Welcome to Cranky Puppy Farm!

This blog belongs to two Gen X-er's smackdab in downtown Kansas City where we've been renovating and decorating two old Victorians built in the 1890's. Our life is filled with 3 demanding Pomeranians (1 of them cranky, of course), honking cars, noisy neighbors and the hustle and bustle of city life but we dream of the day when we can move to our 40-acre farm and hear nothing but the wind and the cows next door. Until then, we're chronicling our triumphs and mishaps here as we try to garden and preserve on 2 city lots, raise chickens, and learn all those things we should have learned from our grandparents. Welcome to our world - we hope you'll stay awhile!

Solar: Sizing Our System

Wednesday, February 26, 2014

Welcome to the fifth post in the series related to our solar energy project.
 Click here to start at the beginning!

Time to get back to talking about our solar project!  When we last talked about this, we were astounded at what big energy hogs we are. At an average daily usage of 45 kilowatt hours (kWH) per day, we're using a good 33% over what the average American household of 4 uses! 

So why is that important?  Because the first decision you make when looking at "going solar" is how much energy you need to generate (or want to generate).  Do you want to try to generate 100% of your energy or just offset 50% of it?

In our case, there were several things to consider:

  • Our utility company is offering a $2.00 per watt  rebate and it's capped at $50,000.  That means we can install a system up to 25 kilowatts and get fully reimbursed.
  • The rebate requires that the system be operational no later than June 30, 2014.  After that, the rebate drops to $1.50 per installed watt.
  • If the system is under 10 kilowatts, the utility is required to approve an application within 30 days.  If it's over 10 kilowatts, they have 90 days to approve or deny.  If they took the entire 90 days, we'd be looking at not having approval until mid-May and that wouldn't give us a lot of time to get everything installed.
  • With a system over 10 kilowatts, the utility and city have additional requirements: more expensive city building permits, we'd be required to have $100,000 in additional liability insurance on the property, etc. 

Since we want to take advantage of the larger rebate amount, we pretty much decided that 10 kilowatts was the max size for our system - we'd be assured quick approval and also it would be manageable in terms of size for us DIYers (as we'll see in just a minute).

Now I'm sure you're thinking "But that's only about 1/4 of the energy you use each day!"

This is where we need to talk about the difference between kilowatts and kilowatt hours.  Solar panels are rated in watts, and there are 1000 watts in a kilowatt.  If you have a 250-watt solar panel, that means they are rated to produce 250 watts per hour under ideal conditions (no cloud, sun directly above them, etc.)  But the sun shines more than 1 hour per day (except for you poor folks up in Seattle or Portland).  When you're sizing your array, the  next piece of information you need is the "peak sun hours" for your location.  Here's a chart for Kansas City:

The sun's higher in the sky in the summer and more directly overhead, so it makes sense that we get over 6 hours per day that are ideal for solar during those months.  Conversely, we get just 1/3 of that in the winter months.  These numbers apply only for our location, so you'll need to find the peak sun hours for your specific location by doing a Google search (there are lots of sites out there).  Here's a map that provides a good rule of thumb for each state:

Now we have all the information we need to determine how much a solar array can generate each day.  The formula is this:

# kilowatts  X  peak sun hours  =  kilowatt hours generated

In our case, we'll plug in the numbers for our proposed system as follows:

10 kilowatt (kW) system  X  4.5 hours  =  45 kilowatt hours (kWH) generated per day

Since we're using an average of 45 kWH and our 10 kW system can generate that much per day on average, then we can come close to offsetting a large chunk of our energy.  But not all of it...solar panels only generate energy when the sun is shining so we'll still need to use power from the electric company during the evening or when it's cloudy outside.  An arrangement where you are tied to the grid through your utility is called netmetering and, in most of these arrangements (including ours), you'll receive credit for the extra energy that your solar panels generate but that you don't use.  That would help offset the energy you'll use at night.  By the way, solar arrays that are tied to the grid are also referred to as grid-tied.

There are also off-grid systems that are stand-alone (not tied to the grid).  They're self-sufficient and are great for locations that may not have utilities readily available.  In order to store energy for use at night, these off-grid systems use a bank of special batteries to store the energy generated by the solar panels during the day.  These batteries are expensive and our utility company frowns on their use for grid-tied systems, so I'm not going to spend a lot of time on them.  If you're interested in learning more about this, just Google "off-grid solar" and you'll be overwhelmed with all kinds of information.

Okay, so what does a "10 kilowatt system" look like?  The answer is that it depends.  Not too long ago, 190-watt panels were all the rage.  But technology marches on and, while 250-watt panels are still favored by installers (at least here in KC), there are now panels available in the 305-watt and 310-watt range.  The higher the wattage per panel, the less panels you will need.  That also means they will take up less space and your cost will be less.

As an example, let's use 250-watt panels and do the math on how many panels we'll need.  Remember, a kilowatt is 1,000 watts.  Therefore, the number of panels we need is:

10,000 watts   /   250 watts per panel   =   40 panels

Now let's look at the same 10 kilowatt system with 305-watt panels:

 10,000 watts   /   305 watts per panel   =   32.8 panels (round down to 32)

Since solar panels are generally around 3.5 feet x 5.5 feet and cost around $320 right now, using the more efficient panels is saving us 126 square feet of space and about $2,560 in the cost of the additional 8 panels. 

This post has gotten pretty long-winded, so I'm going to stop right there for tonight and come back tomorrow and pick up where we left off.  Next up, we need to talk about finding a location for our panels and then we'll talk about the specific system we selected. 

Stay tuned!  If you're interested in more on our solar project, please check the "Solar Project" section on the bar to the right.

I've shared this post with this week's Down Home Blog Hop and Backyard Farming Connection Hop.  Go check them out!


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