Last week, Tesla Motors took a step into being more than just a car company by announcing their residential Powerwall Battery. And it's been met by both fans and skeptics. Our goal here isn't to decide whether this is a game changer or a dud, but to look briefly at how the battery works as a backup power source and how this relates to the transition from incandescent to LED lighting.
The Tesla Powerwall and Solar Panels
We'll point out first that a primary market for this battery seems to be those with solar panels, or solar systems, installed on their homes. In this case, the battery isn't so much an emergency backup battery for the house (though potentially also a backup). Instead, it's a way to store solar energy during the day so it can continue being used at night, extending the value of solar power. We'll leave the details of this, however, to solar experts.
The Tesla Powerwall as a Rate Reducer
Another suggested use for the Powerwall is for those with different energy costs at different times of the day. Residential users of DTE Energy at the time of this writing, for instance, can stick with a standard rate system or -- if they feel they can push most of their energy use to the hours of 7 p.m. to 11 a.m. -- they can choose to pay on-peak and off-peak rates.
In broad brush strokes, if someone chose to do that and somehow pushed all of their energy use to those hours, they could save approximately 2.3 cents per kWh when compared to the traditional rates, saving approximately $20/month in the average home (using just over 900 kWh per month).
Most people could never push all their energy use to between the hours of 7 p.m. and 11 a.m., but with the Powerwall, you could store up 10 kWh during off-peak hours and use them on peak at the lower rate. So if you could limit your daytime use of energy (when many people are at work anyway) to 1/3 of your overall use (assuming 30 kWh per day), then you could fully live on "off-peak" energy rates.
(We should mention that these numbers are really for painting a general picture, and don't take into account things like lost efficiencies of storing and then using the off-peak power.)
Of course this $20 monthly savings comes at the cost of a battery that's roughly $7000 installed. So the Powerwall would have to offer more value than these savings alone. And that's where it comes in as a backup power source.
The Tesla Powerwall as a Backup Battery
The final selling point of the Powerwall is as a backup battery for the entire home. If the power grid goes down, you have up to 10 kWh of energy stored up that you can keep using. Let's get one thing out of the way quickly: central air would eat that up in about 3 hours of use. And an electric heater? Forget about it. But if you've got a gas furnace or shut off your AC when the grid goes down, you could potentially run your house as normal for 6-8 hours. And if you adapted to the need, reducing energy consumption as much as possible, you could extend that substantially. And this is where efficiency in the home -- including lighting -- becomes especially important.
We've already said that air conditioning would have to be turned off. After AC, refrigerators and lighting are the two biggest energy users in the typical home. Based on our research, we've found numbers ranging from 2 to 6 kWh of energy used each day by refrigerators, suspecting that 2-4 is accurate in most cases.
Meanwhile, according to this excellent resource, the typical American household runs about 60 lamps [bulbs] at 1.6 hours a day and at an average wattage of 47.7. (This depends on the time of year, location in the country, and more.) At these numbers, a household's lighting use draws 4.6 kWh per day. Considering a 10 kWh Tesla Powerwall as backup … ouch.
But many of our homes have not yet converted to LED. If we expect that 47.7 average watts in a home includes mostly 60 watt incandescent bulbs along with some at 25 and 40 watts, then a transition to LED lighting could reduce the average wattage from 47.7 to about 8. This then reduces a home's lighting energy use to about .8 kWh per day.
(Likewise, this speaks to the value of more efficient refrigerators and other appliances in the home.)
If you were to limit household energy use during a power outage to refrigeration, some lighting, a couple computers, and a little cooking, you might last a single day on a 10 kWh Powerwall with incandescent lighting; you might last two days if you had switched to LED lighting.
Even without a Powerwall, switching to LED lighting in the example above would save many homes $15 or more per month. Given the Powerwall investment of $7000 to save $20/month, this is a much easier investment -- at Lighting Supply prices, you could replace the 60 bulbs in an average home for $400 to $600 while getting name brand bulbs. (We recommend trusted brands for consistent colors and better quality when it comes to LED.) Those bulbs would pay for themselves in just a couple years … something Powerwall can't really say for itself.
The Skeptic's Look at Powerwall as a Backup Battery
A final thought on the Tesla Powerwall as a backup battery. First, this function is complicated by the other two functions described above. If you're using the Powerwall to carry daytime (solar) energy into the night, or off-peak energy rates into on-peak hours, then you may already be using some or even much of the stored power. So if the grid power goes out at the wrong time, you could have little left to keep your home running. Because of this, if you're using the Powerwall mainly as a backup power source, it may not make sense to also use it for its other purposes.
Some skeptics of the Powerwall have also pointed out that, for $7000, you could get a pretty nice generator powered by natural gas to keep your entire home running regardless of how long the power was out. We don't know what drawbacks that may have, or the costs of maintenance, but this is another point to consider.
So these are our thoughts on the new Tesla Powerwall and one more reason why it's such a good idea to switch to LED lighting. Do you have more information on the Powerwall that should be considered here? Are you for it, against it, or just don't care? We'd love to hear your thoughts on any of what was discussed here.