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Planning for Backup Power in an All-Electric House

A homeowner in Kentucky eyes a solar array to keep his house running in the event of a power outage

Posted on Feb 19 2018 by Scott Gibson

Writing from central Kentucky, Clay Whitenack poses this question: in an all-electric house, what's the best way of providing power when the grid is down?

Whitenack and his family live in a new house, a house that does not have a fireplace or a wood stove. "This leaves us vulnerable in the event of a power outage during a bad winter," he writes in a post at the Q&A forum. "We live in central Kentucky, so the winters here are usually not too bad, but we do have times when the temps get below freezing for extended periods of time."

The house is well-insulated and heated with ductless minisplits; domestic hot water is produced by a heat-pump water heaterAn appliance that uses an air-source heat pump to heat domestic hot water. Most heat-pump water heaters include an insulated tank equipped with an electric resistance element to provide backup heat whenever hot water demand exceeds the capacity of the heat pump. Since heat-pump water heaters extract heat from the air, they lower the temperature and humidity of the room in which they are installed. . Whitenack's first thought about emergency backup power is a solar array.

"I would like to start researching a small solar array that would allow me to keep the essentials going," he says. "Things like heat, hot water, the fridge, stove, and maybe a few outlets around the house for a few lights, charging mobile devices, etc."

He wonders how to calculate the amount of electricity he'd need in a pinch, and what the capacity of a photovoltaic system might be. At least that's where this Q&A Spotlight begins.

How long is an outage really going to last?

A well-insulated house in Kentucky isn't likely to have plumbing freeze-ups even in very cold weather, says Dana Dorsett, so a small woodstove might be enough to keep everyone comfortable during a power outage, and it would not require "punching huge holes in your efficient 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.."

More to the point, Kentucky has a reliable electric grid and power outages that extend beyond a day or two are "extremely rare," Dorsett says, adding, "The capital budget for equipment to cover trailing-edge 0.001% risks should be sized accordingly."

But Whitenack isn't as concerned with a brief outage as he is with an extended grid failure, something caused by a natural disaster that disables the local power plant and leads to a blackout measured in weeks.

"It's not a huge fear of mine," he says. "I'm not obsessed with zombie-apocalypse scenarios, but I have a young family and this is the first house I've lived in that hasn't had some option for backup heat."

Still, that shouldn't be an issue in Kentucky, Dorsett tells him. In the central part of the state, there's plenty of redundancy built into the grid.

"Grid planners in Kentucky are also further along than most states with smart-grid planning and development, making the grid more reliable in general, but also 'self healing' for re-routing power when major links go down," Dorsett says "It's definitely a first-world power grid environment, nowhere near as vulnerable as New Jersey and New York were during Superstorm Sandy."

A PV system isn't cheap editor Martin Holladay, who has lived off the grid for many years, points out that installing a PV system is expensive. Solar panels, batteries, a charge controller, and an inverterDevice for converting direct-current (DC) electricity into the alternating-current (AC) form required for most home uses; necessary if home-generated electricity is to be fed into the electric grid through net-metering arrangements., sized for backup at a house that uses fossil fuels for space heating and domestic hot water could cost between $10,000 and $20,000, Holladay says. "If the house uses electricity to provide space heating and hot water," he writes, "it would be hard to even imagine the size of a battery big enough to get the house through a few cloudy days. Such a system would cost a fortune and would be a maintenance headache. It's far cheaper to spend $1,200 to $2,000 on a gasoline-powered generator."

His own generator is a Honda that can crank out 5,000 watts. It cost a little more than $2,000, and it can produce both 120-volt and 240-volt electricity.

Whitenack admits he hadn't considered buying a generator. While that option does look cheaper and easier to install than a solar array, the long-term financial picture isn't as attractive.

"What about the long-term returns of a solar system vs. a generator, though?" he asks. "I plan on living at this location for another 40-50 years, God willing. A PV system would allow me to offset my energy bills, which a generator would not."

And generators can easily top the $2,000 that Holladay mentions.

Stephen Sheehy, for example, writes that after looking into possible backup power sources for his Maine home, he settled on a 7,000-watt Honda. After hiring an electrician to install an exterior receptacle that could handle the generator's 30-amp output, he ended up spending a total of $6,000.

"I tried to shop around but most dealers didn't have any Hondas available," Sheehy said. "It's a nice, quiet machine with inverter technology that lets me run my minisplits."

Adding batteries to the mix

If Whitenack follows the more expensive solar route, Dorsett suggests looking into a "battery-ready" inverter, which would allow the addition of a battery in the future — when demand charges are introduced for residential electricity customers, for example.

"The cost of lithium-ion batteries is still in free-fall on a double-digit learning curve," Dorsett writes. "At some point in the next decade it will be worth it, even if the primary use is as backup rather than demand-charge mitigation." He points to a Bloomberg report that says the price of lithium-ion batteries in 2016 was $273 per kilowatt hour, a decline of 73% since 2010.

But the addition of storage batteries also raises other questions while making the system more complex and more expensive.

Sheehy says his research for backup power included two quotes for a battery. A Tesla Powerwall would have cost almost $14,000, while a competing battery from LG would have been almost $12,000. Those options would have provided between 12 and 14 kWh of energy storage, and he would still have needed a gas-powered generator.

"Both systems quoted to me could not be charged by the generator," Sheehy adds. "That meant that in an outage I'd need to go back and forth between battery and generator."

Battery technologies are different

Both the Tesla and LG batteries are lithium-ion, while Holladay's system uses older generation lead-acid batteries, which can be charged with a generator.

"Using a generator to charge a battery bank is fairly simple," he says. "Most off-grid inverters include a battery charger. Off-grid homeowners start up the generator when it's been cloudy for a few days. When necessary, I plug my house into my generator. When the inverter senses the generator power, it automatically charges the battery at a rate of about 90 amps (tapering down to 40 or 50 amps when the batteries are close to full). While the generator is running, there is also plenty of extra power to run the washing machine or other appliances."

Although it's not immediately obvious to Holladay why a generator can't be used to recharge a lithium-ion battery — after all, the internal combustion engine in a Prius is capable of charging the car's lithium-ion battery — a little internet sleuthing suggests it's not as simple as it may seem. He finds that lithium-ion batteries may need an automotive alternator rather than a generator for charging, "or a set of old-fashioned lead-acid batteries as a buffer for the generator."

The reason for that is complicated, but Holladay refers Whitenack to an article with a longer explanation.

Calum Wilde disagrees. "If the [Powerwall] is being charged by mains power it shouldn't matter if that 240 volt AC is coming from the grid or your generator," Wilde writes "The only way it could matter is if the hookup method purposefully prevents it. But the battery charger/inverter shouldn't care. If I were you, I'd ask about that setup on Reddit. They're generally a great bunch of professionals with collectively an immense amount of knowledge."

Are batteries really necessary?

One issue with a PV system is whether the electricity it generates can be used during a grid outage without the addition of a battery.

Brian P writes that Pika Energy offers an inverter that appears able to work in a grid-tied system as well as an independent "island" when the grid is disabled. But Pika tells Sheehy that its system doesn't allow PV electricity to be used when the grid is down except through a battery, which to Holladay means it can effectively function as an off-grid system.

Thomas Stone points to two companies that make grid-tied inverters that will operate when the grid is down (and assuming there is sunlight). One is made by SMA, and another is called the Solar Edge.

Kevin Spellman has another suggestion: a system from Schneider Electric, which includes an inverter, a generator and lithium-ion batteries.

"So," he says, "it can be done."

But prices are high. The 19.5 kWh battery that Spellman references costs $14,900, which by Holladay's calculations stores the same amount of electricity you can buy from the grid for between $2 and $4.

"Batteries are expensive," he says. "Grid electricity is cheap."

Our expert's opinion

Peter Yost, GBA's technical director, added these thoughts:

It’s difficult to add much to the practical information and insights already offered by Martin Holladay and others. But I wanted to share some information I have gathered from two conference presentations. One was the 2013 Better Buildings Better Business conference in The Dells, Wisconsin, where my friend and colleague Dan Cautley of Seventhwave spoke. The other was just last week when Marc Rosenbaum presented at the Better Buildings by Design conference in Burlington Vermont. Both were outstanding events.

Dan’s presentation “Electrical Energy Auditing: Typical Use by Appliance,” is a great way to estimate electrical loads when trying to design a back-up system. I pulled these out of his work because Clay Whitenack listed them:

  • Heat-pump water heater: Typical annual use 800 kWh.
  • Refrigerator: Draws 725 watts; typical annual consumption 1000 kWh.
  • Stove/range: Draws 1,500 watts; typical annual consumption 300 kWh.
  • Lighting (50 lamps): Draws 2,364 watts; typical annual consumption 1560 kWh.
  • Phone and camera chargers: Draw 2 watts; typical annual consumption 11 kWh.

For those trying to figure out just what essentials can be covered, Dan’s work is pretty helpful, even if it is a bit dated.

I can report on Marc's presentation only indirectly. I'm sorry to say I didn't get to that session because it competed with another talk I wanted to hear, But friends and colleagues Eli Gould and Jim Duran of PreCraft had these takeaways:

  • Even a thrifty family in a small house would need “huge” storage capacity to cover the starting and sustained power draws from essential appliances and lighting.
  • Reducing the heat load helps a lot, so a wood stove looks mighty attractive for covering power outages.
  • It's really hard to eliminate the need for even a small generator to offset the four-day, cloudy winter scenario.
  • Marc’s calculations on generator emissions show that even the most efficient generators are way worse than the grid.
  • Considering the current limitations in batteries, and the diminishing returns of increasing total battery capacity, you just can’t store enough energy to last through a long outage without the help of the generator.

NOTE: Efficiency Vermont has posted the presentations from this year’s Better Buildings By Design conference. Marc Rosenbaum's presentation was called “PVs and Battery Storage.”

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Feb 19, 2018 4:26 PM ET

Edited Feb 19, 2018 4:36 PM ET.

Ice Storms
by Walter Ahlgrim

Clay’s concerns are not unfounded in this part of the world we have ice storms they happen every 10 years or so.

What happens is we have a long cold snap followed by a cold rain. The rain freezes on contact with any surface when it is bad the ice can be 3 inches thick. The weight of the ice will take down trees, wires and utility poles. The roads will be impassable half the electric grid has been known to go down. Yes repair crews will come from all the joining states and they will repair in an order where each repair will restore power to the most homes. If you are unlucky and the wire from your home to the pole fails you may not get to the top of the list for 10 days.

Clay’s real question is how I can keep my family and pipes from freezing if I am without electric for 10 days?

A small portable generator will run your freezer refrigerator and light but without heat you will be miserable.

How can he run his mini split?

Yes I have personally been unlucky enough to have to have 3 separate multi day power outages in less than 2 years. But most of the blame goes to the regulators that let the utility stop tree trimming for years to save money and bump up profits at the expense of reliability.



Feb 19, 2018 4:46 PM ET

For thermal backup...
by Dana Dorsett

... wood stove. Or even COAL stove.

An ice storm left me without power for 10 days in December 2008, with overnight lows dipping to +10F, in a well below-code-min house. Camping out there wasn't an option, but it was possible to intermittently run the gas-fired kitchen range and fill the bathtubs with the dumb lo-tech water heater we had at the time to keep the pipes from freezing over night.

That experience was an incentive to install a wood stove for auxilliary heat (which I don't regret at all...) in the pre-existing miserable leaky backdrafting open hearth fireplace. Had the wood stove been in place at the time we would have continued to stay there as grid repairs were in progress all around us.

Feb 20, 2018 7:53 AM ET

Running the mini split
by Chris King

Walta I have the same question if you wanted to prioritize a mini split power supply for emergencies is it possible to use a battery backup like a powerwall to power a 2 ton unit. I guess the question comes down to how much ampage does the unit draw on start up and would something like a powerwall be able to kick in quick enough to stop the unit from shutting down if it’s already running which should help keep the amp draw lower. I tried looking at the submittal sheets but I can only find breaker size for the amps not what they use on cold starts so no idea how to calculate how long a battery could keep them going if that was the only thing on them.

Feb 20, 2018 8:08 AM ET

Response to Chris King
by Martin Holladay

The most dependable way to keep your minisplit going when the power goes out -- and the least expensive way -- is to buy a generator.

While it's common to worry that a power outage will make it impossible to run a minisplit heat pump, readers should remember that a power outage will also make it impossible to run a furnace or boiler. All central heating systems are disabled by a power outage. If you want heat during a power outage, take Dana Dorsett's advice and get a wood stove -- or install a propane space heater with through-the-wall venting.

Feb 20, 2018 9:05 AM ET

Thanks Martin
by Chris King

Yes a portable generator is easily the easier and more economical approach I am more just curious if a fully charged powerwall could do it then the practicality behind it. Also in regards to the wood stoves being the only sure fire way way to get heat with no power. Give it a few years I’m sure someone will come out with a wood stove with electronic monitoring to help optimize burning. Then when the power goes out it locks the stove door since it isn’t optimized. Put electronics in everything that’s the way we’re heading....just saw a commercial for a mattress with a sensor mesh in it to monitor sleep.....

Feb 20, 2018 9:57 AM ET

Good advice
by James Morgan

"A warm wood stove evokes fond memories for many Americans. But a wood stove doesn’t belong in a green home. It’s just too hard to balance a wood stove’s voracious appetite for combustion air with a modern home’s need to limit air leakage."

Read more:

Feb 20, 2018 9:26 PM ET

Response to Chris King
by Walter Ahlgrim

Chris my point is if your home is all electric you are vulnerable to extended power outages.

It makes me feel better to have some stored energy on site. At this point I am not ready to drink "battery wall" kool aide.

If you do I would say size your battery for the worst case and be able to power the factory recommended circuit breaker.

If you plan on running a heat pump with a generator it is very unlikely to be a “portable generator”

In my last home we could operate the gas furnace on the portable generator. In my new home the back up to the heat pump is a propane fireplace.


Feb 21, 2018 3:00 PM ET

Better Buildings by Design presentations are available!
by jeffbuell

I wanted to let you know that presentations are available online. Go to:
Click on the button that says "View the Interactive Conference Schedule"
After each session description, there is a header that says “Presentation(s):” and then a button to download the talk. Thanks!

Feb 22, 2018 12:34 PM ET

Edited Feb 22, 2018 11:02 PM ET.

by Malcolm Taylor

Clay initially indicates his biggest concern is heat, then the discussion moves into how to provide enough electricity to do that and run other things - which proves to be a big ask. If you remove heat from the problem, everything becomes a lot easier.

I'd install a propane space heater as back-up. With that, a Honda 2000 portable generator would run enough lights, refrigeration, TVs and computers, and a couple of induction hobs to cook on - all pretty inexpensively.

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