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The premise of the question may be the problem so please let me explain first.

In my home at night we have a bunch of nightlights and we run a small fan in each bedroom for the noise and/or cooling. Most nightlights are LEDs but we still have a few incandescents in fancy or "cute" bedroom nightlights when there is no good LED replacement available.

Every night, usually at exactly 10:00pm all the fan noises lower in pitch and the incandescent nightlights dim. Sometimes it happens 5-10 minutes late, but never early and usually right at 10.

Going out on a limb here, I think the grid voltage is being turned down at night. I don't know when it's being turned back up, maybe early morning. I haven't measured with a meter at all, but the difference in lights and fans is noticable.

Everything still works fine afterwards so I don't mean a brownout. It's just more like it went from 120v to 115v or something.

Why would the grid operator reduce voltage at night? Does it save energy or something? Because I thought usually higher voltage leads to less losses. Alternatively, what might be happening at 10pm to make our nightlights dim and fans slow down?

One alternative explanation I can think of is that some nearby large industrial customer starts up their machines at 10pm. That doesn't sound right in my residential neighborhood with no heavy industry nearby. There is one factory within about 20 miles that runs a cogeneration plant but other than that it's white collar work and light industry only.

I'm on the US west coast grid, a customer of Southern California Edison. I'm not going to ask them so please don't suggest it. I'm just curious.

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    \$\begingroup\$ In some areas you have the option for getting cheaper electricity at night for things like heating and water boiler. Could it be that in your area the night tariff starts at 10pm and a lot of loads get connected to the network? \$\endgroup\$
    – Justme
    Dec 8, 2023 at 6:53
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    \$\begingroup\$ @Aldus, Just going on old experiences as well as current knowledge, it takes quite a whack to see lights dim and fans slow down. I was using a long stretch of wiring that allowed me to change the voltage (drop voltage) at will. So, with my true-RMS meter (newly calibrated by Tektronix) I played around. At 109 VAC no one else in the room made notice when I played. But they did say something about room lighting when dropping down to 105 VAC. Didn't have a fan, though. Why not stick a meter into the plug just before 10 PM. Get the number before and after and report it. I'm curious. \$\endgroup\$ Dec 8, 2023 at 7:53
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    \$\begingroup\$ I'm betting on either timed EV charging or timed hydroponic farming. In southern California I'd guess most have video on demand, but in earlier times when broadcast television was dominant, there were very noticeable surges in power demand (kettles) and water (toilet flushing) during adverts, half-time, ends of TV programmes, especially important football games. \$\endgroup\$
    – jonathanjo
    Dec 8, 2023 at 9:02
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    \$\begingroup\$ A few minutes late sometimes - I wonder if someone's EV app reminds them if they haven't plugged it in. That's the right sort of timescale \$\endgroup\$
    – Chris H
    Dec 8, 2023 at 15:42
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    \$\begingroup\$ Also there can be surprisingly high loads in light industry, but they're likely to be during the site's working hours \$\endgroup\$
    – Chris H
    Dec 8, 2023 at 15:44

5 Answers 5

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Business Time of Use cheap rates start at 9pm according to this.

enter image description here

It's possible a large user is using 10pm as a start, but if you or a neighbor have an electric vehicle and are using scheduled charging, that's also a possibility, especially if you're both being fed off the same distribution transformer.

Note: There is considerably anecdotal evidence that power companies have sized the pole pig kVA to house kVA ratio based on some kind of averaging and many of them are now undersized with EV charging.

For example, this comment from a Tesla forum:

Feb 27, 2016

I'm one of seven houses on a 25kw transformer. Voltage drops to 225 when I charge in the evening. Power company is not concerned.

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  • \$\begingroup\$ Here, we get one phase to the house, and it usually goes ABCABC down each side of the street. Here in the UK 230V and usually TN-S we don't have the extra complication of opposite legs. Voltage drops from big loads should be on the same phase so it's worth having a think about nextdoor-but-2 neighbours. An EV parked outside might make it pretty obvious. Of course sharing a neutral between phases means it carries plenty of current. And all this is more likely if you're a long way from the transformer \$\endgroup\$
    – Chris H
    Dec 8, 2023 at 15:41
  • \$\begingroup\$ This is a good answer and a possibility I didn't think of. But I think Chris H and jonathanjo had more good examples in the comments in the original question. I think those other good examples would look nice incorporated into this answer. \$\endgroup\$ Dec 9, 2023 at 0:22
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    \$\begingroup\$ @SpehroPefhany if it's cheaper to fit extra transformers than to run extra cable, that says something about the distance between properties. Mine's "230V" (I measure 240 normally) back to the 33kV substation a few hundred metres away, but in my village we have lots of 11kV to 220V substations - one big transformer for quite a few houses. I've got a typical 100A main breaker, and I believe the feed is sized to match. Older or smaller houses sometimes have less. But yes, if people are using even close to their full capacity it's for short periods \$\endgroup\$
    – Chris H
    Dec 9, 2023 at 8:21
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    \$\begingroup\$ I went ahead and accepted this one--bottom line, unless I learn something new about my neighborhood, EV charging by neighbors is probably the most likely culprit! Thank you to all! \$\endgroup\$ Dec 10, 2023 at 14:19
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    \$\begingroup\$ @user71659 don't be silly. The NEC definition of "continuous" is 3 hours or more. At that point, all the thermal effects of continuous running are felt. \$\endgroup\$ Dec 12, 2023 at 2:57
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I work for an electric company and I can shed some light on some of your points.

tl;dr: your electric utility isn't turning down your power intentionally but they may be experiencing heavy load that your circuit can't handle well.

Why would the grid operator reduce voltage at night?

It is highly unlikely (read: generally impossible) that distribution company is turning down the voltage. Electric companies don't have dials to turn these things down, because that is complicated and expensive and it's not very useful.

Your other conjecture, that perhaps something is loading the system, is much more likely. (More in a bit.)

As a side note, you (as an ordinary home customer) get your power from your local electric utility company not a grid operator. Southern California Edison also operates transmission and generation, but the energy market, especially in California, is complex. Either way, you're a utility customer not a grid customer. This matters because why electric utilities and grid utilities can (and will) do for, and to, their customers, is very different, specifically on the points that you ask about here.

Does it save energy or something?

Your thought on this is correct. It does not save energy, power, wear, or anything like that. This is why it's not generally done.

One alternative explanation I can think of is that some nearby large industrial customer starts up their machines at 10pm. That doesn't sound right in my residential neighborhood with no heavy industry nearby.

This is quite possibly very close to the reason. You may have the right idea but you may be envisioning the wrong details.

Other than factories, other similar causes, with some relevancy notes:

  • Crypto miners and data centers. Data centers are generally known to the distributor so they're less likely to have a hard cut. Crypto miners tend to pop up in places where power is available (a residential neighborhood may be a great place) and are very capable of suddenly unleashing very high demand on schedule.
  • EV charging. I will mostly defer to @SpheroPefhany about this. It may even be not you or your neighbor, but reduced rates at charging pylons in the neighborhood.
  • Less-industrial uses, like water treatment. There are a great many services like this that could potentially run at night for a number of reasons. This could also imply a change in the nature of the load (power factor). That is, there may be a business that uses a lot of (for example) heavy motors during the day, for which your electric company compensates. At night, when they turn off this load, the compensation behaves as overcompensation.

Intentionality

It is likely that this is not something that your power company is doing but rather something that is done to them. Assuming they're competent (a good assumption because you and very, very many other people in Southern California almost always have workable power), they probably would prefer not to have this happen at 10pm daily. On the short term, they most likely do not have the resources to fix it.

California, and especially Southern California with regard to residential consumption, is a very difficult market for an electric utility. Much of the state (republic, I believe y'all call yourselves) is chronically tight on energy supply for a number of reasons. Anything financial is always a challenge and upgrading a part of a distribution network can be expensive. More remote neighborhoods or less-new developments are even more challenging.

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    \$\begingroup\$ It's highly unlikely to be crypto miners because SCE's cheapest residential rate is 23 cents/kWh. Absolutely no reason to be mining in California when you can double your profit by moving one state over. \$\endgroup\$
    – user71659
    Dec 8, 2023 at 21:48
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While I can't argue with either Spehro Pefhany or Stephan Samuel, I will note that in our city (which has a city-specific power company), the same thing was happening here - at 10PM the line voltage would drop a few volts. I ended up calling our local utility, because the line voltage would actually drop out for a cycle or two and cause my UPS to blip. In our town, it seems, there was an autotransformer between the provider and the civic load that would adjust the voltage downwards a few volts at night as the power drain from the residences slacked off and the load change caused the voltage to rise. The engineer in charge told me later that the reason for the dropout was that some of the transformer contacts had gotten dirty. They have since decided that the cost of maintaining the autotransformer was more than the value of that fine voltage adjustment, and have taken it out of circuit.

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  • \$\begingroup\$ (There was an autotransformer question today (only 7 this year).) \$\endgroup\$ Dec 8, 2023 at 21:52
  • \$\begingroup\$ When you're talking about the load for an entire civic district, an autotransformer does make somewhat more sense than a straight transformer... especially if you feel the need to adjust things. For reasons mentioned in that other question. \$\endgroup\$
    – tsc_chazz
    Dec 8, 2023 at 21:59
  • \$\begingroup\$ Great contribution here, thank you! Until I saw this I was ready to just figure that one or two of the dozen homes we share the service transformer with has a car charger on a timer. \$\endgroup\$ Dec 9, 2023 at 6:24
  • \$\begingroup\$ Actually, I'll note that almost all EVs actually contain timers and can be set to start (and stop) charging at a specific time. And I suppose, some also have a notification function to blip the owner's phone - "You told me to charge but forgot to plug me in!" Which supports Spehro Pefhany's points. But against that I will mention that a normal commute should pull power for less than an hour to get back to full charge... \$\endgroup\$
    – tsc_chazz
    Dec 10, 2023 at 0:36
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A little more data might help, and in particular is important to rule out possibly serious issues. A lost neutral or a lost leg - essentially a partial power outage - can cause lots of strange problems. This is not likely here, but is easy enough to check.

You want to test both legs to each other and to neutral. If things are functioning correctly, your voltage will typically be:

  • 120V each leg to neutral
  • 240V between legs (208V in some apartment buildings and possibly in other types of residential developments)

Any value from 110V/220V on up to 125V/250V is OK, but 120V/240V is most common these days in most of the US. But except for 208V installations, the leg-to-leg voltage should be the sum of the individual leg-to-neutral voltages and the leg-to-neutral voltages should be very close to each other (within a couple of volts).

The catch is that it can take a bit of work to figure out which 120V receptacles are on each leg. If you have an accessible 240V/120V receptacle then it is easy. Most house with electric clothes dryers have one for the dryer, with either three wires (hot/hot/neutral - old, obsolete, dangerous) or four wires (hot/hot/neutral/ground). So the easy way to test this is:

  • Unplug the dryer and check voltages in the receptacle sometime well before 10pm.
  • Next time you see the problem, check voltages right away on the dryer receptacle.

If everything goes down consistently - e.g., 120V -> 115V (for each leg to neutral) and 240V -> 230V (for leg-to-leg) then you most likely have an external supply or transformer issue that is lowering the available voltage. As long as the voltages are within 114V to 126V (120V +/- 5%) and 228V to 252V then you really can't complain. On the other hand, if the voltage drops below 110V (leg to neutral)/220V (leg to leg) then it is worth raising the issue with the electric company.

On the other hand, if you get inconsistent results, such as 120V/120V/240V dropping to 120V/100V/220V or 130V/110V/240V changing to 140V/100V/240V or anything else that shows inconsistency between the two legs then this is a very serious issue and you should contact the electric company right away. This type of problem can be an indication of a loose connection with one of the main wires from the street to your meter. Almost always these problems are outside in utility company wires, because they are subject to extreme weather conditions over time. I don't think this is what you will find here, because such problems are not usually tied to the clock, but some voltage readings are the way to tell.

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Like many others have said, it start searching for the cause in the next apartment/house. If you live in the city, there can be hundreds of high loads per acre. EVs, home batteries, pot farms, local tech startups doing AI model training all come to mind.

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