Analyzing Your Electrical Loads


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Kill A Watt energy meter
An energy meter like the Kill A Watt can help you measure common household 120 VAC loads.
Kill A Watt energy meter

The first step in any energy project is to assess the load—how much energy you are using or are planning to use. Without a load analysis, you can’t make a sensible system design, or know how a proposed system will impact your finances or home’s energy use.

On Grid

On-grid load analysis can be quite easy—just look at your utility bill. Many utilities show the past year’s usage (or more) on each bill. If that information isn’t on your bill, request the past few years’ data from your utility. I like to convert this information into average kilowatt-hours (kWh) per day. For reference, a typically inefficient North American home uses 25 to 30 kWh per day, if space heating is provided by other fuels. Highly efficient homes may use 6 to 10 kWh per day.

Understanding what a kWh is and how many you use per day is a good starting place. From there, you’ll be able to ask RE contractors how much it will cost to generate that much electricity with sunshine, wind, or falling water. Or you’ll be able to calculate how much of your utility bill you can offset with the money you’re able to invest in a system.

Off Grid

Off-grid load analysis is more complicated, and involves measuring or estimating each load. The method can also be used to estimate electricity usage for on-grid homes in the design stage or for backup systems (for sizing battery banks and inverters). It’s also very useful to use this method if you’re on grid, to find out where you are using all that energy. Then you can develop strategies to reduce your energy usage, which is typically the most cost-effective and environmentally friendly use of your dollars.

A spreadsheet is the easiest way to gather the necessary information. For each specific load in your home, you’ll need either its wattage and daily hours of use or its daily kWh use.

Measure each load’s power, since rated or sticker wattage may not accurately reflect actual appliance consumption. When in doubt, round up. Measuring 120 VAC loads is easy with meters such as the Kill A Watt, Brand Electronics Digital Power Meter, or Watts Up?. Any 240 VAC loads will be harder to measure, and you may end up relying on rated wattage or estimates.

The energy use of 240 VAC appliances can be measured with reconditioned utility-style kWh meters. These modestly priced meters can also be installed permanently on major loads like water heaters and heat pumps for ongoing performance evaluation.

Accounting for all these loads may be an eye opener on several levels. You’ll likely be surprised at what is and isn’t a significant energy load. Because energy is power (watts) multiplied by time (hours), high-power loads can be no big deal if they are only used for short periods of time, while “small” loads can add up if they are on most or all of the time. You’ll need to work on your measuring and estimating skills to get accurate results.

Load analysis gives you a view into the biggest factor in off-grid design, and is key to understanding the value of on-grid systems. Careful and detailed attention to this end of your RE project will pay off with better understanding of what you’re getting into—and what you’re going to get out of your renewable energy investment.

Comments (1)

Robert Pollock_2's picture

Thank you for an informative, forthright article. Here in Palm Springs, Here in Palm Springs, where I'm trying to navigate a path to my own successful PV installation, a not-known-enough fact is that grid tied systems will not provide emergency power. It's been 110+ F degrees here for over a week, and we're looking at another 75-100 more days of it. So Cal Edison doesn't have enough product so any system I use, will have to be off grid. Going "off grid" also means you're going "off incentive", because the tax credit and the state incentives only apply to grid connected systems. For me, this single fact changes how I perceive and calculate almost everything in your story.

However, now I'm learning about an inverter which will "untie" itself if the grid goes down. My head is whirling. Why not grid tie, collect the incentives, and then "un-grid tie"? My house, my system, I can do what I want with it, can't I?

Robert Pollock

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