The Tesla Battery (with Units!)

Friends and clients have been asking me questions about the Tesla Powerwall battery that has been getting so much press coverage lately. Can I power my house on one? Will it solve the renewable energy intermittency problem? Is it the game changer, the utility paradigm destroying techno-juggernaut that will usher in the renewable energy era?

No.

It is a good thing, though. What the Tesla folks have done is to package a Lithium Iron Phosphate (LiFPo) battery with a charger, battery management system, and cooling system in a tidy package. They are selling it wholesale at about $350 per kilowatt-hour, which is moderately cheaper than other LiFPo batteries on the market.

Let me stop right there with the term “kilowatt-hour” for a moment. It has been alternately amusing and profoundly frustrating to read news articles that interchangeably use “kilowatts,” “kilowatt-hours,” and the meaningless terms “kilowatts per day” and “watts-per-hour.” A brief tutorial follows. Energy professionals may go get a sandwich; I’ll be done in a few minutes.

Ok, we have AC, DC, amps, volts, watts, kilowatts, and kilowatt-hours (which is abbreviated kWh or kWhr). First, the garden hose analogy:

You’ve got a garden hose with a stream of water flowing out of it, filling a bucket. One factor is the rate of flow, or gallons per minute going through the hose. If you want to get right down to the miniature scale, it’s the number of water molecules per second that are passing a point in the hose. Another factor is the pressure pushing the water. The higher the pressure the more flow you get through a given hose. Then there is the bucket, filled with some volume of water.

Amps are like the flow. An amp is defined as some huge number of electrons passing a point in a wire in one second. Volts are like the pressure. Voltage is the force pushing the electrons through a wire. More voltage means more electrons per second in a given wire. One watt is one amp of flow being pushed by one volt of electrical pressure. Ten watts could be one amp pushed by ten volts or ten amps pushed by one volt, or any combination of amps and volts that multiply to equal ten. A kilowatt is one thousand watts (10 amps x 100 volts or 100 amps x 10 volts, or 20 amps at 50 volts, etc.). If you keep that thousand watts flowing for one hour you have delivered one kilowatt-hour. That’s like the water stored in the bucket, or the energy stored in the battery. Amps, volts, and watts are instantaneous measurements, units of power. Kilowatt-hours are created, delivered, stored, or used over time. They are units of energy.

For a more practical example, let’s say you have a toaster. You turn it over and look at the label, which says “115 Volts, 8.7 Amps.” By happy coincidence, these two numbers multiply to (almost exactly) 1,000 watts, or one kilowatt. Many of your closest friends come over for breakfast and they want toast. You keep the toaster going constantly for one hour. You have used one kilowatt-hour.

That toaster, however, uses AC (alternating current) electricity. Batteries and solar panels produce DC power, not AC. DC (direct current) electricity is like that water in the hose. All the electrons in the wire are continuously flowing in one direction, like those water molecules. With AC electricity those electrons are changing direction 60 times a second, back and forth. That’s because the voltage (electrical pressure) is changing direction 60 times per second. Remember that hum that came out of old fluorescent tube lights? That’s it.

To make AC out of DC you need a box full of electronics called an inverter. I’ll skip over the details here and just say “DC in, AC out.” Some inverters get their DC electricity from batteries, generally at 12, 24, or 48 volts. Others get their DC electricity directly from solar panels, generally at 250 to 500 volts.

Here is the first problem with the Tesla battery. It puts out DC electricity at 350 to 450 volts. There are only two inverter manufacturers, Fronius and Solar Edge, that have inverters that will take high voltage DC from batteries, and Solar Edge hasn’t released theirs on the market yet. That’s a serious limitation; the immature market for high voltage input battery-based inverters.

The second problem is capacity. There are two Tesla battery models, one with 7.5 kWh and one with 10 kWh. Your average American home uses over 20 kWh per day, so a single Tesla battery isn’t going to offer many hours of backup. They can be plugged together to make a larger battery bank, but the “power your home for $3,500” hype is just that.

The third problem is instantaneous power. A Tesla battery maxes out at 2,000 watts. That’s roughly equal to what you can get out of one standard wall outlet. If an air conditioner and a refrigerator come on at the same time the battery will shut itself down. Again, you can parallel multiple batteries, but you have to parallel multiple checkbooks to pay for this.

A more techie problem is that battery based inverters stay under 50 volts DC for a reason. The National Electrical Code gets a lot more stringent about wire and conduit, fuse protection and enclosures when battery banks go over 50 volts. The added safeguards add expense.

On the subject of expense, after paying $3,500 for a Tesla battery you will have to pay that again for an inverter, electrical hardware and an electrician to make it work. That is, if you can find anyone willing and trained to work with a 350 volt battery bank.

My verdict is: A missed opportunity. The battery needs more integration into a product that delivers AC power. Tesla could have partnered with Fronius or Solar Edge to build the inverter and AC interface into that sleek box. Then any electrician could easily hang the unit on the wall and connect it into a household electrical system. With all the capital and engineering prowess at its disposal I am mystified as to why Tesla didn’t.

I guess the energy revolution has been delayed.

(Elon Musk, if you are reading this, I am willing to consult on product design for any old floor model electric roadster you happen to have sitting around.)