Sunday, February 05, 2012

Where Does the Energy Go?

We recently had our first "net zero" electrical energy day, i.e. our solar panels generated as much energy as we used that day (about 9 kWh). Of course, that's totals for the day. Midday, at the solar peak we're generating more than we're using (and selling it back to the grid), whereas the rest of the day we're using more than we're generating. (We don't have any kind of battery storage.)

Since I installed the TED energy monitor it's been interesting watching where the power goes, or more importantly, where the energy goes. A number of things surprised me, not because they didn't make sense, just because I'd never really thought about them before.

Power is the rate at which energy is consumed - measured in watts or kilowatts. Energy is the amount of power consumed - measured in watt-hours (Wh) or kilowatt-hours (kWh). (1 kWh = 1000 Wh) If we were talking about water instead of electricity, power would be the rate of flow e.g. gallons per minute, whereas energy would be the amount of water, e.g. gallons.

Here's some rough figures:
  • 3000 W - clothes drier
  • 3000 W - oven (when the element is on, but that's not the whole time the oven is on)
  • 1500 W - electric kettle
  • 1500 W - microwave.
  • 500 W - furnace fan
  • 500 W - car block heater
If you're trying to get a handle on these numbers, think about an old incandescent 100 W bulb for comparison. If it was turned on for 10 hours, that would 1000 wH (watt-hour) or 1 kWh.

But the figures above are power. What really counts is energy. In water terms again, a dripping faucet can add up to as much water (energy) as turning on the hose for a few minutes. That's why your fridge can be one of the big energy sinks, because it's on all the time.

Here are rough guesses for our January energy usage:
  • 120 kWh - Furnace: 1/3 of the time = 8 hr/day = 240 hours @ 500 W
  • 18 kWh - Oven: 3 hr/week with the element on half the time = 6 hours @ 3000 W
  • 15 kWh - Kettle: 20 min/day = 10 hours @ 1500 W
  • 15 kWh - Microwave: 20 min/day = 10 hours @ 1500 W
  • 12 kWh - Clothes drier: 30 min/week = 4 hours @ 3000 W
  • 5 kWh - Block heater: 10 hours @ 500 W 
This adds up to only 185 kWh, about half our actual usage. There are lots of things I haven't included - fridge, lights, computers, TV, etc. And the numbers are just guesses. But they give a rough idea of what the big energy uses are.

This was for a very mild January. With more normal (colder) temperatures, the furnace would be on more, and so would the block heater.

At night, and during the day when we're at work, we have our furnace set lower (16c instead of 21c). Looking at the usage graphs, you can see the furnace not coming on so often. On the days I work at home, I turn the heat back up, run my computer, and use the kettle and the microwave, and our energy usage is noticeably higher those days.

In January we used about 360 kWh of electrical energy or roughly 12 kWh per day. (I think that's relatively low i.e. most people would be higher.) The solar panels generated about 140 kWh, so our net usage was 220 kWh or about $26 worth of electricity. (Not including the base service charge.) It looks like February might be about half that (less furnace and more solar as the days get longer and the sun gets higher). Recent sunny days the solar panels have generated about 10 kWh of electricity. That should continue to increase towards summer. Based on what we've seen so far, I'm guessing that over the course of a year, we'll generate as much (or more) electricity as we use.

Our solar thermal (hot water) panels are currently producing about 1 kWh on a sunny day, but it takes a surprising amount of energy to heat water so the storage tank hasn't warmed up too much yet. Even so, it means the input to our on-demand hot water heater is at 20c instead of 10c, which reduces the temperature rise to 50c by 25% (50 - 10 = 40, 50 - 20 = 30)

From these figures, it seems like the biggest improvement we could make would be to insulate the house better, although that's not always easy to do in an older house. Another option would be to install geothermal (technically a ground source heat pump). This might not reduce our electrical usage, but it would reduce our natural gas usage and our carbon footprint.

I'd also love to have a plug-in hybrid car so we could use our excess electrical power during the day to charge it up. That would allow us to do our small amount of driving around town virtually for free.

If you're interested in this kind of stuff, I'd recommend Sustainable Energy by David Mackay which you can read on-line or download for free at

View live monitoring from the solar electric system

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