Path-Q Solar

How should battery charging losses be included when estimating solar bill savings?

Answer

Count only the energy delivered after storage losses as available for later use. If round-trip efficiency is represented as a fraction, energy delivered from a full charge-discharge cycle is approximately the energy charged multiplied by that efficiency; also account for standby use, reserve limits, and any conversion losses.

Key points

  • Solar used directly avoids storage losses and should be modeled separately.
  • Stored energy should be reduced by charging, inverter, battery, and discharging losses.
  • The value of recovered energy depends on whether it displaces an import or would otherwise have been exported.
  • Use manufacturer-tested or conservative efficiency assumptions appropriate to the operating conditions.

What it depends on

  • Round-trip efficiency definition and test conditions
  • Charge and discharge power
  • Battery reserve and usable-capacity limits
  • Self-consumption and export value
  • Standby consumption and cycling pattern

Risks and trade-offs

  • Using nameplate energy as delivered energy overstates savings.
  • Efficiency can vary with temperature, power level, state of charge, and age.
  • A battery may increase self-consumption but still have poor economics if its recovered energy replaces low-value exports.

Information you may need

  • Usable battery capacity
  • Round-trip and standby-loss specifications
  • Expected charging and discharging schedule
  • Import and export valuation

Your next step

What to do next

Add a storage-loss line to the savings model and compare direct solar use, exported solar, and battery-delivered energy separately.

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