Power Shift: Apples and Oranges

By , March 17, 2015

Our H40 wind generator is rated at 105 Kilowatt Hours per month (kwh/mo). Our current solar array is rated at 30.7 kwh/mo. If we truly want to change our power structure (if you will) to rely less on the wind generator (see Power Shift: Increasing Our Energy Independence) we need to beef up our solar array by a factor of 3.5 or more.

Our problem here is one of comparing apples and oranges. Both will feed us, but each does so in different ways.

In the windy month of March, I’m inclined to assume that we get a lot more wind than sunlight throughout the year. However, I remember many days when I stood in the hot summer sun, watching and listening for hints that the breeze might freshen enough to drive the wind generator to recharge our battery bank beyond the solar array’s capability.

This leads me to wonder how our wind compares to our sun. While the wind will often blow all night long, we don’t get wind every day. We do get daylight everyday. Even in our temperate rainforest region, we get enough sun through the clouds to collect with a PV array.

I’ve been keeping a weather journal for 5 years now. I could use it to tally the days with charging wind speeds and days with bright sun to see how they compare. Instead, I turned to the experts and used their calculations to estimate what it would take to match our wind energy with solar collection.

I started by making a new energy audit for our “homestead.” I did this once before, but I didn’t think to file it in our power folder. Pity. I started from scratch, listing everything we use that draws power. For each item, I documented or estimated wattage. I figured the amount of time each item gets used each day. For those that get used less than daily, I had to calculate a fraction. For instance, we have an electric blender that we don’t use much. I calculated that its usage time averages .05 hours/day.

I strived for accuracy, but made sure to estimate high when necessary, as I wanted a maximum usage figure when I finished.

I identified 21 power consuming items in our home. Some of these represent a group of tools or appliances for ease of calculation. Our satellite Internet modem and printer share a power strip that comes on with the inverter. We rarely turn the strip off while using the inverter, so rather than identify the draw of each component, I grouped them as one unit.

For each item, I multiplied the power draw by the number of hours it gets used each day. This gives me watt hours per day (Wh/day).

I took the time and effort to make a spreadsheet for this process, which proved wise. I spent about a week correcting values on the list as I discovered mistakes. Changing one item on the spreadsheet adjusted values up and down the calculations, including those in this post and a future one, which will deal with sizing the PV array.

This exercise revealed that currently, we use an average of 1438 Wh/day or less, or 1.4 KWh. That works out to 42 KWh/month.

The calculations above show that averages and ratings only go so far. On paper, our wind generator alone provides 2.5 times the power we need each month. The solar array alone provides 75% of the power we use—on paper. We know we can’t begin to get by on our current solar array by itself. There are some times when we can’t get enough juice from the wind generator to do what we want! But, it gives us an idea of how to plan for shifting the power load from wind to sun. Even if we are working with apples to oranges, we’ve gotten close to a comparison from which to proceed.

It’s appropriate to post on this topic today, our local Vernal Equinox, when we get equal hours of darkness and light. Our solar harvest will increase daily now till the Summer Solstice.

2 Responses to “Power Shift: Apples and Oranges”

  1. Survival Skvez says:

    Where does the 30.7kWh/month figure come from? Is that a rating or have you measured it in practice? I’ve measured solar panel production every 10 minutes across a whole calendar year a few years ago and for the winter months it produced 3% of what it managed during the summer. Be very wary of averages as your batteries may carry you through a day or two of bad weather but they won’t carry you from Autumn [Fall] to Spring!

  2. Mark Zeiger says:

    Skvez, the 30.7 KWh/mo is merely a rating, as indicated in the text. In actual usage, the rating means very little; nor do the ratings of wind generators mean much–they’re just a quick reference point for comparison purposes. Solar coverage is, as you point out (as do I within this topic string) highly variable throughout the year. We will likely end up with more than enough power in the summer, when we receive 18 hours of daylight (of which much less will be actual sunlight) and almost none in the winter. That’s why we will continue to rely on the wind generator for a portion of our power throughout the year, and why we are actively looking for a more reliable wind generator to replace the one we have.

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