## The Hands-Free Solar Panel Estimator Based On Your IP address

**Number of 200 Watt Panels You Will Need to Offset Your Monthly Electric Bill: 8
Configuration:** 200 Watt Panels

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**Energy Consumption:**200 Kilo-Watt Hrs per Month

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**Location:** Cambridge, MA 02125
Not Your Location?

**Latitude:** 42.3646 **Longitude:** -71.1028

**How do we know your location?**

We don't actually know your location. We used your **IP address**, or your **Internet Protocol** address. Every time you click on a link or request a specific web site, part of the information the website receives as part of your page request is your IP address. We're not special, it works this way for every webserver on the planet which receives a request for a page. Knowing the IP address helps to trace hackers who want to gain access to webservers illegally. Most of the time, your IP address changes each time you log in to the internet.

What we did is cross-reference your IP address to a generalized zipcode database, and from this, can usually determine the city from which you are accessing this page. Please rest assured, we don't know anything about you other than the general area from which you've requested this page (your home or work computer). Not Your Location?

**Not Your Location?** Sometimes the IP address is only the IP address of your internet service provider which could be in another city, or even state. If this is the case, you can correct your zipcode in the form to get the right numbers for your area.

**How do we know your monthly electricity consumption?**

We don't actually know your energy consumption. We used the average household consumption in the U.S. which is 200 Kilo Watt Hours per month. If you want numbers more closely reflecting your actual energy use, change the number below in the form. You can find your actual number on your electric bill.

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**Why did we pick 200 Watt panels?**

We picked an average number in the industry. If you would like to change the number, you can change the numbers to reflect a solar panel wattage you are using in the form.

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**Any other assumptions we used in this calculation?**

- We calculated how much solar energy hits your area based on zip code, using numbers from NASA.
- We used a typical solar panel efficiency of 15%. The solar panel efficiency is how efficient the panel is converting sunlight energy into electricity. You can look at the technical specifications of your chosen panel and enter the real number on the form.

For more detailed information on how we calculated this number, see the explanation below.

## How We Calculated Our Numbers

Solar Energy calculations start out with the ** insolation value**. Insolation is a measure of how much sunlight strikes a square meter on the earth in an average day. The map on this page was produced by National Renewable Energy Laboratory NREL, a government research laboratory affiliated with the department of energy. It shows the insolation values for the United States. The insolation values put out by the NREL takes into account weather, sunlight angle, atmospheric scattering of light, etc. Note that these are an average values taken over the years.

### Solar Energy (Insolation) Measuring Units

The units of insolation (sunlight energy) are Kilo-Watt-Hours (kWh) / m^{2} (area) / day.

Insolation = kWh/m^{2}/day

**Example:**

Insolation for Cambridge, MA = 3.84 kWh/m^{2}/day

**To get the Yearly Value, Multiply average by 365 days:**

Yearly Insolation = 3.84 * 365 = 1401.6 kWh/m^{2}/year

**Dividing by 12 to get the average monthly value:**

Monthly Insolation for Cambridge, MA = Insolation * 365 / 12 = 116.8 kWh/m^{2}/month

### Solar Panel Efficiency

A solar panel does not convert all solar energy hitting it to electricity, due to technological restraints. A typical solar panel is about 15% efficient, meaning it converts about 15% of the energy hitting it into electricity.

If we multiply by the efficiency of a solar panel we get the average amount of energy per meter squared a solar panel will produce in a month. Note: this estimate is only valid if the solar panel has absolutely no shadow on it.

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Monthly Insolation for Cambridge, MA after Efficiency = Insolation * Efficiency * 365 / 12 = 17.52 kWh/m^{2}/month

### How Energy Much Do We Need to Produce?

Next we take a look at the desired amount of energy to be generated per month. Note this is an average monthly value. For our calculations, we used 200 kWh - the average American household consumption of 200 kWh/month.

**Monthly Energy Target = 200 kWh/month**

### Surface Area to Produce Energy

The number of solar panels is done by calculating the amount of surface area of the panels needed. In ohter words, the average amount of energy needed per month divided by the amount of energy generated per meter square of solar panel on an average month gives us the area of solar panels required.

Square Area Needed m^{2} = Monthly Energy Needed / (Insolation * Efficiency * 365 / 12)

Let's see how the units cancel each other out in algebra:

m^{2} = (kWh/month) / (kWh/m^{2}/month)

This is the area the solar panels need to cover, operating at the given efficiency.

For Cambridge, MA, Square Area Needed:

**11.415525114155 m ^{2}** = (200)/ (17.52m

^{2})

### Wattage Rating is Dependant on the Size of Panel

For convenience for the user, the output of a solar panel is given in the number of solar panels by Wattage Rating type. ** The Wattage Rating** is a number the manufacturer assigns to the panel based on the Watts the panel generated under laboratory conditions. It isn't a valid means of saying how much the panel will actually generate, other then to say (roughly) that a 200W solar panel will generate twice the energy of a 100W solar panel.

**Wattage Rating = Watts/m ^{2}** (hypothetical)

Fortunately the industry uses standard testing conditions to determine the wattage rating on the panels. Also, the vast majority of solar panels all use the same technology (crystalline silicon). This means there is a strong correspondence of Wattage Rating to area regardless of manufacturer.

We next find the relation between Wattage Rating and the physical surface area of the solar panel. This ratio is expressed as W/m^{2}. For example, a 80 Watt solar panel has a ratio of about 122.57 W/m^{2}. 80W / 122.57 W/m^{2} tells us that a typical 80W solar panel has a surface area of about .65 m^{2}.

### Wattage Rating Curve

We decided to approximate the Wattage rating by using a series of tiers. After sampling many different solar panels from different manufacturers we came up with the following data:

1 W -------> 61.02 W/m^{2}

5 Watt ----> 69.55 W/m^{2}

10 Watt ---> 88.415 W/m^{2}

20 Watt ---> 103.62 W/m^{2}

40 Watt ---> 116.54 W/m^{2}

80 Watt ---> 122.57 W/m^{2}

120 Watt --> 132.20 W/m^{2}

After 120 Watts the ratio stays the same of about 130 W/m^{2}.

Note that these values are averages for each panel type. Almost all the individual values used to compute the averages were fairly close together. Any that were dramatically different were for obvious reasons (such as outdated technology) and eliminated. Also noteworthy is that this data strongly suggests that as the Wattage Rating goes down (the smaller the panel) the less efficient it is.

A default value of a 200 Watts is used for the type of solar panel. This means a default ratio of 130 W/m^{2} is used.

If the user inputs a different value for solar panel type the above data is used and a linear relationship is assumed between each data point. So if the user enters in 30 Watt solar panels to be used the W/m^{2} ratio is assumed to be half way between the 40 Watt and 30 Watt ratios. Note that these calculations are 'ballpark' figures to help the user become educated about solar panels.

Once we have the ratio for the solar panel type we take the Wattage Rating and divide it by this ratio. This gives us the typical size of this type of solar panel. Note we aren't talking about real Watts but Wattage Rating, which is important for identifying what type of solar panel.

Wattage Rating * Ratio = Watts / (Watts/m^{2}) = m^{2}

**Back to Our Example From Using 200 Watt Panels **

Next we take the total area of solar panels needed and divide by our typical solar panel size. Wattage Rating for 200 W Panel:

200 Watts/130 Watts/m^{2} = **1.5384615384615 m ^{2}**

This means you need 1.5384615384615 m

^{2}to get a full 200 Watts

### Finally

Number of Panels Needed = Area Needed / Size of Panels to get Full Wattage Rating.

Back to Our Example From Above for Cambridge, MA

11.415525114155 m^{2}/ 1.5384615384615 m^{2} = 7.4200913242009.

Next we take this value and round up to the nearest integer, because you can't install a partial panel.

8 Panels Needed