Identifying the Best Solar Panel Calculator





Businessman standing by solar panels

There are plenty of solar panel calculators out there for understanding your solar energy potential and cost savings, as we discussed in a

previous article. The criteria for selecting the best calculator can be condensed into two for simplicity. First, does the solar calculator cover your region?
As noted, PVGIS developed by the Joint Research Center in Italy and funded by the European Commission is excellent for Europe and North Africa but no good for North America since its database is regional.
Second, is the tool simple and intuitive to use? Many tools such as Solar Prospector or System Advisor Model (SAM) are too complicated for most homeowners to use and make sense of the data.
Both Solar Prospector and SAM are developed by the trusted US-based National Renewable Energy Laboratory (NREL). RETScreen is another example of a powerful but complex tool.
Developed by National Resources Canada, it can compare between many renewable energy sources and generate both power and financial charts.
However, it requires the user to download a desktop software which then requires the user to input a variety of data before generating any results.

So for a primarily North American audience,
let's compare the two main contenders: PVWatts vs. WhatNextNow Solar Discover.

Presenting PVWatts: Free, Simple and Intuitive


For the United States, one of the best available tools is PVWatts.
Developed by NREL, PVWatts is a free, simple and intuitive tool which allows you to put in a home or business address.
The tool finds the closest location to you which has a weather data file in its database. If there are multiple options, you can choose which data source to go with. Then,
it gives you several boxes to change the solar panel system info, as well as the utility cost.  Economic incentives for solar power such as tax credits and utility cost reductions are also shown.
Under the solar panel system, you can click on "Draw Your System" to draw a bounded figure within an interactive satellite map to designate the size of your solar panels.

This leads to a chart detailing the amount of incoming solar radiation,
the solar electrical output, and the value of this output in dollars for every month, and overall during the year. In addition to the chart, a dollar per kilowatt-hour (USD / kWh) comparison between the grid system cost and the solar panel system cost is available.
This gives you a baseline to decide whether solar panels will be worth it. Data tables can be downloaded for each month or even each hour.

WhatNextNow Solar Discover: An even simpler alternative with some added benefits


Another tool that is useful for both Canadian and American residents is our very own WhatNextNow Solar Discover.
The user interface for WhatNextNow Solar Discover is even simpler than that of PVWatts. Simply, go to WhatNextNow Solar Discover
and input an address or postal code. Unlike PVWatts which uses localized ground stations in the United States and its territories, limiting its analysis to the United States,
WhatNextNow Solar bases its data analysis on a global data set allowing the tool to be used anywhere in the world.

After you select your site, WhatNextNow Solar Discover provides the monthly average solar radiation based on the last 20 years of data. Next, users can check out the Energy page which shows the total energy produced in kWh for each corresponding month.
You can also change the specifications of your solar PV system, such as the slope, power capacity, orientation, PV efficiency, and system losses. The energy graph updates automatically as you change each parameter. At the bottom of the screen is an interactive satellite map where you can draw a bounded figure to calculate the size of your solar panels.
The power capacity is updated automatically to reflect the size of the solar panels you draw.

Next, the Finance page yields a graph of revenues and expenses for the next 25 years based on financial data including the cost of electricity in USD / kWh, system cost in USD, project lifetime,
and inflation. This tool gives you a comprehensive look at the cost of your solar panel project and how much money it will generate for you over the years. Once again, the graph updates automatically with each adjustment.


A bonus feature of WhatNextNow Solar Discover is the ability to save your session with the key information on your site, the solar PV system you configured and the resulting financial analysis.

Battle of the solar panel calculators:
Lancaster, CA


Now let's look at a specific example to compare WhatNextNow Solar Discover and PVWatts data for a home in Lancaster, California. Lancaster has recently href="http://www.forbes.com/sites/peterdetwiler/2014/05/13/can-a-city-get-to-net-zero-lancaster-california-mayor-thinks-so/" target="_blank" rel="noopener"> passed a law requiring that starting from January 1, 2014,
all single-family homes must have a solar PV system generating 1 - 1.5 kilowatts (kWs). Presently,
one in five newly-built homes in California is solar powered and this trend is projected to grow.

First, we will set up the same site using both online solar panel calculators to estimate solar radiation and electricity output using identical solar panel configurations.


Let's start with WhatNextNow Solar Discover:

Clicking the right arrow goes to the Energy page where we'll tweak a few parameters to better compare with PVWatts:

WhatNextNow Solar automatically changes the monthly average energy in kWh and the annual energy produced as these values are changed. For the parameters selected, the annual energy produced is 8457 kWh. WhatNextNow Solar also has a cool feature showing a few statistics about what this amount of energy can be used for,
such as charging a smartphone for 241 years, powering a refrigerator for 1 year, driving a Tesla Model S for 25,371 miles, and powering the International Space Station for 4 days.

Clicking the right arrow again leads to the Finance page where the levelized cost is given as 0.14 USD / kWh with a capacity factor of 24%.  href="/home/solar/what-is-the-levelized-cost-of-solar-energy/" target="_blank" rel="noopener">Levelized cost refers to the cost of building and operating this system over a period of time. The
capacity factor is the percentage of time that the system is operational at full capacity, based on the availability of the sun.

Now, let's use PVWatts for the year are also displayed. Here, you see that 8,568 kWh with an energy value of $1,351 is generated by the solar panels by the end of the year.
The average cost of electricity purchased from the utility is 0.16 USD / kWh while the cost of electricity generated by the system is 0.14 USD / kWh. PVWatts unfortunately does not have a more detailed financial analysis tool like WhatNextNow Solar and presents no information on levelized cost, capacity factor, annual revenues and expenses, etc.
Furthermore, only the data tables can be downloaded.

A side-by-side screenshot comparison of the results is shown above (please note that there are two screenshots for WhatNextNow Solar since unlike PVWatts, WhatNextNow Solar combines the inputs and outputs on one page but separates each section).

Our verdict: Both are free, both are fun to use
- but WhatNextNow Solar Discover offers some added benefits


WhatNextNow Solar Discover has two different pages for energy and finance to allow the user to play around with either the energy parameters or the finance parameters for instantaneous feedback on either page. The final results show that WhatNextNow Solar predicts 8,457 kWh generated in one year,
whereas PVWatts predicts 8,568 kWh generated. These two energy values are close enough to warrant a look at both tools. However, those who are interested in a more detailed look at the revenues and expenses for their solar investment may want to choose WhatNextNow Solar Discover.