Hey all. I am trying to size a generator and I was looking at peak consumption in KW. I noticed when I looked at Nov 1 until today my peaks showed 19.1 KW. I expanded my start date to the beginning of my iotawatt install and the peaks didn’t show as high. It is an average that brings down the peaks? The two graphs can be seen here: Imgur: The magic of the Internet
Any insight would be appreciated. Thanks in advance!
Yes, average peaks are generally lower than short term peaks. I have the data from my Iotawatt going to InfluxDB from there I can run a query (I am using Flux, but IQL can probably do it too) to get the max over the window and then get the max of that.
I will say that my peaks are about 7X higher than the normal load. I have a 16KW whole house generator that is capable of about 65A on both legs. It is fairly rare to be drawing more than 20A on one leg and half that on the other, except on laundry day. I sized the generator to handle the dryer, well pump, water heater, septic pump, and normal house loads. I was concerned that I might need to do some demand side load management (some of the newer generators have this capability as an option) to stay within limits. I actually have the opposite problem. There is too little load and the voltage is a little high. Since outages are generally short, it isn’t an issue worth addressing.
I would suggest that you look at shorter periods of time (12 hours gives you a 1m average, 24h is 2 min, 48 is 5 min). I look at 12 hours at a time to get the easy visual peak of the main amps. You can scroll backwards through the days and weeks. That will be quicker than setting up and learning how to use InfluxDB. It isn’t hard if you are comfortable with computer applications, but even then there is quite a bit you need to learn to be able to trust the data.
Since I didn’t have any of this when sizing my generator, I did it the old fashioned way. I figured out what all I wanted to be able to run at the same time and then figured out how much power I needed. Water heater is just under 5KW. 2HP pumps are about 2-3KW each. Dryer is about 4KW. That puts it at about 13KW minimum. The 13KW one didn’t have quite enough circuits (though that ended up not being an issue the way I had it installed) so I ended up with the 16KW one.
Your house is likely very different in usage patterns and you have to decide if you want to be able to use every circuit when main power is out or just critical ones.
Thanks for the details. In my home in Florida I had a 75 KW generator that we were forced to run for 11 days straight. The power that comes ff these things is very dirty. I didn’t have current reading or voltage reading but I had dimming lights that blinked like crazy. In our house here in NC we want to be able to run entire the house unlimited timeframe if needed. I do plan on adding load to the generator with an addition and more HVAC but I would like to know what the current peak usage is. Some people would argue do you really need to run your pool when the power is out? My answer is yes Why? Just the other day, here in NC, the power went out, and our pool pump obviously didn’t work - the water froze in our pool heater and expanded and blew out the copper. My pool guy said more modern pool heaters can be drains etc but it can be a costly mistake if you don’t do it in advance and here in the winter the heater in a spa is useful.
So in Florida when sizing we called FPL and they gave us peak power consumption. My power company here doesn’t know that 
Thanks for the insight!
Back in 2006 we had the Storm of the Century and we’re without power for 8 days. We were running on the generator the whole time, except for planned shutdowns to check and add oil.
Our whole house runs off the generator, so I definitely understand the value. You do too, so you are right to get an answer to the question of “how big a generator do I need?”. That is a tricky question and can have a very expensive answer. Part of it depends on if you are willing/able to change behaviors during an actual outage. I wasn’t willing to make much change, so my peak to average ratio is fairly high at about 7X. That peak is also rarely for more than a few minutes every few days. The highest peak I have seen is 77A, which is more than my generator’s capacity. But that peak only shows up when just about everything is running. My pumps typically only run for 2 minutes at a time and only once to 8 times a day depending on water usage. Had I had this knowledge (of just the peak) before I bought the generator I might have bought the 20KW one instead. But, I already knew about typical peak ratios, so went with a more conservative 16KW. The 13KW one would have been fine with some demand side management (make sure both water heaters, 3 now, don’t come on at the same time) to shave any really high peaks.
But, at 75KW you are in quite a different league. I would start by scrolling through the main amps data graphs in Iotawatt at the 12 hour or 24 span. The other trick is to use a larger span and zoom in on each of the peaks. That should allow you to see the details of the peaks quicker. See how long they are and if you really want to size based on them.
As far as voltage regulation goes, mine is not great either. I have some electronic devices from the 70’s that won’t work on the generator at all. The voltage is very high at my typical load and drops whenever there are peaks of load. Standard dimmable LEDs don’t handle this well, as they assume they are being asked to dim by a cheap dimmer. I got rid of all the dimmable lights and use only regular non-dimmable (harder to find these days) or smart lights (mostly Phillips Hue). That helps quite a bit, but there is still some flicker.
Finally, the generator is noisy when it is running, but you already have experience with that likely. I am mostly used to it now, but it used to keep me up at night. During the day it is not so noticable, have to open the door to make sure it is still running.
Thats what I did. Here in NC - no surprise peak usage since I put the iotawatt in, was July. I went to that peak day and then realized it was 25kw currently. We are going to add another 200 Amp Service for the addition and that will have a couple of AC units that won’t be geothermal like we have now. The interesting thing is that a 48 kw isn’t much less than a 60 KW. Neither are available so there’s no true benefit to getting a smaller one. Seems like greater capacity would be useful if loads (AC or other appliances change)?
Putting on my mechanical engineering hat, I believe there can be a big difference if they are not inverter type modulating) heat pumps when doing AC. There are two types of heat that need to be removed to make a space comfortable: Sensible and Latent.
Sensible heat is the heat embodied in the temperature of the air. Latent heat is the heat embodied in the moisture contained in the air (humidity). Comfort in a conditioned space is a function of the temperature at a given relative humidity. The air can be quite warm, but with low RH you feel comfortable because you’re body’s natural cooling system works great releasing a little bit of water (sweat) which absorbs a bunch of heat from your skin as it evaporates.
Conversely, if the air is cool, or even cold, but the RH is high (all things being equal the RH of air increases when it is cooled) you may feel uncomfortable and “sticky” as the sweat that your body produces does not evaporate. Give me a warmer low RH environment anytime. Witness the “dry heat” that is talked about in the southwest.
So air conditioning must remove both the sensible and latent heat from the room. Latent heat is removed as condensation in the air-handler. When the duty cycle is long, or continuous as with an inverter heat-pump, moisture is condensed continuously. When you have an oversized AC unit, it may cycle to keep the air temperature low, but the duty cycle to accomplish that may be only 25% or so and so the moisture removal is much less and the RH remains high.
Rightsizing the unit is important to achieve economy and comfort. Even inverter units have a minimum output and can have this problem, especially where the outdoor humidity is high and the temperature differential between outside and the conditioned space is small. My feeling is that I would rather have a slightly undersized unit that runs continuously in those conditions and than an oversized unit. I’d rather have higher air temps with lower RH on a muggy day than an icebox with high RH.
I have a 3,000 sq ft home in northern New England. We have a one ton (12,000 BTU) mini-split in the middle of the house. On the few really hot muggy days, we run it continuously and the house may be 78°F or so but it’s extremely comfortable at low humidity. I can see the condensation literally pouring out of the drain line.
I think he meant generator not heat pump. You are very correct about heat pumps used for cooling.
The larger generator will have a somewhat higher no/low load utilization. Not sure what the difference is, but it was about 10% or more the last time I looked.
I think once you get to the water cooled generator you are in another class and the difference between ones in the same class is not that great.
I did see lead time on 48KW and larger is 6 months to a year. 30KW appears to be more available and cheaper too. So, think about what you want and when you can get it. 30KW is a lot of power, but it might not be enough to run everything at the same time. Perhaps the ability to shed some large loads that are less necessary to always be on, will enable using a small generator but still provide you what you need. Since availability is an issue, this might not be an issue you can just throw money at.
Right I meant the price difference might be 10% more for a 60W generator vs a 45 KW generator…