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 heater. 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 envelope.”
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
GBA 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 inverter, 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 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 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 would have cost almost $14,000, while a 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 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 . 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 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 , and another is called .
Kevin Spellman has another suggestion: a , 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 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 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