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Community and Q&A

Energy Star Appliance Peak Loads

paulneumann | Posted in General Questions on

If this has been answered and anyone point me towards the right direction I would appreciate it but couldn’t find it. 

My question is for a project I have been working on and with some energy modeling I have been trying to do for an off grid home and storage system. My biggest concern is trying to understand the peak loads for different appliances. Ideally there would be a smart management system to turn off circuits to whatever we set the max to (similar to the Lumin Smart Panel) but still I am trying to get a realistic understanding. I have read through so many different cut sheets for appliances as well as a few energy models like BeOpt and with Open Studio but still would love if anyone has any insight or actual data. 

Realistically the problem is only when the peaks aline. Like the classic “duck curve problem” with a family all coming home plugging in their electric car, cooking dinner, showering, doing laundry I still think that’s somewhat a realistically scenario. So for peak loads that’s a concern when its all electric. 

My biggest mystery has been with the electric range either induction or just traditional. I know the ratings are the maximum for each burner but for instance like the oven at 400F is their any way to get any idea on the load from that without testing it? 

If anyone has any data or insight into energy star rated appliances and their peak loads I would love to see it because I am trying to see basically how many appliances could be turned on before we would pull from the grid and etc..

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  1. Expert Member
    BILL WICHERS | | #1

    Energy star doesn't really get into what you're looking for. Basically whether or not the Energy Star program applies doesn't really address the peak loads, especially between different appliances. What you're thinking about is usually known as "demand factor", and it usually comes up in multiple occupancy buildings (think apartment complexes, stuff like that), and the electric code provides methods and formulas for working out the expected load from a group of similar appliances. Basically this means if you have 50 units, you don't have to allow for all 50 electric ovens running at the same time, so you can reduce the total available capacity somewhat and still be OK.

    For a single occupancy, demand factors don't usually apply, since they technically assume multiple numbers of similar appliances. In your case, it sounds like you want to know what you'd get in normal operation of the entire home, and that depends too much on the behavior of the occupants to be able to be calculated in a standardized way. It is unlikely, but not impossible, for everything to be on and at peak at the same time, but you can't really know for sure what that peak would be without measuring things. I think it's reasonable to expect that someone might be cooking dinner while running the laundry while their car is charging, so those loads would all be additive. What is not likely is that they probably won't be doing those three things while also having EVERY light in the house turned on.

    The rated load for an appliance is supposed to be the max it will ever draw. This means an electric cooktop, for example, is rated with all the burners on at the same time -- the rating is the "worst case" load rating. In reality, those burners will cycle on and off to maintain whatever they are set to, so while they may all be on at the same time initially, they will gradually taper off as they cycle and the max load of the cooktop as seen by the electric service will be something less than the max rating of the appliance. That is something that can't really be calculated though, since how things cycle depends on the size of the pot, what's in the pot, etc, assuming the cycling is tied into maintaining some constant temperature in the pot.


  2. walta100 | | #2

    Seem to me that people don’t change much so monitor your current usage patterns will give you the best answer to what you are likely to need and adjust for improved technology like induction, heat pumps and EVs.


  3. Danan_S | | #3

    I wonder about the wisdom of going both all electric and off grid. Is this just for you or for a family with kids (meaning lots of cooking, hot water, laundry)?

    But if you do, make sure you plan that your backup battery's is rated for the current output of all th devices you will run simultaneously. That will be very expensive, but if money isn't a consideration, go for it.

    In my case - grid connected but battery can only do 20A) - I had to take care not to put my induction stove, electric oven, and EV charger behind the battery backup circuit.

    However, you might want to consider propane cooking (on a grill or in the kitchen) instead of oversizing your solar and battery system to accommodate multiple high power appliances. Or perhaps smaller 120v/15A plug-in induction stoves connected to a smart load panel that will shut it off if needed.

    If you don't want to rely on even the propane supply chain (that's an energy grid also), or want no fossil fuel burning, get a very efficient wood burning cook stove and use biomass harvested from your land.

  4. paulneumann | | #4

    Hey all—appreciate the answers and input but I think I didn't explain my question clear enough.

    I am mostly trying to figure out the appliance peak loads for an energy model I am building for a non realistic house specifically. Just trying to test the bounds and go with a few different scenarios of occupancy but really am trying to get an idea of peak loads for batteries and super capacitors in a home. I know it's not very practical or realistic.

    Was hoping someone might have some data on it from their own house or had maybe known about the peak loads of the large appliances (Washer, Dryer, Range) because I've gotten our HVAC system pretty dialed in.

    I know the NEC220 goes over the Demand Factors for service sizing but I am more curious about like for instance how many appliances could be on with a 6kw max discharge from a storage system or 8kw or etc. And messing around with it like that.

  5. Expert Member
    BILL WICHERS | | #5

    If you want to know how much will fit in a 6kw power budget, you just take each thing you want to run and subtract it from that 6kw. When you run out, that's your limit :-) Peak loads don't matter as much here as steady state loads, since things like motors starting are of very short duration. What I would do here is to make a guess at what you'll be doing. An easy example is to take a typical 4-burner electric cooktop. These cooktops usually tell you how much each burner uses in watts. Assume you have two small burners of 1.5kw and two big ones of 2.5kw each. If you run one big and one small as your "typical" load, that's 4kw for that cooktop. That's pretty close to what you're likely to actually see with the usual cooktop that wants a 40A, 240V circuit. If you are boiling water, that big burner is probably going to be running for at least a few minutes to get the pot to a boil, so you might as well count the entire burner running (and not cycling) for your load estimate.

    You can do the same thing for everything else. To approximate the watt draw of an EV charger, multiply the current by the voltage. A typical Level 2 EV charger will have a 40A circuit, but it can only provide 32A to the car. The exact load will be determined by the car, since not all cars are capable of using the full 32A -- my volt, for example, is limited to about 12A. Using my Volt as an example, that 12A at 240V is just shy of 3kw. That plus our cooktop example puts you at about 7kw, and now you're over your 6kw power budget with nothing for laundry.

    Clothes washing machines are probably going to draw around their rated current for the entire time, since they usually are running their motor most of the time during a wash cycle. A dryer will probably cycle somewhat, but probably not very much. I've never taken actual measurements on either though, so I'm just making educated guesses -- although they would be worst case in terms of load, which is safer for your esimating purposes.

    Remember to allow a few hundred watts for some basic lighting too.


    1. paulneumann | | #7

      Hey Bill—thanks so much that was exactly what I was looking for really appreciate it! Trying to get an idea of balancing smart controls or demand response controls with smaller storage or if larger storage will be needed and you'll just be cutting appliances too much basically is the idea.

      I wish cooking food took less energy (realistically that is besides slowly). I had seen a few threads about the differences between the rated burner wattage and the differences between induction and resistance but wasn't sure if for instance high would be closer to the rated wattage and for instance the burner on low smaller. Yes these are minute differences but was second guessing I guess.

      The fridge I know has a duty cycle but is still so minimal hasn't been a concern of mine and I found data for dishwashers on the different wattages for High versus "Eco". For clothes washers they typically just use DHW from the water tank right?

      For the dryer I could find pretty accurate wattages for different temps but wasn't sure if it would cycle and was considering looking at a heat pump dryer as well but I know that complicates it more with performance curves.

      Really appreciate it!

      Paul N

      1. Expert Member
        BILL WICHERS | | #9

        Heat pump dryers are more complex since they work like an air conditioner in reverse. I would just assume them to operate near their rated draw for most of the cycle and play it safe. It's always safer to OVER estimate your loads.

        Fridges draw surprisingly small amounts of power these days, maybe a few hundred watts. I bought a new freezer this past January and the very low draw really surprised me.

        Cooking food is what it is, because physics says it takes x units of energy to raise y mass of food z degrees. I like to say "the one true law of the universe is that physics is a b****", because there is no cheating physics, unfortunately. What you can do is be more efficient. Induction is very efficient, because it essentially focuses almost all of the input energy into the target area, making the pot itself the thing that heats up (the induction "burner" is just a coil of wire, the heat is generated as the coil produces an electromagnetic field that basically wiggles the atoms in the metal of the pot to make heat). Other burners (IR, resistance, etc.) produce a lot of heat that bypasses the pot, going around the sides, and not doing anything useful. The result is that input energy to the induction cooktop will heat things up faster, because less of it is wasted. You use the same amount of energy, assuming a similarly watt-rated cooktop, but you save time since the energy is utilized more efficiently to achieve the desired function of making your food hot.

        Battery storage is very poor in terms of energy density. You'll find that it takes a LOT of battery capacity to do very little useful work. To support a typical home (average about 1kw load) for one day requires 24kw/h of battery stoage, which, using a 48v string, is going to be around 500 ampere hours worth of battery capacity. That is a pretty minimal setup too, if you really go off grid you want more battery runtime than that to be able to deal with at least a few days worth of storms or overcast skies. That's a big part of the reason grid tied is recommended over off grid setups anytime you have the ability to be connected to the grid. Keep in mind that the battery string is a consumable too, with a finite number of years of life, and also a limit number of charge/discharge cycles.


  6. Expert Member
    Akos | | #6

    Here is a month snapshot from an all electric apartment. It has standard vented dryer and ceramic cooktop. Cooling is with a 9k wall mount mini split.

    You can see the highest load is around 7kW. I would guess it is a combination of running the dryer plus cooking. You can see the baseload increase on a hot day to run the AC.

    I took a quick zoom out and don't see anything higher than 8kW. The 7kW peak seem consistent for two other similar places.

    1. paulneumann | | #8

      Awesome thanks so much I really appreciate it!! That is very helpful as I am a graduate student and have an old apartment and can't currently track it.

      Good to know as well about the highest peak you had.

  7. GBA Editor
    Martin Holladay | | #10

    Zephyr's point is key: "Battery storage is very poor in terms of energy density. You'll find that it takes a LOT of battery capacity to do very little useful work." Zephyr is right.

    As someone who lived in an off-grid house for decades, here's my advice: If you are designing an off-grid house, don't use electricity for cooking, drying clothes, or charging a vehicle.

    1. Danan_S | | #11

      They way I've started thinking about this is that we only have few ways of generating high concentrations of heat. One is to pass a ton of electricity through a wire, which is expensive when not connected to a grid, and the other is to burn a hydrocarbon.

      All the efficiency games we play with heat pumps, etc are only relevant at the temps that we like to live at, not the ones we need to cook food at. This occurred to me when my kid asked why we don't have a heat pump stove to go with our heat pump HVAC/water-heater and clothes dryer. Kids sometimes ask good questions :)

      That said, nobody has mentioned solar ovens, which are a thing I'd consider for a (hypothetical) off grid lifestyle. I figure if you are going to accept the inconvenience of no grid, the inconvenience of configuring one of those isn't terrible. There are some schnazzy ones on the market these days.

      1. GBA Editor
        Martin Holladay | | #12

        If you are responsible for preparing food for your family, you will quickly learn that families get hungry three times a day -- even on cloudy days.

        1. Danan_S | | #13

          > you will quickly learn that families get hungry three times a day -- even on cloudy days.

          Don't I know it, and maybe it's worse on cloudy days because they aren't getting as much solar energy :)

          I'm fully grid connected though, and I'm of the impression that the grid is the only way to achieve a low carbon yet high quality lifestyle at current population scales.

          I do think we can learn a lot about how to stretch cooking energy from societies where energy is more scarce and unreliable by using low power input techniques like soaking, drying, pickling, and fermentation.

          That may sound dystopian to some, but as much as I believe we need the grid, we also need to prepare for it to be more variable in supply (and hence price) over time.

          For example, it could be that in the future, soaking beans overnight before cooking them to reduce their cooking time becomes about saving energy - like it used to be - instead of just saving time. Or we could set our smart instant-pots to monitor the price of electricity and cook the dinner when it's cheaper and cleaner.

          1. paulneumann | | #14

            I completely agree and part of why I am an advocate for Microgrids and localized storage. Especially for things like thermal energy and PCM batteries that store heat rather than electricity.

            Yes I understand the grid but I also don't think we can continue to "use the grid as our battery". In California we are already seeing the challenges of all electric and renewables. IMPO unless we had nuclear as a base load the grid will never be clean. That leaves one way off reducing building energy usage and that is at the grid, micro, or building level.

            I am curious about the idea of companies and manufactures using super capacitors in appliances to balance peaks. Like for instance in an induction range having a small capacitor that could meet those peaks without needing to store energy like a battery. I mean we already have electronics before the electrons pass in the device why not add more and solve the battery storage issue?

          2. charlie_sullivan | | #15

            Paul, I don't think it will ever make sense to add energy storage to a cooking appliance. If you invest in perhaps 2 kWh of storage, you would be better off adding that to a main battery at the household level, so that it can be used by any appliance, not just the stove. And then there are the safety and reliability issues that would arise from locating it near the high temperature stuff, particularly in an oven.

          3. Expert Member
            BILL WICHERS | | #16

            Capacitors don't have great energy density either. What they have are much better cycle lives, but they degrade rapidly in high temperatures. There is no point in local energy storage within appliances, in part because that local energy storage can't do much to help with peak loads elsewhere. You're much better off with centralized storage.

            Microgrids are ging to be less efficient generally too. The point of the large power grids is to balance loads, which lets the most efficient (which tend also to be the cheapest to operate) generators run near full load as much as possible maximum efficiency, and to allow excess generation in one area help to cover shortfalls in other areas. As you shrink down those grids, the peak load relative to the base load becomes higher as a percentage, which means the generation has to run with more margin, meaning it runs at a lower percent average utilization where it's less efficient. Note that this applies to solid state things like inverters in the same way as it applies to mechanical things like turbines, it's just less severe with solid state things. The second issue, sharing resources to help with capacity shortfalls and broken things, is also a bigger issue with microgrids due to their relatively limited resources.

            The solution isn't to go back to cave man days here, it's to improve the larger systems. Maintaining a good quality of life is important too, so you can't focus on just emissions or anything other single issue in this complex problem.


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