It's All About the Loads

Intermediate
Utility bills can show your home's energy usage.
Measure your loads with a kW/kWh meter.
Load calculations spreadsheet example

When discussing renewable energy (RE) with students, clients, readers, friends, and family, it’s very common for the conversation to focus on RE generation sources like sunshine, wind, falling water, biomass, etc. and the technologies, such as photovoltaic (PV) modules, wind generators, and hydro turbines.

While the technologies are interesting—what’s next, how much they cost, and how they work—what should drive the design of any energy system are the loads. When a client asks how much a PV system will cost, I don’t ask about the square footage of the house, how many people live there, or even how much sun they have. I ask them how much energy their home’s loads use, which determines an RE system’s design. This logical starting point is too often bypassed as we get excited about the glamorous generating technologies.

To understand loads, some basic electrical terminology literacy is helpful. “Watt-hours” (Wh) measure electrical energy generation and usage; 1,000 Wh equal 1 kilowatt-hour (kWh), which is the measurement we use in our homes, pay the utility for, generate with our RE systems, store in batteries if desired, and send back to the utility when our RE systems provide surpluses.

On-grid folks can usually pull out a recent utility bill to determine these numbers for their electrical energy usage, and, if they are on the fossil-fuel grid, for their monthly natural gas or propane consumption. Off-gridders have a bit harder task, though modern metering tools can provide excellent information. But if we don’t have metering or if we’re trying to plan for a future home, a load spreadsheet, on which the wattage and daily duration of each load is recorded, is necessary.

Handy tools for this analysis include watt/watt-hour meters like the Kill-A-Watt brand; hard-wired utility-type meters; and the utility’s existing kWh meter. Each is used to measure the instantaneous wattage or the cumulative kilowatt-hours of individual loads, specific circuits, or your whole house. See “Using Energy Wisely Off-Grid” (in this issue) for more information and examples of off-grid loads.

Loads for a residence in the planning stages require estimation. Since you cannot measure the wattage of electrical appliances and loads that aren’t purchased yet, you need to research their energy requirements and estimate the daily usage of each to arrive at the total anticipated loads. This is something a skilled RE installer can help with.

Understanding electrical loads and how they are measured is the right first step toward using RE in your home. Wrap your brain around the terms and then do the sleuthing to find out how much you use in your home. You’ll be the life of a nerd party with this knowledge and better prepared for purchasing an RE system.

Comments (5)

Anthony Maciorski's picture

I couldn't find out what the difference was between U.S. and Canada either. How could the same model use less KwHrs in Canada? Different test conditions? What voltage does Canada use?

Marc Fontana's picture

Apparently, the U.S. EnergyStar rating changed how it estimates energy usage, I suspect they require different test conditions, resulting in higher kWhrs per Year. For me, the lower Canadian average annual kwhrs rating is closer to what I've measured on my fridge so far.

Anthony Maciorski's picture

I just replaced a 14 year old GE Eterna 20 cu. ft. fridge estimated 745KwHr/ year. A few months back, we plugged it into a KwHr meter. For the first few months it used 2.15 KwHr/ day, or 785KwHr/ year. Just yesterday we replaced it with a new GE 24 cu.ft. French door freezer under ( yellow tag 610 KwHr/ yr ) and I checked the watt meter for the last time. It was now using 2.54 KwHr/day, or 927 KwHr/ year. The meter was plugged in for 108 days, so I don't know if was true or not.

Michael Welch's picture
Maybe different testing standards, but I don't know that for sure.
Marc Fontana's picture

I would add that some appliances such as refrigerators are variable loads, at times drawing almost no power and at other times such as when the compressor is running or the defrost cycle is on, drawing a larger amount of power. To get an accurate number, you really need to measure the energy use using a watt-hour meter over several days and calculate the average per day. When comparing different fridges for energy efficiency, you can use the EnergyGuide Estimated Yearly Electricity Use number. I only use this estimate to compare models, but I don't trust it as a real world energy use number for my calculations because I have found it to be much different than what I typically measure. To make matters worse, since 2014, the U.S. EnergyGuide estimate is different than the Canadian rating printed on the same EnergyGuide card ! For example, It's not uncommon to see a fridge rated with a Yearly estimated electricity use of 584 kWh on the Yellow side of the card and to see a Canadian rating of 423 kWh on the White side of the card for the SAME MODEL fridge ! What's up with that? I've tried to get an answer, but so far, no one seems to know. Prior to 2014, both numbers were the same.

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