawn-mower type pull-rope start, (and both demand & production have skyrocketed since the Fukushima event). But even were that unit available in the US I'd worry more about freezing the heat exchangers on the beast if you ran out of propane even during normal Maine winter weather.

    Two adult occupants would provide ~500BTU/hr even while sleeping, which is enough heat to make a difference in PassiveHouse, if not so much in a "pretty-good house". Throwing a party with a couple dozen guest could exceed the design condition heat load.

    Pretend that Maine is in a first-world country with a grid that won't go down for more than a few days at time- spending ANYTHING on auxiliary power is just silly.

    During the ice storm of 2008 my central MA home was out of power for over a week, when overnight temps were running ~+10F for the first three days without breaking the freezing mark. This is a 1923 antique with known gaps in the insulation and almost zero solar gain due to shading factors. I monitored peak & temps with battery operated RH & temp monitors in several rooms, and while the coldest room was the (at the time) nearly-uninsulated upstairs bathroom, the temp in that room never dropped below 38F. That's not a lot of margin, to be sure, but again, this is no PassiveHouse, (or even a padded out rancher.)

  9. GBA Editor
    Martin Holladay | | #9

    Response to Dana Dorsett
    You wrote, "Martin, seriously- the average mid-winter daily temps in Fairbanks is lower than 99% outside design temps for most of Maine."

    Your argument is unnecessary, since I agree with you. At no point did I say that the weather in Fairbanks was similar to that in Maine. I was responding to Ed Dunn -- I wasn't discussing Roger Normand's project in Maine.

    Ed Dunn never told us his climate zone, which made his data on interior temperatures impossible to evaluate. That was my point.

    Happy new year, Dana.

  10. CalBungMelb | | #10

    Battery backed storage for PV's
    Lead acid batteries are a pain to manage but there's a bunch of other technologies at various stages of commercialisation. I'm doing a case study for a large isolated grid installation. I've been doing some costing for a 500kWh battery storage using one of the newer technologies. I was pleasantly surprised at the amortised cost/kWh over a 20 year lifetime. And it substantially increases the fraction of demandr that can be supplied by PV's. The maintenance costs for the newer battery technologies are lower as well.

    This might not scale all that well at the moment to individual residential storage installations but when implemented at a community scale, larger storages could be used to supply individual houses.

  11. DoctorBeer | | #11

    What was the price of the PV battery backup system you refer to?

    I have been reading and commenting in various GBA forums on and off over the past few years in the course of designing and building a super energy efficient house in VT just north of $towe (that's Stowe). More recently I've been working at a reseller of Solar PV, Thermal, Direct Hot Air and Wind Turbines. Since you don't mention what the cost of the PV system is or its capacity it's hard to comment on whether the system you refer to was expensive because it was over sized wrt backup needs or not. You yourself make the point at the outset of your article that power outages are rare and it's unusual for them to be many days. PV grid tie with battery backup systems don't have to be extremely expensive and there are lead acid based battery technologies such as AGM or Sealed Gel which produce no off gassing during recharging (the way flooded lead acid batteries do) and are very safe and stable to have inside a dwelling.

    Wrt to backup generator vs PV let's take a couple of recent situations. A couple of years back there was an ice storm in New England. My brother in law lives west of Manchester, NH. The numbered state route he lived on had trees downed everywhere and it was 5 days before they could clear them in either direction so that he could get in and out of his house. If he had been relying on a gasoline generator he would have been out of luck since he'd have run out of fuel quickly. The recent hurricane and associated fuel shortages on Long Island were another example of where the reliance on fuel left people in the lurch.

    One thing you can always count on (even in New England in winter) is that sooner or later the sun will shine.

    In addition when you design a backup system most people don't size it to run their entire house. Instead you utilize a sub-panel that has the most essential circuits on it and only run those. That's how I'm designing the grid tie battery backup system for my VT house. Typically things like lights in key areas, refrigerator and heating circuits are what are included.

    So here's an example system. Solar PV panels are running around $0.89 to $1.25 per watt these days. If you assume only 3 hours of sun and put 2KWH on your roof you'd be looking a about $2000 in panels. A high quality pre-wired (ie very easy to install) inverter power panel that can work with a 24 or 48 volt system and handle 3000 to 3600 Watts per hour load goes for around $3500. That includes the battery charge controller, system controller, breakers and safety switches, etc. Battery capacity for a single string of AGM batteries (not flooded lead acid) at 48V that can hold 6KWH (taking into account 50% depth of discharge) would run around $2000. When you include mounting hardware and other costs (wire mostly) you're probably looking at around $10,000.

    A friend of mine spent well over $10,000 to put in a natural backup generator to run his whole house. For the same $10,000 when you put in a battery backup grid tie solar PV system while you don't get enough capacity to run the entire house you certainly get enough capacity to run it over a long winter night. If you have a few days without sun you will, admittedly, either have to conserve energy or do without. But when the sun does shine you will charge the battery bank back up.

    Also consider that with the generator you are locked into the capacity of the generator to produce electricity and, unless you add a battery bank to work in concert with it (as many of my customers have been doing) it has to run constantly in order to produce power and typically produces more than you need while sucking down fossil fuels as it does so. Whereas with a PV based battery backup system you can add additional strings of batteries over time as your budget allows. If over time you upgraded the single 48V string I mention above to 3 strings you'd be able to store 18KWH of power and with smart usage that could last many days.

    However the one really important thing that the $10,000 in a PV grid tie with battery backup system gets you that a generator never will is payback. Everyday that the grid is up and the sun is shining is a day that a generator would sit there idle after having cost you piles of money. But for a grid tie solar PV with battery backup every day that the grid is up and the sun is shining is a day you are recouping the cost of the system by producing electricity and selling it back onto the grid while simultaneously reducing your fossil fuel consumption and CO2 production. Over time the PV option will pay for itself. The generator never will.

    Which way you go is a personal choice based on your lifestyle and ability to modify it in an emergency. For me personally it's really no choice because the PV approach, while not necessarily a flawless backup system, embraces the reason d'etre of this forum and the reason everyone who reads it or comments on it does so, which is to use less energy.


    Jay Hersh

  12. DoctorBeer | | #12

    Martin's comment about the achilles heel of grid tie systems
    Hi Martin,

    In my position as a reseller of PV equipment I hear about great stuff hitting the market before most others. Companies that manufacture grid tie only systems have not been unaware that after paying all that money for panels and an inverter it sticks in the craw of customers when the power goes down and they can't take advantage of the PV generation capacity they have during an outage.

    The solution for this is something they refer to as AC coupling. As you mention the Achilles Heel of grid tie systems is the anti islanding safety feature that prevents line workers from injury or death by making certain that PV grid tied systems disconnect from the grid when it goes down. The way AC coupling works is that an additional box is purchased to work with the system. This box sits between the grid tied inverter and the main AC panel. However it also ties into the emergency backup electric sub-panel and a battery bank. When the grid goes down the box draws power from the backup batteries and has a small inverter in it that is enough to make power that fools the grid tie inverter into thinking that it is still on the grid. Simultaneously it actually disconnects the grid tie inverter from the grid and routes the power that it produces to the backup emergency sub-panel and the battery bank system. If there is no sun then it stops trying to fool the grid tied inverter and instead just pulls power from the battery bank and routes it to the backup sub-panel.

    Essentially it is a best of both worlds approach and the devices that are out or about to come out can work with existing grid tied inverters and be installed retroactively. The folks that produce these systems are no dummies. They're very smart people and while it didn't take much time to recognize the drawbacks of the prior system architectures it has taken some time to design and test hardware that could address those shortcomings. Fortunately at least 2 vendors I know of will have solutions and within a year or two most or all of them probably will...



  13. GBA Editor
    Martin Holladay | | #13

    Response to Jay Hersh
    As someone who has lived with a battery bank in his house for the last 33 years, I predict that very few grid-tied Americans will want to buy the lead-acid batteries, and to engage in the required battery maintenance activities, that would be required to run your equipment -- especially once they find out the price and the hassle involved.

  14. GBA Editor
    Martin Holladay | | #14

    Another response to Jay Hersch
    I now realize that you posted two comments in a row. My last comment was in response to your comment #12. This comment is in response to your comment #11.

    You have skewed the figures several ways in your $10,000 example.

    First of all, discharging your batteries to 50% of full charge is irresponsible. It shortens the life of the batteries. At 50% discharge, the battery voltage will be quite low, anyway. Most homeowners with systems that combine PV and batteries aim to discharge their batteries to no more than 80% of full charge.

    Secondly, who needs a $10,000 generator? My 5-kW Honda generator cost $2,000. It provides more than enough power to run my whole house, and it is likely to last for 18 years. Yes, it's a good idea to stockpile gasoline in anticipation of an ice storm.

    Your puny PV and battery system will provide only a fraction of the output of my Honda. And if you think you'll get 3 hours of sunshine each day after an ice storm, you're dreaming. We go for two weeks in November and December with no sun at all.

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