You Don’t Need a Tesla Powerwall Battery, Unless…

It doesn’t make sense to invest in a Tesla Powerwall battery for you home unless you have these specific needs.


What is a Tesla Powerwall?
Tesla, the company that makes electric cars, also makes batteries for use in houses. The Tesla Powerwall is a 6.4 kWh lithium ion battery that’s designed for household use. It’s intended to be installed on a wall in your home and tied into your existing electrical system to it can provide power for your devices and appliances.

The battery can be ganged with other Tesla Powerwall batteries for increased capacity. The physical dimensions of one battery are  33.9″ wide, 51.2″ tall and 7.1″ in depth. It weighs 220lbs. Tesla “launched” (announced) the product on April 30th, 2015 and is now (early 2016) taking reservations for the product.

If you’d like to know what kWh means.

Why Would You NOT Need a Tesla Powerwall?
At $3,000, the Tesla Powerwall is affordable for people who love “green” gadgets – anything that appears to be sustainable. That $3,000 is just the purchase price. It doesn’t include other components or installation.

Despite it’s apparently low cost, there are some considerations for anyone thinking about buying one. Here are four reasons NOT to buy a Tesla Powerwall (or any other battery for your home)…

1. It’s not base load power; it’s a bridge. A battery for your home my be able to meet some electrical loads like lighting and plug loads for gadgets: televisions, computers, modems, routers, phones, etc. You’ll have to adjust your behavior to avoid draining the battery with normal household chores like cooking and laundry.

The Telsa Powerwall is a 6.5kWh battery. An average clothes washer uses 2.3 kWh for each load. An average clothes dryer uses 3.3 kWh for each load. An oven uses about 2 kWh for one hour of cook time at 350 degrees. So, you’d probably need more than one Telsa Powerwall battery for your home if you’re busy or have a big house and|or family.

The catch is that none of these loads includes the power demands for heating and cooling systems. The Tesla Powerwall is probably not going to get you through a cold winter night or a hot summer evening after sunset. You’ll need more capacity for that.

For example, a central air conditioner may consume 3,000 watts of power every hour. If you’re running it for nine hours a day in the hot months, that’s about 31 kWh per day of power consumption. You’d need an extremely efficient house to reduce the heating and cooling loads enough to get by on battery power when your PV system isn’t producing power to meet your demands.

2. The numbers don’t work for your home. You can gang more than one Telsa Powerwall battery together to increase the power capacity to meet the electrical loads of your home, if needed, though that probably wouldn’t be cost effective compared to just buying power from your local utility. The Telsa Powerwall comes with a 10-year warranty. Even if you could get by with one battery, it would need to pay back your purchase and installation costs within its warranty period to be worth the investment.

3. You don’t have PV panels on your roof. PV stands for photovoltaic – what many people call “solar panels”. PV panels collect energy from the sun in the form of DC (direct current) power though utilities provide power in the form of AC (alternating current) power. Houses are wired for AC power. To transition from DC to AC power, you need an inverter.

The literature for the Telsa Powerwall describes its typical installation as being paired with PV panels. The Telsa Powerwall is a DC battery so it can be charged by the PV panels without the need for an inverter. You could charge your car if you have a DC-powered car like a Telsa Model D or a Nissan Leaf though you’d need to convert that power to AC to run much of anything in your home.

The $3,000 price for the Tesla Powerwall doesn’t include the inverter or the installation so the actual “installed cost” is higher. The inverter and installation will likely cost at least a couple thousand more dollars. The addition of the inverter brings the cost of the Tesla Powerwall much closer to competing home battery systems (that come with an inverter) from companies like Sonnen.

4. You’ve not invested in passive systems. The RTA blog often addresses the importance of passive systems like air sealing and insulation. They’re not glamorous though they can significantly improve health, comfort and efficiency of homes. If you have a leaky, poorly-insulated home it makes no sense to buy expensive battery technology. You should consider commissioning a HERS rating to figure out what needs to be addressed, set your priorities and invest your money into passive systems that have no operating or maintenance costs.

Why Would You Want a Tesla Powerwall?
There are multiple reasons to consider a Telsa Powerwall (or a similar product). Here are three:

1. As Tesla suggests, if you already have a PV system with an inverter, you should be able to install a battery to store excess energy from your PV system. The battery should also be able to bridge drops in production due to rain, cloud cover, etc. If you already have a PV system and inverter, the installation costs of the battery should be minimal because you’ve already got the system wired. You’ll just have to find some place to install the battery or batteries.

This could be a significant issue because of the price difference between power you purchase and the power you sell to your power company. If you have a PV system and it produces more power than you need in your home, you may be able to sell your excess power to the power company. The rate at which the power company buys power from you is often significantly less than the rate at which they sell power to you.

If you can store your excess power, you won’t have to buy as much at a higher rate from your power company. That can help you pay off your investment in PV and battery technology more quickly so it should be carefully considered when designing the house and its systems.

2. If you have issues with the stability of your power supply, a battery may be a good investment for you. Living in an area with frequent storms may cause disruptions in your power supply that are quickly repaired. Living in an area where the population growth has outpaced the local utilities’ construction of power production can also result in blackouts and rolling brownouts where demand on the system exceeds its capacity. You may also live in an area where the power company struggles to maintain its infrastructure so frequent failures of aging equipment like transformers cause small and temporary power outages.

Any of these reasons could cause power outages that disrupt the lives of all the people affected. It’s hard to put a cost on these disruptions so the decision to purchase a battery for your home to get you through frequent yet short power failures will be more of an emotional decision. (If you have a home office, you can check with your accountant to see if the purchase and installation of a battery, even if it’s just charged with power from your utility, is tax deductible.)

3. You can do your own peak shaving. RTA has previously published a blog about how utility companies are using (big banks of) batteries to shave peak demand by storing excess power to be used during periods of peak demand. If your power costs are high enough, it may make sense for you to invest in a home battery. You’ll have to run the numbers to see what saving you can achieve with a battery.

Don’t forget it’s important to look at a year’s worth of anticipated use because your power consumption and the rates you pay change throughout the year. For example, your rates in the summer are probably higher so that needs to be factored into the equation.

The Future is Now, or Slightly Later
It’s important to understand your needs and the available solutions before you make a decision. This blog post encourages you to study your needs to determine if the Tesla Powerwall is the right solution for you though there are many other solutions in the market. For example, a bank of iron-phosphate batteries may be a better solution for your needs than one or more lithium ion batteries from Tesla.

Designing and constructing to meet building codes means you’re working to meet the minimum acceptable standard. The insulation values required by the building code probably aren’t going to yield a house anywhere near efficient enough to reduce your heating and cooling loads to the point you could operate mostly from renewable power that you collect and store on your property.

Improvements in design tools, technologies (like batteries + PV panels) and verification of construction practices means it’s possible to effectively integrate energy storage into home design now. You’ll need to make a more significant initial investment in design and construction costs up front though you could realize a great return on your investment compared to just meeting the building codes if you’re careful about evaluating the design solutions and managing your project budget.

That’s the premise behind the Zero Energy Ready Home (ZERH) program by the U.S. Department of Energy. It encourages participants to invest in critical passive systems like air sealing and insulation that can’t be easily changed later. You prepare to add renewable energy sources like PV panels and energy storage when you’re ready. If you’ve studied your options, you should know the purchase, installation and operating costs needed to make the numbers work so it’s a matter of waiting for the market to reach your thresholds.

In Conclusion
For years, energy storage has been regarded as the next big thing. It has the potential to give each of us energy independence. An efficiently-designed house with on-site power generation (a PV system on the roof) and energy storage (a battery or batteries) could mean you’d pay upfront and maintenance costs for your own system to cut your power bills to almost nothing. You’d only need power from your utility to get you through maintenance outages and the worst weather.

It will be interesting (to say the least) to see how the evolution in this area impacts how we layout neighborhoods; how we orient our homes on their lots; and how we design our homes. The combination of increased efficiency in our gadgets + appliances and the ability to store energy should be disruptive in a great way.

As always, if you’ve got a comment or question, please share it below. Thanks!

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