Towing and charging

ajdelange

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I thought we were talking about whether the CT would handle a whole house in an emergency. I am saying that based on the fact that I weathered many a power outage on a 7 kW generator for years and that I expect, therefore, that I would be able to do it with the 7 kW inverter in the CT (as long as the battery lasts anyway). It is so obvious that some sort of load management is/would be
required that I didn't feel it necessary to discuss it explicitly until it appeared this was an unfamiliar concept to you and so I didn't until No. 29.

So let's go a bit more into how you would manage loads manually if running my whole house on a 7 kW inverter. In particular let's talk a bit about how I would have done that on the most demanding day of last winter which was February 7th on which day average consumption was 5.71 kW with a crest factor of 2.5 (peak demand 14 kW). Load was less than 7 kW 61.4% of the time but it was over the remaining 38.6. Here's the history:
Tesla Cybertruck Towing and charging Loads2_8_21


The long pole in the tent is the heat pump. It's probably obvious that its compressors are the cause of the 5 kW "pulses". It's clear that in the middle of the night the baseline load is 1.5 kW and it is also clear that loads other than the heat pump climb to around 4 kW later in the day. To run on the 7 kW generator or inverter we have to keep the total load below 7 kW (unless the duration is very brief).

So we suppose that on that day we awake and it's colder that a well diggers behind because the power went out. What do we do? Haul out of bed, go out to the garage, fire up that generator, go outside and open the disconnect to the panel. Go to the panel, open its feed breaker and all breakers except for some light, comms, the well, some outlets, the refrigerator and the heating systems control transformer. This removes all the loads except the baseline 1.5 kW and you should be able to go 100 hrs on 100 kWh from the battery. But no heat? Not acceptable. What would happen chez mois is that the thermostats would detect that their calls for heat are going unanswered and they would call for E-heat thus firing up the backup propane boiler and the house would come to temperature.

So what happens now. The heat pumps won't run and you have no heat. After a few minutes of no heat the thermostats will call for E-heat. That doesn't do anything for those heatpumps that have electric E-heaters but the one in my house that is backed up with an gas boiler (which also does instant on DHW) fires up and the part of the house you are in warms up (and warms up faster than it does with the heat pump).

Given this I would hope you would be able to see that I could have run my house on that coldest winter day on a 7 kW generator and did so many time largely because the heat pump gets taken out of the equation. In summer time there is no gas backup for the heat pump so the solution is to not run the A/C or run it when all the other loads have been turned off by flipping their breakers. This clearly requires a lot of manual intervention and is a big PITA but it can be done. I've done it. The ability to do it depends in no small measure on the fact that the system is designed to be operated on backup power supplies. That's why the DHW and heating backup are from a gas boiler which operates on only a few watts of electricity.

I will comment that if you have an electric whole house instant on hot water heater you are pretty much out of luck with respect to emergencies. Some of these things draw 36 kW and to run it you need a 36 kW generator. This is not a small generator but many houses do have them in that size and even larger.
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Crissa

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As I said, a central AC unit will use almost all the power it can supply. If the water heater kicks on at the same time, it's not going to be able to handle the load.
You A) need a more efficient A/C system. B) can have these two not kick on at the same time. They're both controlled by little computers.

Personally, my plan is to have two layers of critical loads: stuff on uninterruptible power, like the expensive PC and the network; stuff on the critical loads panel. Which will just be a special outlet in each room, for the most part. That way I can choose what's critical at the spur of the moment.

Eventually I want a whole-house backup, but I need to be able to handle extended outages and ration power.

-Crissa
 

ajdelange

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The tendency today is to approach the problem from the opposite direction. Instead of picking a handful of circuits which you consider essential you pick the biggest loads, prioritize them and connect them to your panel(s) through contactors. Everything else is connected directly to the panel(s). The big loads are few. They are things like E-heaters, heat pumps, clothes dryers, EVSE, electric ovens and any special purpose things like pottery kilns. While the average consumption due to these loads is often not much more than the average baseline load the peak demand when several of them are on at once can be 7 - 10 times greater. The strategy is to keep several of them from coming on at once. This is done by monitoring the emergency power source load and if it rises above say 80% of the supplies capacity the lowest piority big load will be shed. For example, if the heat is going full bore all over the house, there's a load in the drier, cookies in the oven and someone plugs in 2 Teslas to charge the generator will shut down the E heat for a couple of heat pumps, the ovens and the clothes dryer to keep the generator loaded less than 80%. The main heat pump stays on as do the refrigerators, lights, phones, computers, TV's, toasters, blenders,...When someone notices the generator is on, says "Hey the Telsas are charging" and goes goes and unplugs them the generator senses the load has gone down and starts restoring those loads it has shed. I have 96 kW service. All the loads I just mentioned plus the baseline (all the other loads) total 74 kW. I have a 48 kW generator and the inspector is happy with it because the system can shed 50 amps worth of loads if it needs to getting me down to 24 kW. This makes it clear that my 48kW genny is twice as big as it needs to be! It would not be twice as big as it needs to be if I had a 36 kW instant on water heater.
 

ajdelange

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This is yet another level of sophistication where UPSs are involved for critical loads which cannot be interrupted for even the briefest period of time. When the utility goes down a relay connected to it deenergizes closing a pair of NC dry contacts. These are wired to the generator which using its battery operated controller starts counting. After about 30 seconds the controller starts the generator and when the generator voltage, as sensed at the transfer switch, has been stable for about 5 seconds, the transfer switch operates. During this 35 second waiting period there is no primary power. A critical load like your home computer or a respirator is often powered by batteries (within a UPS) charged by the primary power. If it goes down for a few minutes the load runs on the batteries.
 

jhogan2424

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This is yet another level of sophistication where UPSs are involved for critical loads which cannot be interrupted for even the briefest period of time. When the utility goes down a relay connected to it deenergizes closing a pair of NC dry contacts. These are wired to the generator which using its battery operated controller starts counting. After about 30 seconds the controller starts the generator and when the generator voltage, as sensed at the transfer switch, has been stable for about 5 seconds, the transfer switch operates. During this 35 second waiting period there is no primary power. A critical load like your home computer or a respirator is often powered by batteries (within a UPS) charged by the primary power. If it goes down for a few minutes the load runs on the batteries.
That’s exactly the setup at my house. Whole house backup with auto switch and then I have several UPS placed in a few places I don‘t want to shut off for the short time the generator is starting and switching like internet router, POE security cams, DVR, PCs, and just a couple led table lamps. You can get away with the super cheap UPS because you only need it to hold power for the 30 seconds or so before generator kicks in.
 


Ogre

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We just built our house my two big focuses were minimizing maintenance over the next 50 years, and minimizing power we need to survive. Didn't do "Great" at either, but did well enough at the latter than we've gone through the hottest summer in the Willamette valley in decades (ever?) with more than half a dozen 100 degree+ days with no AC. (Heating I didn't do quite as well on unfortunately)

Personally think it's irresponsible that builders are allowed to make new houses which require high powered HVAC systems during normal weather. It means during extreme weather events, homes need extreme amounts of heating or cooling. A modern built home should not turn into a furnace or a freezer if the power goes out.

... sorry what were we talking about again?

</triggered>

If your home can get by with minimal HVAC, you don't need a ton of power to keep things running if the power goes out.
 

AZCYBER

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Isn't a NEMA 14-50 rated for 40 amp continuous (80%x50)?
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