Window Energy Savings Calculator
Estimate your potential annual energy savings by upgrading to more energy-efficient windows. This calculator considers your current window performance, new window performance, local climate data, and energy costs to provide an estimate.
(Typical old single-pane: 1.1-1.2, old double-pane: 0.6-0.7)
(Typical ENERGY STAR: 0.25-0.30)
(Find for your location online, e.g., NOAA)
(Find for your location online, e.g., NOAA)
(1 Therm = 100,000 BTU, common for natural gas)
(Average electricity cost)
(e.g., 0.85 for 85% AFUE furnace)
(Seasonal Energy Efficiency Ratio, e.g., 14 for a typical AC)
Understanding Window Energy Efficiency
Windows play a significant role in a home's energy consumption. Old, inefficient windows can be a major source of heat loss in winter and heat gain in summer, leading to higher utility bills. Upgrading to modern, energy-efficient windows can drastically improve your home's comfort and reduce energy costs.
Key Terms Explained:
- U-Value (U-Factor): This measures how well a window prevents heat from escaping or entering. A lower U-value indicates better insulation and less heat transfer. It's typically expressed in BTU/hr-sqft-°F. For example, a single-pane window might have a U-value of 1.1, while a high-performance double-pane window could be as low as 0.25.
- Heating Degree Days (HDD): A measure of how much and for how long the outside air temperature was below a specific base temperature (usually 65°F). It's used to estimate the energy needed for heating. Higher HDD values indicate colder climates and greater heating demand.
- Cooling Degree Days (CDD): Similar to HDD, but it measures how much and for how long the outside air temperature was above a specific base temperature (usually 65°F). It's used to estimate the energy needed for cooling. Higher CDD values indicate warmer climates and greater cooling demand.
- SEER (Seasonal Energy Efficiency Ratio): This rating measures the efficiency of air conditioners and heat pumps. A higher SEER value indicates a more energy-efficient cooling system. It's calculated as the total cooling output (in BTUs) during a typical cooling season divided by the total electric energy input (in Watt-hours) during the same period.
- Heating System Efficiency (AFUE): For furnaces, this is often expressed as Annual Fuel Utilization Efficiency (AFUE), a percentage that indicates how efficiently the furnace converts fuel into usable heat over a typical year. An 80% AFUE furnace converts 80% of its fuel to heat, with 20% lost up the chimney.
How the Calculator Works
Our Window Energy Savings Calculator uses these key metrics to estimate your potential annual savings. It calculates the difference in heat loss (during heating season) and heat gain (during cooling season) between your existing windows and the new, more efficient ones. This difference is then converted into energy units (Therms for heating, kWh for cooling) and multiplied by your specified energy costs to provide a dollar savings estimate.
The formulas used are based on standard energy modeling principles:
- Heat Transfer (BTU/year) = U-Value × Total Window Area × Degree Days × 24 hours/day
- This heat transfer is then adjusted by your heating system's efficiency or your cooling system's SEER rating to determine the actual fuel/electricity consumption.
Realistic Examples:
Let's consider a homeowner in a moderate climate (e.g., 4500 HDD, 1500 CDD) replacing 10 old double-pane windows (U-value 0.65) with new ENERGY STAR windows (U-value 0.27). Each window is 15 sq ft. Heating cost is $1.50/Therm, cooling cost is $0.18/kWh. Heating system is 85% efficient, and AC is 14 SEER.
- Old Windows Heat Loss: 0.65 * (10*15) * 4500 * 24 = 13,162,500 BTU/year
- New Windows Heat Loss: 0.27 * (10*15) * 4500 * 24 = 5,467,500 BTU/year
- Saved Heat: 13,162,500 – 5,467,500 = 7,695,000 BTU/year
- Saved Heating Therms: 7,695,000 / (0.85 * 100,000) = 90.53 Therms
- Annual Heating Savings: 90.53 Therms * $1.50/Therm = $135.80
- Old Windows Cool Gain: 0.65 * (10*15) * 1500 * 24 = 4,680,000 BTU/year
- New Windows Cool Gain: 0.27 * (10*15) * 1500 * 24 = 1,944,000 BTU/year
- Saved Cool: 4,680,000 – 1,944,000 = 2,736,000 BTU/year
- Saved Cooling kWh: 2,736,000 / (14 * 1000) = 195.43 kWh
- Annual Cooling Savings: 195.43 kWh * $0.18/kWh = $35.18
Total Estimated Annual Savings: $135.80 + $35.18 = $170.98
These savings can add up significantly over the lifespan of your new windows, making them a worthwhile investment for both your wallet and the environment.