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.

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