EEBA Newsletter

The views expressed in these articles are solely those of the author and do not reflect the views of EEBA staff, officers, or board members. EEBA welcomes guest articles from qualified authors, and we offer these articles as a service to the high-performance housing industry as a way to encourage discussion and collaboration between industry professionals on relevant issues.

Posts From July, 2018

Home Battery Q&A

Answering the most common builder questions
Home Battery Q&A

You've decided you want battery storage. How do you go about choosing the right one for your customers? Battery makers will help you through the process, but you're more likely to make the right choice if you go into those discussions with some basic knowledge.

According to Adam Weinstein, a regional sales manager with battery-maker Sonnen, builders tend to ask a lot of the same questions when deciding what battery type and system to choose for their homes. We asked him what those questions are and how he generally answers them.

 

Q: What main types of home batteries are on the market today?

A: The two most common types for home storage are Nickel Magnesium Cobalt (NMC) and Lithium Iron Phosphate (LFP).

NMC batteries are made for rapid charge and discharge, and also pack a lot of power into a small, mobile package. Those qualities have made them popular with some electric vehicle manufacturers. However, news stories about thermal runaway, or explosions, in electric vehicles have made some homeowners reluctant to install them.

LFP batteries weigh more and cost more, so they're unsuitable for electric vehicles. However, this type of battery has a longer lifespan and is designed specifically for stationary applications with no chance of thermal runaway, making the chemistry inherently safer and ideal for home installation.

 

Q: How long do home batteries last?

A: Manufacturers' warranties are a good way to compare the service life of two batteries but the actual lifespan will depend on battery chemistry and how often it gets cycled (from completely charged to completely discharged and back). Manufacturers of LFP batteries claim their batteries to last for 10,000 to 15,000 cycles, or over 30 years on a home that charges and discharges it every day. An NMC battery, by contrast, might be limited to a few thousand cycles, or 5-10 years on the same home.

 

Q: How do I size the battery?

A: It depends on a few key factors: the size of the solar array being integrated into the home, the size of the loads that the homeowners intend to back up with the battery in the event of an outage and the number of daily loads needing power.

Here are three sample battery sizes along with the loads they will power in backup mode for a typical home.

Battery size: 4kW/6kWh

  • Refrigerator
  • Microwave
  • Three lights
  • One computer
  • One tablet or phone
  • Three extra outlets

 

Battery size:7kW/10kWh

  • Refrigerator
  • Microwave
  • Three lights
  • One computer
  • One tablet or phone
  • One television
  • Three extra outlets
  • One single-room air conditioning unit

 

Battery size:8kW/16kWh

  • Refrigerator
  • Microwave
  • Three lights
  • One computer
  • One tablet or phone
  • One television
  • Three extra outlets
  • One single-room air conditioning unit
  • Two fans
  • One Washer/Dryer combination

 

Q: Where can my battery be installed?

A: The biggest consideration is temperature and moisture. Most batteries will deliver full power in a temperature range of 40F to 120F, so if you build in an area that gets hot in summer or cold in winter you don't want to put them in an unheated room or in direct sunlight. Most people install them in a semi-conditioned basement or garage.

Batteries are available with outdoor-rated enclosures that protect them from the elements, but an outdoor rating is not a failsafe option.

Some batteries, specifically LFP, are safe enough to install in a conditioned living space with the wiring totally hidden. In fact, many of the LFP batteries Sonnen has installed in German homes are in living spaces as they're considered a status symbol.

 

Q: Do I have to wire the house differently?

A: If you intend to use the battery for backup power, the only difference is that the electrician will need to wire a sub-panel, called a protected loads panel, with the circuits you want to power in the event of an outage.

 

Q: What is the difference between an AC-coupled and a DC-coupled system?

A:  An AC-Coupled system has two inverters: one for the solar panels and one for the battery. All power flowing to and from the battery—whether from the grid or the solar panels—flows through the battery inverter.

AC-Coupled is the easiest way to add a battery to an existing PV system and also provides redundancy—if one of the inverters fails, you don’t lose both your solar and storage. Also, the AC-coupling allows homeowners to charge their batteries from multiple power sources: solar, the grid and even gas generators.

In a DC-coupled system, the battery is charged directly from the panels. The panels and the battery also share a single "hybrid" inverter. Because this system type requires one inverter, equipment costs are lower, making it popular for new construction specifically with new solar installations.

With a DC-Coupled system, the batteries will only be charged by the solar array.

 

Healthy Growth for Solar Battery Storage

While the West Coast still accounts for most installations, the technology is taking hold in all parts of the country.
Healthy Growth for Solar Battery Storage

It used to be that a typical grid-tied solar electric system only worked when the grid worked.  If a storm took out the power lines the homeowners ended up in the dark, despite those shiny panels on the roof.

That's changing thanks to more efficient and cost-effective battery technology, as well as to utility involvement.  Greentech Media Research (GTM) estimates that for the first time, total installed home battery capacity of 15.9 megawatts has reached near parity with utility-scale deployments of 16 megawatts.

It's on track to get even bigger. "The residential [storage] market this year is going to be over five times the size of the market last year," said GTM Senior Analyst Brett Simon. And while California and Hawaii account for 74 percent of that capacity, installations are also growing elsewhere, with products from companies that include LG, Panasonic, Samsung, Sonnen, and Tesla.

Distributed Grid

Electric utilities are helping to drive this growth.  For instance, Walpole, NH-based Unity Homes recently built a home in Guilford, Vermont with a 10-kW solar system and a Tesla Powerwall 2.0 battery supplied by Green Mountain Power (GMP) at an installed cost of $1,500.  GMP Communications Director Kristin Kelly says that the batteries usually cost $7 to $8,000 installed, but that the utility wants to create a distributed storage grid it can draw from during peak demand times.  It began installing batteries in late 2017 and will have 2000 in place by the end of 2018.

Of course, most homeowners expect tangible benefits from that $1,500, so it's no surprise that most batteries are going into rural homes where winter storms often cause outages.  The batteries provide an estimated 8-12 hours of backup power, and the subsidized cost makes them cheaper than a whole-house generator.

Outage protection certainly drives demand for storage where there's severe weather, such as winter storms in New England or hurricanes in Florida and the Gulf states.  But without a utility incentive, they can still be a hard sell.  For instance, Sean Buckley, Owner of Harvest Sun Solar, a solar installer in Tisbury, Mass. says that with no subsidy only 10% of his customers have been willing to pay full price for a backup battery.

If customers in Buckley's market didn't pay a fixed electric rate, he might get more interest.  According to Energy Sage, an online solar energy marketplace, natural markets for storage are where the utility’s rate policy includes any of the following: time of use rates, demand charges, the lack of net metering, or net metering reimbursement based on avoided rather than retail costs.

Staying Under Threshold

Some utilities charge customers a higher rate if they exceed an energy use threshold.  Others have net metering arrangements that don't reimburse homeowners at the retail rate but rather at avoided cost—what the utility would pay to buy or generate that power.

Both apply in the Milwaukee area, where Tim O'Brien Homes recently broke ground on Wisconsin's first Zero Energy neighborhood.  Homes at Red Fox Crossing in New Berlin will be certified under the Department of Energy’s Zero Energy Ready Home program as 40% to 50% more efficient than a typical new home.  They will also include rooftop solar panels.

While these homes don't include battery storage, Craig North, the company's VP of Product Innovation and EEBA board member, says the company plans to include it in future projects.  "Our utility providers have put a tariff on over-production of solar electricity," he says.  "Homeowners also only get 25 cents on the dollar for excess power put back into the grid."

Avoiding Peak Rates

In time-of-use areas, the system can be configured to avoid buying power when it's most expensive.  That describes much of California.  "The differential between Summer on-peak and off-peak in San Diego is as high as 32 cents per kWh," says Michelle Mapel, Senior Director of Sales and Marketing at battery-maker Sonnen.  "If you can use the battery for the four hours of peak, you will definitely save money."

That potential savings led Bakersfield, California builder Dave Packer to complete a Net Zero Electric model home with solar panels and a Powerwall 2 battery system.  When Pacific Gas & Electric's rates are low in midday, excess power from the panels flows into the battery until it's fully charged.  The home can draw from this stored power in late afternoon and early evening when rates triple.  The battery also provides backup power but Packer says that outages in his area are rare and short-lived.

PG&E, like Green Mountain Power, offers batteries at a subsidized rate of $1,500, which Packer believes will motivate more homeowners to buy.

Even with time-of-use rates, a lot of homeowners aren't going to pay $7,000 or more for a battery option.  Gene Myers, CEO of Thrive Home Builders in Denver, and EEBA Board President, has a solution.  Rather than a battery option, put them in all homes and build the cost into the selling price.

Thrive will certify all of the 240 homes it completes this year under the Department of Energy's Zero Energy Ready Home program.  All homes include rooftop solar panels and a storage battery on a wall in the garage.

The battery helps mitigate time of use rates and provides backup power.  One battery can handle basic lighting, the refrigerator and freezer, as well as enough outlets to charge electronic devices.  It won't run the heat pump, but Thrive's homes all have backup furnaces.

So far there hasn't been any customer pushback.  "Having a Zero Energy product is part of our brand," says Myers.  However, he also admits that given his homes' price point of $500,000 and up, the extra cost isn't the big deal it would be in a lower-priced home.

Thrive Homes in Denver is one of a growing number of builders who are outfitting homes with solar panels and storage batteries.  Shown here: the ZEN model.

Community Power

Thrive isn't the only company that has decided to make batteries standard.  Sonnen is working with Mandalay Homes in a 2,900-home development in Prescott Valley, Arizona.  Every house will have an individual solar system and a battery that together add about one percent to the home cost but save the homeowners 20 to 30 percent on electricity, thanks to a utility policy that gives a lower rate to customers who can completely power their homes for a portion of the day.

Mapel says that Sonnen is negotiating with the utility to configure storage in other communities in a way that meets the utilities' load-shifting and other needs.

In Birmingham, Ala., the utility has actually taken the lead on a community-wide storage effort.  Alabama Power is working with Signature Homes on a 62-home "Smart Neighborhood."  It will include enough solar capacity to run all the homes, which will have HERS scores of around 45, or 35 percent more efficient than standard new Alabama homes.  It will also have Samsung lithium-ion battery storage.

What makes the Smart Neighborhood unique is that rather than being installed on individual homes, the panels and the storage will be centralized in a 2-1/2 acre site, creating what the utility calls a microgrid.  Customers won't be able to switch to battery power during peak demand times but have agreed to let the utility adjust their loads to reduce total electric use via smart thermostats and other controls.

Jim Leverette, a Research Engineer with Southern Company, Alabama Power's parent company, says that the utility envisions battery-enabled microgrids as part of homebuilding's future, and is using the neighborhood as an R&D project.  "We want to evaluate the pros and cons and to get a better understanding of technical challenges."

 

Batteries such as those from Sonnen help mitigate peak demand charges and act as a backup power supply. They also look good.