How to Use the PVWatts Calculator
Now you’re ready to use the actual calculator and get your numbers for monthly solar radiation, AC energy in kWh your solar panels will produce, and energy cost savings at your location.
You should see the following screen, which is the only page you have to complete:
On this screen, the NREL has entered default values for all these fields. We’re going to take you through each one to help you understand what they mean, and how to fill it in.
DC System Size
This will play the greatest role in the numbers you get from the calculator. Most residential systems will be less than 10kW. Commercial systems and large farms can be quite a bit larger, even into the megaWatts range (1 MW = 1000 kW).
How do you know how big your solar power system should be? Eventually you’ll need to talk with an expert. But if you look at your power bill, you can see how many kilowatt-hours you’re using each month.
A 1 kW solar energy system means it produces 1 kilowatt of energy per hour – of direct sunlight. So in southeastern states like Georgia, South Carolina, Florida, and Mississippi, you get a lot of sunlight. If you average 8 hours of direct sunlight per day, then a 1 KW solar system will produce 8 kWh of energy per day. Multiply that by 30 and you get your kWh per month.
Here’s a simple formula you can use:
x KW system * hours of sunlight per day * 30 = approximate kWh per month
See how much sunlight Georgia averages per day
If you look at your current bills, you might want to do this in reverse to find out how many total kWh you need to produce. If you want to produce 300 kWh of solar energy per month, then divide that by 30 to get your kWh per day. Then divide that by the average hours of sunlight per day. That tells you about how large a solar power system you’ll need to get free electricity.
Here, you have just three choices – standard, premium, and thin film. “Standard” is the most common type of solar panel used in the industry – polycrystalline silicone, with efficiency of about 15% (though it’s advancing higher with new technology). “Premium” would most likely be monocrystalline silicon – more expensive but also more efficient, usually 20% or higher. Thin film is a different kind of technology, usually cheaper but not as durable.
Learn more about thin film vs crystalline solar panels here
This affects your output because of the efficiency. More efficient means more power output for the same sized system.
You can choose between fixed, roof, and several types of trackers. Trackers cost more money, but these are solar panels that adjust their position throughout the day to maintain maximum direct sunlight as the sun’s position changes in the sky. So trackers are more efficient, but more expensive.
Most residences will choose roof-mounted panels, but sometimes ground mounted is better depending on shading and other factors. For now, choose the one you think you’re most likely to go with.
This is the most complicated part of the calculator. To adjust this number, you click on the “Loss Calculator” button. The NREL has provided default values for all these metrics. For now, just use their values and don’t change this number.
We’ve created another whole article on how and when you can adjust these values. Read the article here.
For now, the biggest ways to lower this number (which is good) is to install microinverters or power optimizers.
Geometry time! Get your protractor!
Tilt is the angle of your solar panels compared to the ground. So, panels lying flat on the ground (or flat on a flat roof) would have a tilt of 0o. Panels standing straight up and down (which would be silly) would have a tilt of 90o.
So, most roof-mounted panels usually have angles between 20o and 40o, depending on the ‘pitch’ of your roof and the latitude of your house on the earth. Why? Because the higher up in latitude – the closer to the North Pole – the more your panels have to be tilted to receive the most direct sunlight.
To use an extreme example, in Alaska, the sun is at a very low angle in the sky most of the year. So if someone in Alaska wanted solar panels, they’d have to tilt them at a much higher angle than most other places on earth need to do. It’s all about maximizing direct sunlight.
This again is why some people install trackers. As the sun’s position in the sky changes – and not just by day, but by season – the tracker keeps the solar panels directly in line with maximum sunlight.
You’ll notice the PVWatts calculator uses a default tilt angle of 20o. If you don’t know what your tilt will be, just leave it at that.
But if you intend to install panels on your roof, you can go measure the “rise” and the “run” of your roof with a tape measure.
Just go outside and measure a length along the horizontal of your roof (the “run”). Then, from the end point of the run, measure up vertically until you reach the roof itself. In other words, you’re making a right triangle, and you’re measuring the horizontal and vertical sides. The roof itself would be the “hypotenuse”, if you remember your geometry, but you don’t need that number.
Then, use the table below (the same table shows up if you click the “i” on the PVWatts calculator), and calculate your rise/run to find your tilt. And again – this assumes you’ll be laying your panels flat on your roof. If you ended up angling them differently than the pitch of your roof, that would adjust this number. But you’d only do that if it would lead to collecting more direct sunlight, which would make these numbers work out more in your favor. So this is a “worst-case” scenario.
Tilt referred to the angle of the panels from the ground. Azimuth is the angle from the sky – the bird’s eye view.
Why does this matter? Same reason. The sun rises in the east. So an east-facing solar panel will get more sunlight in the morning, and a west-facing panel gets more in the afternoon.
A perfectly south-facing solar panel has an azimuth of 180o. For northern hemisphere houses, this is ideal. But if you don’t use trackers, it not that big a factor. Facing west is 270o, and east is 90o. The table below shows the values for other directions.
If you’re not totally sure, just eyeball it and estimate the direction your roof faces (if you’re planning on roof-mounted panels). Again, if you’re off by 10o, it’s not going to matter much.
DC to AC Ratio
Click the “Advanced Parameters” section to see three more metrics, including this one.
The DC to AC ratio is power output. A larger system increases the ratio, and the cost. The typical range is from 1.10 to 1.25. For really large commercial systems this number can be higher. Higher is good.
The PVWatts calculator assumes a 4 kW system and a 1.10 ratio. (See the DC System Size section above for more about the size your system might be), which is in line with a common residential system. If you have a farm or other agricultural business, you’ll probably be looking at a minimum of a 10 kW system. But that doesn’t mean this ratio will change.
To get a higher number usually costs more, but it also depends on orientation (azimuth and tilt), and on the location of the house, farm, or business.
Unless you have technical data saying otherwise, just leave it at 1.10 for now.
The inverter converts the DC solar energy into the useful AC current that your power outlets use. Every solar power system has to have an inverter.
Each inverter is rated based on its DC to AC conversion efficiency. You can see dozens and dozens of inverter ratings in the Renewable Energy Handbook, available for viewing here.
The PVWatts calculator assumes a 96% efficiency, but these can range a fair amount above 90%. Like the “loss calculator” we kind of skipped by earlier, microinverters and power optimizers can improve this number, increasing it to 99% in most cases.
Ground Coverage Ratio
If you’re using roof or ground-mounted solar panels, this term doesn’t apply. This ratio only matters if you’re planning to use trackers. If so, consult the technical manuals.
The simplest one of all for this calculator – it’s either residential (homes) or commercial (businesses). If you have a small farm, you still might go with residential. But for larger agricultural operations like poultry farms, you should go with commercial.
Average Cost of Electricity
This helps determine how much money you’ll save with solar power. Look on your electric bill and find your per-kWh costs. This rate is what you want to reduce or eliminate with your solar panels.
The PVWatts calculator assumes you will recover 100% of your energy costs, but does not include net metering or things of that nature, which some states and cities offer.
Get Your Report!
Once you’ve filled all these values in, get your solar payback report. Here’s a sample of what it looks like. This report is for a farm in Vidalia, Georgia, planning to install a 10kW solar power system.