Photovoltaics Economics Calculator

The photovoltaics economics calculator is an asynchronous CGI and JavaScript application co-authored by programmer Karen Pease and actuarial student Elaine Mahler. This web-based tool allows you to describe in detail your solar setup and provides a detailed breakdown of what sort of power you'll get out of it and how economical of an investment the system will be. It is provided free under the GNU Public License v3 (sources here: (1)(2)(3)(4); the TMY2 solar data is from the Renewable Resource Data Center and goes in a directory called tmy2), with no warranties, either expressed or implied.

Fill in as many blanks as possible with information relevant to your desired solar power setup (location is required). Fields that are automatically calculated have a checkbox next to them. You can override any automatic calculation by filling out the blank yourself. The checkbox next to it will check itself. To have that field go back to being automatically calculated, simply uncheck the checkbox. To reset everything, reload this page.

How it works: All simple calculations, such as determing the cost of a given number of cells at a given price or the total power generated in a given month based on the average daily generation in that month, are done in Javascript and can be viewed by the "view page source" option in your browser, in the function "recalculate". The actuarial calculations are done there as well. The elements that need to be simulated -- power in from and out to the grid, minimum, maximum, and average daily generation -- are done in the asynchronous CGI component based on the values that you provided (click on the link to view the program's source). The program reads a TMY2 file from nrel.gov and simulates a typical year, hour by hour. The amount of sunlight on a given amount of panel is based on the direct normal radiation times the cosine of the angle between the sun and the panel, plus the horizontal diffuse radiation times the amount of sky the panel is pointing at, plus the amount of global radiation times the albedo of the ground times the amount of ground the panel is pointing at. All of this is adjusted by the efficiency of the panel at a given temperature, where the temperature is determined by the radiation equilibrium temperature averaged with the ambient temperature by an amount dependent on the level of panel insulation and the windspeed.

Given the same conditions, power generation results tend to be only a few percent off from the data produced by PVWatts

For programming additions/fixes, contact Karen Rei Pease at meme@daughtersoftiresias.org. For an improved calculation featureset, contact C. Elaine Mahler at galadriel@queenoflorien.net.



Select a city and state (required):
State City

Photovoltaic panel options: Customize these settings to match the listed values for the model of panels that you are considering purchasing. If you are unfamiliar with any of the options, just accept the defaults.
Panel cost Nominal watts
per panel 		Cost per
nominal watt 		Weight per panel Panel width Panel height Panel area Nominal efficiency Nominal operating
temperature 		Power degradation
due to temperature
$ W $ kg m m m^2 % °C %/°C

System configuration: Set how much area there is to place photovoltaic cells ("Available Width" and "Available Height"), how much obstruction of the light there will be in a typical day from sunrise to sunset ("Obstructions") (include obstructions all the way to the horizon and obstruction by other cells)**, and how tightly the cells fit together ("Cell coverage"). If you plan to have your cells track the sun, use a low coverage percentage in order to give them room to turn. The derate factor indicates what percentage of the generated power isn't lost due to inefficiencies. Insulation is a rough percentage to indicate how well the backside of the panels are thermally insulated from their surroundings, such as being up against a roof (100% = no heat transfer) Ground albedo is how shiny the ground is (fresh asphault=0.05, old asphault=0.12, grass=0.2, gray concrete=0.4, white concrete=0.75); don't include snow, but do include things like the sides of buildings and trees that are "in view" of the panels). Emissivity is how well an object radiates heat.
(** - Even with 100% obstructions, the calculator will still indicate a small amount of power production. This is not a bug. It comes from ambient lighting -- the global horizontal radiation times the ground (and thus obstruction) albedo.)
Cell coverage
(% each way) 		Available Width Available Height Obstructions (%) Derate factor Insulation Panel emissivity Ground albedo Ground emissivity
% m m % % % % % %

System calculations: These fields are typically calculated from the type of panels and their layout (as specified above), but you can customize them here if you so choose.
Panels wide Panels high Total panels Total area (m^2) Total nominal wattage Total weight
m^2 W kg

Angles and tracking: Collector tilt is the angle formed between the cells and a flat surface, such as the ground (an angle similar to your latitude is usually best). A panel pointing straight up at the sky has an angle of 0°, while a panel pointing at the horizon has an angle of 90°. If your system is to use heliostats (sun tracking), select which axes it tracks and specify a cost per square meter.
Collector tilt
(Optimal =
30.00°
Heliostat type Heliostat cost
per m^2 		Heliostat cost
° $/m^2 $

Batteries: On-grid users shouldn't typically worry about battery banks unless they either want backup power for their house or their utility won't purchase power back from them. Off-grid users will need batteries. To get the power of a battery in kilowatt hours, multiply its rated amp-hours times the battery voltage and divide by 1,000.
Battery capacity Battery price per
kWh 		Battery voltage
Battery costs Battery lifespan Annual battery
replacement cost
kWh $/kWh V $ years $

Maintenance: Things break over time. Some things, like many kinds of batteries, go bad at predictable intervals. Terminals corrode. Solar cells slowly degrade. Accounting for these costs will be done by analogy with annuities (funds that pays you at regular intervals) with an infinite number of maintenance payments that go up along with the inflation rates. The cost of these annuities will be rolled into the system purchase price. This section is divided into two parts: an annuity for general maintenance and one for the batteries specifically. Each can have its own inflation rate. Thus, if you think the price of the occasional replacement solar panel and wiring is going up at 5% per year but batteries are going down at -2% per year, you can set those respective inflation rates. Notice: If any inflation rate that you set is higher than the rate of interest (next section), unlimited maintenance on the system will never be affordable.
Battery inflation
rate 		Battery annuity
cost 		Other components
maintenance cost 		Other components
inflation rate 		Other components
annuity cost
% $ $/yr % $

Costs: A typical solar setup will involve costs beyond batteries, sun tracking, and the cells themselves. These can include inverters, charge controllers, installation, and so forth. Charge controllers are only needed when you use batteries. Inverters convert DC to AC. Different states and municipalities offer different tax benefits for solar power installations; if you can estimate how much such breaks will reduce your total costs, enter that percentage below. Tax breaks are assumed to not apply to maintenance.
Inverter power
requirements 		Inverter price
per watt 		Inverter price Charge controller
amps 		Charge controller
price per amp 		Charge controller
price 		Installation Panel costs Tax breaks (%) Total costs
kW $/W $ $/Amp $ $ $ % $

Economics: You can see how much you average paying per kilowatt hour of electricity on your electric bill, but will need to look up whether you can resell it at the same rate. To take into account the hidden costs of typical grid power sources (like coal) on the environment (pollution-induced health problems, destruction of habitats, etc), increase the costs for both electricity purchase and resale. For offgrid homes, don't worry about these fields. The annual nominal interest rate is the rate that you would have to borrow money at to fund this purchase (if you already have the money for your solar setup on-hand, it can be viewed as the rate you could be making by investing that money in the market instead). The electricity inflation rate is how you expect electricity prices to change as time progresses.
Annual nominal
interest rate 		Electricity
purchase cost 		Electricity
resale cost 		Electricity
inflation rate
% $/kWh $/kWh %

Hourly power usage: A person's use of electricity varies over the course of the day. At night, the typical household uses little power -- only their baseline of phantom loads, always-on appliances like refrigerators, non-peak air conditioning, and so forth. During the waking hours, more power is drawn for lighting, cooking, entertainment, etc. A major load during the summertime is air conditioning, which peaks during the afternoon. Select values that you feel will roughly represent your yearround average draw at different times of day, or just accept the defaults.
Time Percentage of
max consumption
12:00 AM - 1:00 AM %
1:00 AM - 2:00 AM %
2:00 AM - 3:00 AM %
3:00 AM - 4:00 AM %
4:00 AM - 5:00 AM %
5:00 AM - 6:00 AM %
6:00 AM - 7:00 AM %
7:00 AM - 8:00 AM %
8:00 AM - 9:00 AM %
9:00 AM - 10:00 AM %
10:00 AM - 11:00 AM %
11:00 AM - 12:00 PM %
12:00 PM - 1:00 PM %
1:00 PM - 2:00 PM %
2:00 PM - 3:00 PM %
3:00 PM - 4:00 PM %
4:00 PM - 5:00 PM %
5:00 PM - 6:00 PM %
6:00 PM - 7:00 PM %
7:00 PM - 8:00 PM %
8:00 PM - 9:00 PM %
9:00 PM - 10:00 PM %
10:00 PM - 11:00 PM %
11:00 PM - 12:00 AM %

Monthly power usage: This section graphs where power and money are flowing throughout the course of a "typical" year. You will generally only want to customize the first column -- your monthly consumption figures -- by what your electricity bill reports you as consuming. If you are planning on an off-grid house, anywhere that your minimum daily charge reaches zero would be a blackout. If your "power out to grid" is still significant in a month where your charge gets down to zero, you need more battery capacity. If not, you need more generation capacity, less power consumption, or an alternative power source.
Month Consumption Power in from grid Power out to grid Minimum daily generation Maximum daily generation Average daily generation Minimum battery charge Total generation
(AC) 		Power surplus Cash flow Savings
January kWh kWh kWh kWh kWh kWh kWh kWh kWh $ $
February kWh kWh kWh kWh kWh kWh kWh kWh kWh $ $
March kWh kWh kWh kWh kWh kWh kWh kWh kWh $ $
April kWh kWh kWh kWh kWh kWh kWh kWh kWh $ $
May kWh kWh kWh kWh kWh kWh kWh kWh kWh $ $
June kWh kWh kWh kWh kWh kWh kWh kWh kWh $ $
July kWh kWh kWh kWh kWh kWh kWh kWh kWh $ $
August kWh kWh kWh kWh kWh kWh kWh kWh kWh $ $
September kWh kWh kWh kWh kWh kWh kWh kWh kWh $ $
October kWh kWh kWh kWh kWh kWh kWh kWh kWh $ $
November kWh kWh kWh kWh kWh kWh kWh kWh kWh $ $
December kWh kWh kWh kWh kWh kWh kWh kWh kWh $ $
Annual: kWh kWh kWh kWh kWh kWh kWh kWh kWh $ $

Results: The payback time is the simple determination of how long it would take for what you spent to be equaled by your savings on electricity. However, one shouldn't consult payback time alone. For an economically sound purchase, look for a mortgage length that isn't marked "Infeasible". The mortgage considers how long it would take for a purchase of a solar power setup to be a more sound investment than in another investment (see "Annual nominal interest rate" near the top of this page). The shorter the mortgage length, the better. Another method is the Internal Rate of Return, or "IRR". Set the length of the period you would like to stretch the investment over, and it will calculate the effective annual rate of return you'd get for your investment over that period.
Payback time Mortgage length IRR Period IRR
years years years %