Path-Q Solar

Would electric water heating create enough daytime load to change my preferred solar system size?

Answer

It can, but the effect depends on the water heater's annual energy use, control strategy, and schedule. A heat-pump water heater may add substantial electricity use while creating flexible daytime load; a resistance heater may use more energy. Size solar from the added annual load and evaluate whether shifting heating to sunny hours changes the preferred system or storage design.

Key points

  • Estimate hot-water demand, inlet-water temperature, setpoints, standby losses, and heater efficiency.
  • A timer or compatible control can shift some heating to solar-production hours, but household hot-water needs and comfort remain constraints.
  • Daytime use can improve direct solar self-consumption without necessarily requiring a larger array.
  • The system should also account for other simultaneous loads and the heater's electrical circuit requirements.

What it depends on

  • Water-heater type and efficiency
  • Household hot-water demand
  • Heating schedule and controllability
  • Existing electricity use and solar production profile
  • Whether storage or load-management equipment is planned

Risks and trade-offs

  • A larger array may not be justified if the heater can use existing daytime surplus.
  • Aggressive scheduling can reduce hot-water availability or increase backup operation.
  • Resistance heating can create high instantaneous demand even when annual energy use is predictable.

Information you may need

  • Current and proposed water-heater specifications
  • Household size and hot-water usage pattern
  • Available interval data or estimated annual energy use
  • Solar production by time of day
  • Electrical service and circuit constraints

Your next step

What to do next

Compare the annual energy and hourly load profiles for the current and proposed water-heating systems, then evaluate array sizing with and without daytime load shifting.

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