Solar panels are a big investment, so before you install a PV system, you’re going to have to figure out if it’s worthwhile. Remember, expensive doesn’t mean wasteful, you’re just going to have to do some math to see where the clean energy equation balances for you.
While the discussion below is primarily about installing panels on a roof, placing panels on a pole mount is another option. It increases the cost of the system, but the benefit of being able to face your panels perfectly south (instead of being constrained by your roof’s orientation) might make it worthwhile for your situation.
Shading
First things first– do you have a good roof for solar? You should have little or no shade during prime sunlight hours, 9 a.m. – 3 p.m. Solar installers use a device called a Pathfinder to indicate where shading occurs during each time of the year. This will help you determine if shading will be a problem or not. You may need to move your panels to one side of the roof or the other, or you may need to consider trimming a tree.
Roof pitch
If your panels will remain fixed, the slope of your panels should be about the same number as your latitude. For example, Baltimore’s latitude is 39°. At about 45°, my roof is pitched about 6 degrees too steep for optimum energy collection, but because a steeper tilt is better for winter collection – when the sun shines the least – I’m okay with it. If you’re looking for optimizing collection in a particular season (i.e., if the panels are for your summer house, lucky you) then check out The Energy Grid, which offers a more detailed explanation of the proper tilt for each season.
Roof orientation (azimuth)
It’s best if your roof faces south. Determining this is trickier than it sounds, because compasses lie about north like an actor does about his age - it's close, but not quite the whole story. This is because compasses only measure magnetic north, which is different from true north. True, or geographic, north is the axis around which the earth rotates, and the one that determines your orientation to the sun. The magnetic north pole is in a spot hundreds of miles away from true north, and migrates around the general area of the North Pole.
Depending on where you live, you need to find out the declination (amount of degrees you add or subtract from the magnetic north reading) to get true north. If all this seems way too complicated, ask your solar installer to figure it out for you. Or, check out your house on Google Maps and eyeball it to get a rough idea of your orientation before you call for an estimate. Also, most GPS's can be set to orient to either true north or magnetic north. Interesting note: if you’re in Chicago, true north and magnetic north pretty much line up.
Calculate yourself
The next step is determining how many panels you need. There are two factors to consider here: your electricity usage, and cost.
It’s important to know how much electricity you actually use per month. Then find out how much you pay. The amount you use and what you pay per kilowatt hour (kWh) change during different times of the year. Look at a year’s worth of bills and figure out an average. Looking at multiple years is even more helpful. The great news is that electricity prices are generally higher in the summer – just when solar panels are putting out the most energy.
Next, stop using so darn much! The cheapest kilowatt is the one you never use. We used a Kill-a-Watt to determine which appliances were really costing us. Unfortunately, many were things like the fridge, which we really need. But some were expendable, like the space heater we thought we were just using “every now and then” in a cold bedroom. Turns out it was sucking down 10 kWh a day. At 80 days a year of use, this translates to about 800 kWh, or $80, a year. We paid an HVAC company $450 to create larger & better insulated duct work for the rooms that were too cold in the winter and too hot in the summer. We should make the money back in six years, but more importantly, we’ve retired the space heater and all the carbon it spewed. Plus, the rooms are more comfortable.
Over the past two years, we’ve cut our average electric consumption rate by about 310 kWh a month. In our area, this translates into a savings of $490 a year. Most of this was due to replacing about 75% of our light bulbs with CFL’s and then yelling at each other to turn off the lights. It’s become a friendly competition, where no one wants to be the idiot who left the carbon burning. With bedroom lights, it’s easy to fix the blame, but in common areas like the basement - it’s a witch hunt. It only takes one person to change a light bulb, but it takes an entire family to keep it off.
Next, we plugged our TV, DVD player, and game consoles into one power strip with a switch that we turn off when they aren’t being used. This reduces our phantom load – the electricity remote controlled components use when they are turned “off.” I forget to turn off the power strip far more than my teenagers do, which gives them a chance to show me up.
Calculate your Panels and Payback
Now that you have a pretty good idea of your electric usage, you’ll want to know how much you can afford to offset with solar power. This is probably the most fun part of the process. Go to the website for PV Watts, input your location data (including the roof orientation and pitch you calculated above), and the size of the system you are considering, and it will give you a good estimate of the annual output you can expect. This is particularly useful when comparing panels with different power ratings from different contractors. You can use the annual output to determine the amount you would pay for each kWh you will produce. Here’s what you need to do:
- Subtract all incentive programs from the total system cost to determine your “net cost.”
- Use PV Watts to determine your annual electricity cost savings.
- Divide your net cost by your annual savings to determine the number of years needed to recoup the net cost.
- Repeat steps 1-3 for a few different system sizes until you find the payback time frame you require.
Because of limits on the incentive programs, (for example the Federal tax credit was for 30% of the total cost of the system, but was capped at $2,000) there was an upper limit to the number of panels we could place on our home and still break even in less than 20 years. However, some of these limits have been raised (Maryland) and completely eliminated (Federal) for 2009, so the current payback time for system like ours has been cut down to under 10 years.
Finally, don’t forget to calculate the carbon payback. Over 20 years, we will have prevented 82 tons of CO2 from entering the atmosphere. I would need to plant 12 trees a year, each year, for 20 years to have the same effect. Twelve trees might not seem like much, until you think about the amount of space a tree actually takes up when it’s fully grown. Now think of 240 of them. I don’t have near enough space for those trees on my lawn, but I did have room for 18 panels on my roof.