Energy Factor (EF) is the older federal efficiency rating for residential water heaters. The U.S. Department of Energy replaced it with the Uniform Energy Factor (UEF) in a test-procedure rule published in July 2014, so current ratings and ENERGY STAR listings use UEF, while older equipment may still show an EF figure.
What Does Water Heater Energy Factor Rating Mean for Your Home?
Energy Factor measures water heater efficiency by dividing energy output (hot water delivered) by energy input (fuel consumed) under standardized laboratory conditions. DOE's 2014 rule created UEF as a single efficiency descriptor and updated the simulated-use test, including new draw patterns.
Uniform Energy Factor (UEF) is the federal efficiency rating for residential water heaters and replaced the older Energy Factor (EF). A higher UEF means a more efficient water heater, but UEFs are only directly comparable between models tested in the same draw pattern.
Under the UEF test, each water heater is assigned to one of four draw patterns based on its first-hour rating (storage models) or maximum flow rate (instantaneous models): very small usage (10 gallons per day), low usage (38 gallons), medium usage (55 gallons) or high usage (84 gallons). The water heater is tested on that one pattern, which is why UEFs should be compared only between models in the same draw pattern.
How Does Energy Factor Rating Affect Your Monthly Utility Bills?
Use our free rebate calculator to see rebates available at your address.
Which Energy Factor Rating Works Best for Your Climate Zone?
California's building energy code divides the state into 16 climate zones, and climate matters for heat pump water heaters because they draw heat from the surrounding air. Federal UEF testing for heat pump water heaters is done at a fixed 67.5°F and 50% relative humidity (10 CFR Part 430, Subpart B, Appendix E), so a unit installed in a space that is warmer or colder than that will perform differently from its rated UEF.
Manufacturers publish an ambient operating range for each heat pump water heater model. Check the spec sheet before installing in an unconditioned garage or basement, and confirm the space meets the manufacturer's minimum air-volume and condensate-drain requirements.
What Are the ENERGY STAR UEF Minimums by Water Heater Type?
| Water Heater Type | ENERGY STAR Minimum |
|---|---|
| Integrated heat pump water heater | UEF ≥ 3.30 |
| Integrated heat pump (120V/15A circuit) | UEF ≥ 2.20 |
| Split-system heat pump | UEF ≥ 2.20 |
| Gas storage, 55 gallons or less | UEF ≥ 0.81 (medium draw), ≥ 0.86 (high draw) |
| Gas storage, over 55 gallons | UEF ≥ 0.86 |
| Gas instantaneous (tankless) | UEF ≥ 0.95 |
| Solar water heater | SUEF ≥ 3.00 (electric backup), ≥ 1.80 (gas backup) |
Can You Get Tax Credits or Rebates for Installing a High-Efficiency Water Heater?
The federal Energy Efficient Home Improvement Credit (Section 25C) ended for property placed in service after December 31, 2025 (IRS FS-2025-05). Federal tax credits for this work have ended, but state, local and utility rebates may still apply; check what is available at your address with the DuloCore rebate calculator.
Utility rebates vary by service territory and change during the year. As of October 2026, LADWP's Consumer Rebate Program lists heat pump water heater rebates of up to $2,500 per unit, tiered by tank size and UEF (minimum 3.3 UEF and 30 gallons; new construction, additions and ADUs are not eligible; apply within 12 months of purchase). SMUD lists rebates of up to $4,000 for gas-to-electric heat pump water heater conversions and $1,000 for electric-to-electric upgrades. SoCalGas lists $300-$575 for high-efficiency gas storage water heaters (UEF ≥ 0.64, 55 gallons or less) and $80-$1,500 for tankless models replacing a tank in a single-family detached home, first-come first-served through December 31, 2026. Check your own utility's current rebates, or visit our heat pump rebates guide.
Official Sources
- ENERGY STAR Residential Water Heater Key Product Criteria — ENERGY STAR UEF minimums by water heater type
Related Reading: Learn more about Energy Audit Priority Recommendations and Energy Audit Report Example.
Frequently Asked Questions
What does energy factor rating mean for water heaters?
Energy Factor (EF) was the older federal efficiency rating for residential water heaters. DOE replaced it with Uniform Energy Factor (UEF) in a 2014 test-procedure rule. Higher numbers mean better efficiency, but compare UEFs only between models in the same draw pattern (very small, low, medium or high usage).
How does energy factor rating affect my water heater rebate eligibility?
The federal Energy Efficient Home Improvement Credit (Section 25C) ended for property placed in service after December 31, 2025 (IRS FS-2025-05). Utility rebates set their own efficiency minimums. For example, LADWP's heat pump water heater rebate requires at least 3.3 UEF, and SoCalGas's storage water heater rebate requires UEF ≥ 0.64. ENERGY STAR's own minimums are UEF ≥ 3.30 for integrated heat pump water heaters and UEF ≥ 0.81–0.86 for gas storage models.
What's the difference between energy factor and thermal efficiency?
Energy Factor measures total system efficiency including standby losses and cycling patterns, while thermal efficiency measures only combustion or heating element efficiency. Heat pump water heaters achieve UEF >1.0 because they move heat rather than generate it.
Do higher energy factor ratings save more money on energy bills?
Generally yes. For two models in the same draw pattern, the one with the higher UEF uses less energy to deliver the same hot water. Actual savings depend on your fuel, rates and hot water use.
What energy factor rating do I need to qualify for federal tax credits?
The federal Energy Efficient Home Improvement Credit (Section 25C) ended for property placed in service after December 31, 2025 (IRS FS-2025-05). Water heaters placed in service in 2026 do not qualify for a federal tax credit; only state, local and utility rebates remain.
Ready to find your savings? Use our free rebate calculator to see how much a high-efficiency water heater will save on your utility bills and discover available rebates in your area. Get your personalized estimate in 60 seconds.
Last reviewed: October 8, 2026. Reviewed by DuloCore Energy Specialists. About the team.
Check your California water heater rebate eligibility — see the programs you qualify for and the dollar amounts on our California water heater rebate finder.