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Frequently Asked QuestionsHOMERLogo

General Questions

1 What is the difference between HOMER and Hybrid2?

2Why doesn't the Overall Optimization Results table show all of thesystems I asked HOMER to model?

Getting Data In And Out

1 Where can I find data to run HOMER ?

2 Can HOMER produce an input summary report?

3 Can HOMER produce a report summarizing simulation results?

4 Can I export HOMER's graphs into my own documents?

Economics Questions

1 Why does HOMER rank systems by net present cost?

2Why does HOMER require a base case to calculate payback and othereconomic metrics?

3 Is there a way to calculate simple payback by hand?

4 Does HOMER account for inflation?

PV Questions

1 Does HOMER account for the effect of temperature on the PV array?

2 How does HOMER calculate the output of the PV array?3 How does HOMER calculate the PV cell temperature?

4 How does HOMER calculate the radiation incident on the PV array?

5 Can HOMER model a maximum power point tracker on the PV array?

Battery Questions

1 How do I specify the DC bus voltage?

2 Can HOMER model a battery charge controller ?

International Issues1 Is a Spanish language version of HOMER available?

2 Can I use imperial units in HOMER?

Miscellaneous Questions

1 Why do I have to scale load and resource data?

2 Why would I do a sensitivity analysis?

Page 1 of 12Frequently Asked Questions

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Written by: Tom Lambert ([email protected])Last modified: June 6, 2008

Finding Data to Run HOMER

One of the biggest challenges in using a model like HOMER is finding the numbers to put into it. This page is meant to help you find the information you need. If you know of any other helpful sources,please let us know so we can add them. Note that NREL cannot guarantee the accuracy of any datafrom external sources, and does not endorse any manufacturer or retailer.

Electric Load Data

The only source of hourly load data that we're aware of is the DG Marketplace, which you can visit atwww.dgmarketplace.com. This website sells typical residential, commercial, and industrial load data for

locations across the US.

Geography

To find your latitude and longitude, check www.multimap.com or www.maporama.com. Or check thelist of world cities at www.infoplease.com/ipa/A0001769.html.

To find your time zone, check www.worldtimezone.com.

Solar Radiation Data

HOMER will accept hourly or monthly solar radiation data. There are many sources of solar radiationdata:

l NASA's Surface Solar Energy Data Set provides monthly average solar radiation data foreverywhere on earth at http://eosweb.larc.nasa.gov/sse/.

l This help file contains a table of monthly solar data for selected worldwide locations.l The HOMER website provides hourly solar data for 239 US locations at

www.nrel.gov/homer/downloads/TMY2_Solar_Data.zip. This file contains 239 .sol files(containing data extracted from the TMY2 data set) which you can use directly in HOMER.

l The South African Renewable Energy Resource Database provides solar, wind, hydro, and

biomass resource data at www.csir.co.za/environmentek/sarerd.l The US Renewable Resource Data Center provides US data and maps at

http://rredc.nrel.gov/solar .l The World Radiation Data Center provides worldwide solar data at http://wrdc-mgo.nrel.gov.

Note that data from this website need a lot of processing to be useful in HOMER.l The HELIOGIS website provides solar data for Mexico at www.heliogis.com.

Wind Speed Data

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It can be difficult to obtain measured wind speed data. Proper measurement of wind speed is expensiveand time consuming, and average wind speeds can vary markedly over short distances because of terraineffects. For these reasons, it is often necessary to synthesize wind data from estimated monthly averagewind speeds.

Many countries have published wind atlases:

l Wind maps for many US states are available atwww.eere.energy.gov/windpoweringamerica/wpa/wind_maps.asp.

l The Canadian Wind Atlas is available at www.windatlas.ca.l The Brazilian Wind Atlas is available at www.cresesb.cepel.br/atlas_eolico_brasil/atlas-web.htm.

A number of other websites provide wind speed data:

l The US Renewable Resource Data Center provides information on wind data athttp://rredc.nrel.gov/wind.

l The website www.weatherbase.com provides monthly average wind speed data for many citiesaround the world.

l The Windustry website maintains a list of US wind data resources atwww.windustry.com/resources/windmaps.htm.

l The US National Climatic Data Center provides monthly average wind speed data for many UScities at www.ncdc.noaa.gov/ol/climate/online/ccd/avgwind.html. To convert from mph to m/s,divide by 2.23694.

l The Technical University of Denmark maintains a database of wind characteristics atwww.winddata.com. You can convert the 10-minute data from this site to hourly average data forHOMER using a conversion utility available athttp://130.226.17.201/software/HOMER/index.htm

l The European Wind Atlas, containing wind data for all of Europe, is not available on the Internet, but the book can be purchased from www.windatlas.dk .

Renewable Power System Components

Several retailers sell components for renewable power systems . The websitewww.ecobusinesslinks.com maintains a list of renewable power retailers around the world. A few havevery helpful websites providing cost and performance data for PV panels, wind turbines, hydro turbines, batteries, converters, and other system components. Check out:

l Real Goods at www.realgoods.coml The Alternative Energy Store at www.altenergystore.coml SolarEnergy.com at www.solarenergy.coml The Solar Biz at www.thesolar.bizl Bitterroot Solar at www.bitterrootsolar.coml The Energy Development Co-operative at www.unlimited- power.co.uk

Other sources of cost and performance data for renewable power system components include:

l The California Energy Commission, at www.energy.ca.gov/distgen/economics/economics.htmll The Renewable Energy Technology Characterizations, a detailed report on the performance and

costs of solar, wind, and biomass power systems, available atwww.eere.energy.gov/power/techchar.html. This report even includes predictions of future



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performance and costs out to the year 2030.l SolarBuzz, which provides price data for PV modules, inverters, batteries, and charge controllers

at www.solarbuzz.com.l Wind Power Online, an online marketplace for wind turbines at www.windpoweronline.com.

Generators

l A very useful document covering the technology, emissions, and costs of natural gas-firedreciprocating generators, microturbines, fuel cells, and Stirling engines is the Gas-FiredDistributed Energy Resource Technology Characterizations, available at www.eea-inc.com/dgchp_reports/TechCharNREL.pdf .

l The Distributed Generation Information Center provides summary technical and economic data onreciprocating engine generators, microturbines, and fuel cells, as well as a large library of links, atwww.distributed-generation.com.

l America's Generators provides prices on a wide range of diesel and natural gas generators andaccessories at www.americasgenerators.com

Emissionsl This help file contains a table of US grid emissions factors.

The US Environmental Protection Agency provides emissions coefficients for CO2, SO2, and

NOx for US locations at their Power Profiler website at

www.epa.gov/cleanenergy/powpro/screen1.html.

The EPA's eGRID website contains even more emissions data, including state-by-state averageemissions factors for all the pollutants that HOMER models.

l Additional data is available The EPA document Emissions Factors, Global Warming Potentials,Unit Conversions, Emissions, and Related Facts, November 1999, atwww.epa.gov/appdstar/pdf/brochure.pdf .

Policies and Incentives

l For information on current net metering policies across the Unites States, see the US Departmentof Energy web page on the topic at www.eere.energy.gov/greenpower/markets/netmetering.shtml

l For a list of renewable energy incentives across the United States, see the Database of StateIncentives for Renewable Energy at www.dsireusa.org.

Written by: Tom Lambert ([email protected])Last modified: October 7, 2005

Why Do I Have To Scale Load AndResource Data?

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People often wonder why they have to enter scaled averages for load and resource data. Can't HOMERust use the specified data without scaling it? Yes it could, but then you wouldn't be able to do a

sensitivity analysis on the load size or the wind, solar, hydro, or biomass resource. We introduced thescaled averages to allow users to perform sensitivity analyses on these important variables.

The first time you specify synthetic load or resource data, and every time you specify a data file,HOMER sets the scaled average equal to the average of the baseline data. So you only have to enter thescaled average when you want to scale the baseline data or perform a sensitivity on it.

Written by: Tom Lambert ([email protected])Last modified: May 26, 2004

Why Would I Do a Sensitivity Analysis?

There are good reasons why you might want to enter multiple values for a particular input variable.First, you might be uncertain as to what the exact value of some variable should be. By specifying arange of values, you can determine how important that variable is, and how the answers changedepending on its value. In other words, you can determine the sensitivity of the outputs to changes in thatvariable.

For example, image that a modeler doing a preliminary analysis of a wind-diesel system was uncertainabout three variables: the annual average wind speed, the average fuel price over the life of the project,and the interest rate. To determine the sensitivity of the system's cost of energy to those three variables,she did a sensitivity analysis using HOMER. Her best estimate for the wind speed was 6 m/s, for the fuel

price was $0.50/L, and for the interest rate was 8%. But she entered multiple values for each variable,covering the range of uncertainty of each. HOMER produced the spider graph shown below, showingthat the cost of energy is most sensitive to the wind speed (the wind speed line is the steepest). As aresult, the modeler chose to invest more time and money to obtain a more accurate estimate of the windspeed.

Another reason for performing a sensitivity analysis is to make a single analysis applicable to more thanone installations. For example, image you are designing small renewable power systems for six telecom

sites. If the sites are similar in most respects but there is some variation in wind speed from one site toanother, you could specify several wind speeds spanning the appropriate range. Then a single analysiswould be sufficient to design all six hybrid systems. The results of such an analysis might look like theexample shown below, where HOMER has suggested a wind/battery system for the site with the highestwind speed, a PV/battery system for the site with the lowest wind speed, and wind/PV/battery systemsfor the sites with intermediate wind speed.

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See also

Definition of sensitivity analysis

Written by: Tom Lambert ([email protected])Last modified: June 18, 2004

Does HOMER Account For Inflation?

Yes, but it assumes that all prices escalate at the same rate. With that assumption, inflation can befactored out of the analysis simply by using using the annual real interest rate rather than the nominalinterest rate. All costs are therefore in constant dollars.

Since HOMER assumes all prices escalate at the same rate, it not possible to model a situation where the

fuel price (or any other cost) escalates at a different rate. It is possible, however, to explore the effects ofan escalating fuel price by doing a sensitivity analysis on the fuel price.

Written by: Tom Lambert ([email protected])Last modified: November 1, 2004

Hybrid2

Hybrid2 is a hybrid power system simulation model developed by NREL and the University ofMassachusetts. It performs a more accurate simulation than HOMER does, but it does not dooptimization or sensitivity analysis.

Hybrid2 has several capabilities that HOMER does not:

l It can simulate many dispatch strategies.l It tracks the changing voltage of the DC bus and its effect on the PV array, DC wind turbines, and

the battery bank.l It accounts for the fact that the inverter efficiency is not constant, but rather gets worse at lower

power output.l It uses a more detailed algorithm to estimate the life of the battery bank.l In modeling the operation of the generators and the energy flow into and out of the battery bank, it

accounts for short-term fluctuations of the load and resources that happen within the time step.

On the other hand, HOMER has several capabilities that Hybrid2 does not:

l It can optimize the system configuration (simulate many system configurations at once and rankthem according to net present cost).

l It can perform sensitivity analyses, where you enter multiple values for an input to determine its

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effect on the outputs.l It can model grid-connected systems.l It can model run-of-river hydro turbines.l It can model biomass gasification.l It can model fuel cells.l It can model systems that use hydrogen for energy storage.l It can model systems produce hydrogen instead of electricity.

In some instances, it may make sense to use HOMER to design an approximately optimal system, thenuse Hybrid2 to refine the system design and further investigate its performance. In other instances, itmay make sense to use only Hybrid2 or only HOMER.

For more information on Hybrid2, please see http://www.ecs.umass.edu/mie/labs/rerl/hy2/intro.htm.

Written by: Tom Lambert ([email protected])Last modified: October 26, 2004

How HOMER Calculates the PV ArrayPower Output

HOMER uses the following equation to calculate the output of the PV array:

If on the PV Inputs window you choose not to model the effect of temperature on the PV array, HOMERassumes that the temperature coefficient of power is zero, so that the above equation simplifies to:

See also

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where:

YPVis the rated capacity of the PV array, meaning its power output under standard testconditions [kW]

f PV is the PV derating factor [%]

is the solar radiation incident on the PV array in the current time step [kW/m2]

is the incident radiation at standard test conditions [1 kW/m2]

aP is the temperature coefficient of power [%/°C]

Tc is the PV cell temperature in the current time step [°C]

Tc,STC is the PV cell temperature under standard test conditions [25 °C]



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PV Inputs window

PV derating factor

PV temperature coefficient of power

Calculating the radiation incident on the PV array

Calculating the PV cell temperature

Standard test conditions

Can HOMER model a maximum power point tracker?


Does HOMER Account for the Effect ofTemperature on the PV Array?

Yes. The PV Inputs window gives you the choice of explicitly modeling the effect of temperature on thePV array. If you choose to do so, you need to enter ambient temperature data, which HOMER uses tocalculate the cell temperature in each time step. If you choose not to explicitly model the effect oftemperature, you should still account for the temperature- related discrepancy between real and rated power output by reducing the PV derating factor .


See also

How HOMER calculates the PV array power output

How HOMER calculates the PV array cell temperature

Can HOMER Model a ChargeController?

HOMER does not model the battery charge controller as a separate component. So you must include itscost and efficiency in the values you specify for other components.

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If you are simulating a PV-battery system or a grid-connected PV system, the best place to include thecharge controller costs and efficiency is the PV aray inputs. In the cost of the PV array, include the costof the charge controller. Then reduce the PV derating factor to account for the efficiency of the chargecontroller. For example, if you want to use a derating factor of 90% to account for losses in the PVarray, and the efficiency of the charge controller is 95%, then change the derating factor to 90% * 95% =85.5%.


Why does HOMER require a base caseto calculate payback and other economicmetrics?

HOMER calculates payback by comparing one system with another. In general, payback tells you howmany years it will take to recover an investment. You invest a certain amount of money up front, thenearn income from that investment, and the payback is the number of years it takes for the cumulativeincome to equal the value of the initial investment.

But you can only rarely define the "income" of a power system without comparing to some alternative.If, for example, a simple grid-sales system does not need to serve any load, but simply sells power to thegrid, then you can define the income of the system as the money it makes by selling power to the gridminus the expense of operating the system. In that case, you could compare the initial cost of the systemwith its income to calculate the payback.

For distributed power systems, it's usually not that simple. Say you are designing a system to provideelectricity to an off-grid house. A pure diesel system might have low capital cost and high operatingcost, whereas a PV-diesel-battery system might have high capital cost and low operating cost. Neithersystem produces any income. In both cases you spend money up front to build the system, then you keepspending money each year to operate the system. The concept of payback has meaning only if youcompare one system to the other. You can define the payback of the additional investment required forthe PV-diesel-battery system by comparing the difference in capital cost with the difference in operatingcost. HOMER does exactly that in the Compare Economics window.

So, to calculate the payback of one alternative, we must compare to another alternative. Even to

calculate the payback of the simple grid-sales system mentioned above involves an implicit comparisonwith the base case, which is to do nothing. The do-nothing alternative is easy to compare to, since all itscosts are zero, but if we have an electric load to serve, the do-nothing alternative is not an acceptableone.

A renewable power system designer will often use a non-renewable power system as the base case, butHOMER lets you choose any base case you want. You could compare a system with two wind turbinesto a system that has one wind turbine to calculate the payback of the second wind turbine. Or you couldcompare a PV-diesel-battery system with a PV-diesel system to calculate the payback of the battery.

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See also

Compare Economics window

Is there a way to calculate simple payback by hand?

Written by: Tom Lambert ([email protected])

Last modified: June 9, 2008

Is a Spanish language version ofHOMER available?

Not yet, but this help file contains a Spanish-English glossary of terms.


Can I Use Imperial Units in HOMER?

No, HOMER uses only metric units. This help file contains a table of unit conversions to help convert between imperial and metric units.


Can I Export HOMER's Graphs IntoMy Own Documents?

Yes. Just right click on any graph and copy the image to the clipboard or save it to a file which you canimport into a word processor.

If you plan to print the image, the metafile format is probably best because it is a vector format andtherefore looks good even at the printer's fine resolution. A metafile image will look good even if youscale it up so it is larger than the original image. But not every program will accept metafile images, and

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since they do not use any image compression, the files can be relatively large.

The bitmap format is a simple raster format that most programs will accept. You can copy a bitmapimage to the clipboard and paste it into any word processor or into Paint to modify it. Bitmap images arenot compressed.

The PNG (portable network graphics) format is a compressed raster format that most word processors

and image processing programs will accept. PNG files are extremely small, and the compression doesnot cause any distortion. If you plan to email the image or post it on an HTML page, the PNG format isdefinitely the best format.

Note that right clicking on an image also allows you to modify its properties or export its numeric datato a text file.


How Do I Specify the DC Bus Voltage?

HOMER now allows you to set the DC bus voltage by defining a number of batteries per string. On the battery input page, choose a battery type and specify the number of batteries per string. HOMERdisplays the bus voltage, which it calculates by multiplying the battery's nominal voltage by the numberof batteries per string. In the sizes to consider table, instead of entering numbers of batteries, enternumbers of strings. (Remember to include a zero in the table if you want to consider systems with no batteries.)

See also

Search space

Battery Inputs window


Is there a way to calculate the simplepayback by hand?

Yes. Pick two systems that you want to compare from the Optimization Results and divide thedifference in capital costs by the difference in operating costs. That gives you a "simple payback" that isa little simpler than the simple payback that HOMER shows on the Compare Economics window

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because the hand calculation does not account for the fact that costs vary somewhat from year to yeardue to occasional replacement costs.

See also

Compare Economics window

How does HOMER calculate payback?

