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Electric Tankless Water Heaters: Hidden Power Quality Challenges for Modern Grids
This white paper examines the significant impact electric tankless water heaters (TWHs) have on residential power quality. Through real-world data and analysis, it demonstrates how the high instantaneous power demands of TWHs can cause voltage sags, flicker, and transformer overloads, especially when multiple units are installed on the same distribution transformer. The paper also discusses mitigation strategies and compares TWHs with traditional electric water heaters, highlighting the unique challenges posed by the widespread adoption of TWHs. Using field data, the paper shows a single tankless unit can draw 20–30 kilowatts (and more at higher flow), causing abrupt step changes and rapid cycling at partial flow that drive light flicker and root-mean-square (RMS) voltage variation.
Key topics include:
- Introduction to Tankless Water Heaters
- Power Requirements and Load Characteristics
- Voltage Quality Impacts of TWHs
- Real-World Data and Analysis
- Mitigation Strategies for Voltage Problems
- Comparison: Traditional vs. Tankless Water Heaters
Why utilities should care:
Widespread adoption of electric tankless water heaters can lead to transformer overloads, voltage sags, and flicker, impacting service reliability and customer satisfaction. Understanding these effects is crucial for planning, infrastructure upgrades, and maintaining power quality standards. On a 25 kilovolt-ampere (kVA) transformer, a 24–36 kilowatt unit can depress secondary voltage; with multiple homes sharing a transformer, flicker spreads. At low gallons-per-minute (GPM) flows, fast element cycling sustains modulation; at shower flow, continuous high current drives undervoltage. Verify loading with root-mean-square (RMS) min/average/max stripcharts and correlate power steps to usage. Mitigation includes upsizing service conductors or, when necessary, the distribution transformer; monitoring confirms results. Because these appliances are sold and installed without utility notification, screening and customer guidance reduce complaints and protect transformer life. Document seasonal variations, cold-inlet effects, and parallel installations to prioritize upgrades and avoid surprises, service calls.
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