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Solar Hot Water System Calculator

Use the FEMP solar hot water calculator to estimate what size of solar system will work best for your Federal facility and how much it will cost.

The Energy Independence and Security Act (EISA) of 2007 Section 523 requires new Federal buildings and major renovations to meet 30% of hot water demand using solar hot water equipment if it is life-cycle cost effective. This tool can help meet that goal.

Follow the steps below to calculate approximate solar hot water system size and cost needed to meet the Energy Independence and Security Act (EISA) of 2007 Section 523 solar hot water requirement for new Federal construction and major renovations.

Select the appropriate building type from the drop-down menu. Tips on average Federal facility hot water load will be displayed to help complete the remaining fields. Then, enter the desired cold and hot water temperatures. Common temperatures are pre-entered for convenience, but can be changed to match your conditions.

Water Usage Estimates
Office: 1 gal/day/person
School: 2 gal/day/person
Barracks: 10 gal/day/person
Dormitory: 13 gal/day/person
Residence: 30 gal/day/person
Food Service: 2 gal/meal
Motel: 15 gal/day/room
Hospital: 18 gal/day/bed

Annual energy and cost savings are calculated based on the current hot water heater fuel type, fuel price, and water heater efficiency level. Select the appropriate fuel type from the drop-down menu. The average efficiency level and fuel cost is provided, but can be changed to match your conditions.

Based on the data provided, the results for your facility includes the following. Note that these outputs do not include available incentives or rebates.

Site Information

Project Name
Nearest City
ZIP Code

Input Values

Building Type
Amount of Water Usage
Number of person(s)
Cold Water Temperature (°F)(Tcold)
How Water Temperature (°F)(Thot)
Water Heater Fuel Type
Water Heater Efficiency
Average Fuel Price


System Size m2
System Cost
Annual Energy Savings kWh/year
Annual Cost Savings $
based on $/$
Simple Payback years
Solar Fraction %
Annual Greenhouse Gas Reduction lbs. of CO2