Immediately after Tesla released Tesla Powerwall, the Internet was flooded with lots of articles discussing the real pricing of the new Tesla product. You do not have to be a genius to notice that Powerwall is not economically efficient. Even the economics behind lead acid batteries based systems is debatable, although these batteries cost three times less than the Li-Ion batteries used by Tesla. Sure, lead acid batteries are bulkier, however this does not matter much in a house. They might not resist to so many discharge cycles like Li-Ion, but that’s just theoretical. At the same time, lead acid batteries have some advantages too. For example, they can easily sustain a high discharge current, which makes them appropriate for fast charging jobs (like charging your car really fast).

I do not want to get into the economics of Tesla, but as some articles noted, the Tesla system does not include the inverter. The inverter is the device that can convert from DC (stored in batteries) to AC (used by household equipment). Just because home equipment like TV, laptop, refrigerator etc. must use AC current. This device is expensive, for example a modest inverter able to drive 3500W (basically the peak power of the 10kW Tesla Powerwall) costs around US $1,000.

Why do we need this inverter?

Simply because batteries store DC and we need AC in our homes. Well, that’s not quite true anymore, in the 21th century most devices in our home actually require DC. Yes, they plug in to AC, but they incorporate transformers that reduce voltage and convert AC to DC. All electronics use DC and even lighting is more efficient with DC. AC is still used to drive motors; for example, motors in refrigerators use AC. But this does not mean that DC motors cannot do the job.

And there is huge penalty in converting from AC to DC and reducing voltage (remember your charger getting hot?). Every time a conversion takes place, for example in your nice flat TV, about 20% of the energy is lost. And this happens in every single case. So basically, the Tesla system looks like this:

• Tesla Powerwall supplies DC power.
• Inverter converts it to AC. In this phase, typically more than 10% is lost. While manufacturers claim 90% efficiency on this step, this happens on peak power. When the load on the system is around 10-20%, a whole lot more gets wasted.
• Power gets from the plug to your TV and another conversion takes place – again 20% of the energy is lost.

This does not look right – 30% of the energy is lost. And what happens when you charge Powerwall from the grid AC power, which is of course necessary if you do not have solar panels? Again, 10% of the energy is lost. So basically, even when you use the refrigerator (which does not require an AC to DC conversion), about 20% of the energy is lost in Tesla Powerwall recharging cycle.

This is not something new, it’s actually well known by anyone working with UPS systems – a lot of energy is wasted, especially when the UPS is not fully used. Because all systems are more efficient close to their peak power rather than on 10% load.

Is this Tesla’s fault?

Of course it’s not. Tesla had to adapt to our household environment, but with all the discussions around green energy and stuff, it can make a decisive move by being a strong advocate for DC power. The debate regarding DC/AC is very old. DC was invented by Edison and AC by Tesla, as a more efficient solution to power transmission over long distances. In the 19th century, it was very hard to raise the voltage of DC in order to make it transportable over long distances, but these arguments are quite different with today’s technology. Because physically it is more efficient to transport DC.

And since the equipment in our homes is mostly DC, there might be multiple benefits from switching to DC even if we do not use battery buffers. This is a simplistic approach without considering the entire grid system that starts with the very way power is produced. There are many devices that deal with power conversion in the grid until the current reaches the end-user device and each one is losing energy in the process.

Someone could say that we should find a more efficient process to convert between AC/DC. In fact, the industry has made huge progress in the past 20 years, but the most efficient transformers are expensive due to their complexity. Not even your high-end TV will use the most efficient transformer available, as no one will prefer a TV just for this reason.

What can we do?

While there is no perfect solution, there are a few measures otherwise not very difficult that could be implemented.

Standardize DC voltage

This should be done for all electronics. EU started such an initiative with mobile phones, but unfortunately poorly documented, as they cared more about the connector and ignored the 5V voltage, which is too low to drive any serious power (even to charge phones fast, but that’s another discussion).

And this standardization must require that all devices have an external and optional (paid extra) power supply. So basically, if you still have AC, you must buy the power supply. Otherwise, you can just plug in the device to the DC plug. This might ring a bell to economically justify at least some DC circuits in any home.

Purists note – I know that there is no such thing as universal DC voltage. Some voltage drop/boost will still be necessary with some devices and this is still not 100% efficient.

Make lighting circuits DC

Moving lighting to decent voltage DC is very easy as the current infrastructure can sustain the amperage. For example, 48V circuits can drive very serious LEDs even right now and technology is rapidly improving.

I think this could be the most important measure because it can also streamline the path to home automation with important cost savings.

 
Lots of interesting stuff can be built with LEDs – in the pictures above, you can see some images of one of our playground projects. Because LEDs and the Cloud have a lot in common 🙂

It’s time for lobby

As a matter of fact, DC power has been tried in Data Centers and had good results. It’s not a new idea and I take no credit for it.

Tesla has a “green” problem with Powerwall. When used in the grid, out of the box 20% of the stored energy is lost as heat. When charged by solar panels it looks OK, but it could be much better in homes with DC circuits – even charging electric cars will be more efficient. It’s time for some serious market education and political lobby.

UPDATE: As this article generated a lot of comments and because some require an extensive response, I have posted a follow-up article available here.