About 6 months ago I installed a solar power system with 2 Tesla Powerwalls for backup. Today, we got to test out the system with a power outage from an ice storm in the southeast. (We are still on battery power as I write this.)

My kids came downstairs to tell me that it was time for bed (9PM), but when I looked at my phone, it was only 8:17PM. They were reading the time off the microwave. I checked the alarm clock in my room and it also read 9PM

I know that the way power is "pushed" around between solar, utility, and the Powerwall is by modifying the frequency of the ac signal. I also know that many digital clocks keep time via utility frequency.

So, my question is this: if our power went out at roughly 10AM, and by 9PM our digital clocks were off by almost 45 minutes, what frequency is the powerwall delivering backup power at? That large of a shift in time seems excessive (purely speculative).

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    \$\begingroup\$ If the powerwall has line voltage outputs they should be set to line frequency. What does the powerwall specification/nameplate say? Are you 100% certain that the clocks didn't simply reset during the momentary dropout before the UPS/PTS kicked in? \$\endgroup\$
    – K H
    Jan 14, 2019 at 2:01
  • \$\begingroup\$ It maybe skipped pulses from switching between power sources. They all may have accurate 60 HZ +/- 0.1 % timing, but it can take several milliseconds to switch power sources. Most manufactures guarantee to switch in 10 mS or less, but even that is a 1 count error. That and built in errors on some equipment can easily add up to 45 minutes in 11 hours. \$\endgroup\$
    – user105652
    Jan 14, 2019 at 2:02
  • \$\begingroup\$ "I know that the way power is "pushed" around between solar, utility, and the Powerwall is by modifying the frequency of the ac signal." No. It's by modifying the voltage and phase. If you modified the frequency, you'd be going into and out of phase slowly! \$\endgroup\$ Jan 14, 2019 at 4:59

3 Answers 3


Units such as APC and Tripp-Lite UPS detect 60+/-3 Hz as a power good and either reduce AC load outside this frequency or switch off AC charge to rely on battery backup. They also use voltage thresholds.

However, these UPS units have a relatively short backup time compared to the PowerWall2 (PW2). There are UPS's with +/-6Hz tolerance for input power detection such as the Minuteman.

Therefore to reduce non-essential loads that have short-term backup such as UPS, the PW2 runs > 65 HZ intentionally (or > 7.7% fast).

Your clocks were reading (21:00-10:00) * 60min/h = 660 minutes when you were expecting (20:17-10:00) * 60min/h = 617 min or 43 minutes fast = 43/617*100% = 7.0% fast which was the actual increase on the PW2 or 64.2 Hz.

The PW2 was 10% within the 65 Hz that I expected, but I do not have their actual specifications.

I am aware of this but cannot prove it.

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    \$\begingroup\$ So what you are saying is that the Power Wall 2 intentionally "activates" UPSes by running at a higher frequency? What’s the purpose of that? Wouldn’t it just drain the UPS pretty quickly, making the equipment connected to it shutdown? \$\endgroup\$
    – Michael
    Jan 14, 2019 at 9:26
  • \$\begingroup\$ @Michael: Indeed, and that (controlled) shutdown is intentional. UPS'es have two goals: to bridge short outages, and to cleanly shut down the connected computer(s) in case of longer outages. The PW2 isn't designed as a UPS and cannot shut down computers directly, but in this way it can delegate that function. \$\endgroup\$
    – MSalters
    Jan 14, 2019 at 10:03
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    \$\begingroup\$ This is almost right! \$\endgroup\$ Jan 14, 2019 at 10:03
  • \$\begingroup\$ But what if you want to supply your computers with the PW2? \$\endgroup\$
    – Michael
    Jan 14, 2019 at 10:15
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    \$\begingroup\$ If you have a PW2 then you have access to the specifications and software updates which affected these results. Some users I noted in the PW2 forums are aware of this change and elected to switch brands, It is all software controlled so I suspect there are options on all units to change this feature. But how I cannot say for sure. If you wanted to operate your computers on a PW2 then you only need a UPS for the transient shutdown then you can turn-off the UPS and draw off the PW2. \$\endgroup\$ Jan 14, 2019 at 17:21

The Powerwall 2 intentionally runs at between 64Hz and 65Hz when it is supplying power if its battery is full. This has the side-effect of making clocks run faster. It also causes UPSes to run on battery.

Now, the first question you might be asking is how it could be the case that the Powerwall is supplying power and its battery is full. Simple. The house has solar power and the solar power exceeds what the house is drawing. This will mean the Powerwall's battery will charge and, if this condition continues, eventually get full. Since the utility power is off, extra power can't be sold.

Now, you can quickly imagine a problem. The Powerwall's battery is full. The house isn't drawing as much power as the solar panels are creating. Somehow, the Powerwall has got to stop the grid-tie solar system from trying to supply it with power. It does this by bringing the frequency out of specification for the solar panel inverters. Typically, it takes a 64Hz to 65Hz frequency to do this.

So, essentially, this is how the Powerwall shuts the solar panels down when its battery is full and it cannot use all the power the solar system is trying to supply because it cannot sell it to the grid.

  • \$\begingroup\$ I would think the PW2 operates at 60Hz when the battery is full and solar charger is charging , but then sheds UPS loads by running up to +5 Hz when not charging. This may account why the average clock rise was only 7% instead of 7.7% \$\endgroup\$ Jan 14, 2019 at 17:28
  • \$\begingroup\$ Definitely explains the why. And thanks for straightening me out on the the things I "know"... This is the best information in the thread as of now, but didn't specifically answer the question - but validates the answer SunnySkyGuy. Wish I could split a vote! \$\endgroup\$
    – Matt
    Jan 14, 2019 at 19:16
  • \$\begingroup\$ This is an overly complicated way to do what a discrete pair of wires could do. And the discretes would eliminate problems with things like clocks and potential damage to appliances with motors. \$\endgroup\$
    – user71659
    Jan 14, 2019 at 23:58
  • \$\begingroup\$ @user71659 It is a reliable way to do it. I agree that discretes have a lot of advantages, but this approach only relies on existing standards rather than having to construct a new one and get buyin from all solar inverters which will work with the Powerwall. Similarly, my UPS has a discrete wire (well, a USB connection) which can be used to tell my computer to shut down gracefully. However, failing to properly configure all of that, the computer often is shut down when the UPS runs out of energy. \$\endgroup\$
    – Cort Ammon
    Jan 15, 2019 at 0:24
  • \$\begingroup\$ @CortAmmon The problem is you're violating standards of everything that plugs into it. Power line frequency is supposed to be +/- 0.5 Hz. You're running the risk of damaging equipment by exceeding spec by 10x. This is exactly why UPSes and things like motor drives trip off: to protect equipment. \$\endgroup\$
    – user71659
    Jan 15, 2019 at 1:55

45 minutes over a span of 660 minutes is an error of 6.8%. That's rather high. I would have expected a crystal-controlled frequency with an error of 100 ppm or less — i.e., less than 4 seconds over 11 hours.

And power isn't "pushed" by modifying the frequency per se, but rather the relative phase of the various sources.

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    \$\begingroup\$ Most cheap devices don't use crystals, but instead use the very reliable grid frequency \$\endgroup\$
    – Ferrybig
    Jan 14, 2019 at 15:34
  • \$\begingroup\$ @Ferrybig: I'm talking about the circuitry on the Powerwall that controls the line frequency that it generates. \$\endgroup\$
    – Dave Tweed
    Jan 14, 2019 at 15:38
  • \$\begingroup\$ The PW2 software upgrade provided this feature to the PW2 according to users. Being Xtal controlled it is very precise but being software controlled so is the controlled rate of change and target line frequency according to supply and demand. \$\endgroup\$ Jan 14, 2019 at 17:24

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