Savings Estimation: Common Questions

HVAC and Appliance Efficiency Considerations

Appliance and lighting upgrades save less in heating climates

• Lighting and unvented appliances convert 100% of their electricity use to heat, adding this heat to the conditioned space.
• This reduces HVAC load during heating seasons but increases it during cooling seasons.
• The impact depends on the length of heating vs. cooling seasons and the HVAC efficiency.

EXAMPLE:
Consider a house with electric baseboard heat and central AC in a heating-dominated climate.

• Assume the year is split: 50% heating, 15% cooling, 35% neither.
• An upgraded refrigerator saves 300 kWh annually in plug load.
  • During heating hours, replacing lost heat offsets some of the refrigerator's savings, reducing net savings to 150 kWh.
  • In cooling season, AC removes extra heat. If the AC has a 10 SEER efficiency (~300%), it removes 15% of 300 kWh = 45 kWh.
  • With 300% efficiency, this saves an additional 15 kWh of cooling electricity.
  • Final total savings: 165 kWh/year (300 saved plug load - 150 lost heating + 15 saved cooling).

Adding Window Tint in Las Vegas Increases Energy Use?

Issue: Adding low-e tint (reducing SHGC from 0.65 to 0.50) to dual-pane aluminum windows resulted in negative energy savings.

Explanation:

• The film was applied to SW and SE windows but not NE and NW windows.
• Summer solar intensity is fairly even, but winter solar gains are much higher on southern facings.
• This impacted more solar gain in winter than summer, leading to net increased energy use.
• However, due to higher electricity costs, there was still a small positive dollar savings.
• Applying the film to all windows improved savings.

Questions to Consider:

• Was the SHGC the only change?
• Was the change applied to all windows or just one facing?
• Were overhangs modeled correctly?
• Where are you looking at the savings?

Cooling Load Increases With Envelope Improvement?

Issue: Why does kWh usage increase when I improve the envelope of the home?

Response:

• In cooling-dominated climates, solar gains are a major factor.
• If the house lacks natural ventilation, there are many hours where:
  • Outside temperature < setpoint, yet cooling is still needed due to solar gains.
  • The envelope reduces cooling load, but reducing it too much can increase net cooling needs.
• The increase is usually small but may be notable in specific cases.
• Good news: Peak load reductions still save demand costs and lower utility expenses.