Home battery rate shifting after round-trip losses

Start with usable capacity

Nameplate kWh can exceed the energy a system allows the household to discharge. Use the manufacturer or commissioning value for usable capacity and retain any reserve needed for backup.

Losses increase charging energy

If a battery delivers 10 kWh at 88% round-trip efficiency, more than 10 kWh had to enter it. The charging cost must apply to that larger input, while the avoided high rate applies only to delivered energy.

Cycles describe throughput

A full equivalent cycle can be composed of several partial cycles. Do not assume one cycle every day when the household has insufficient solar excess or tariff spread.

Gross value is not payback

A positive energy spread is only the first line. Purchase, installation, degradation, power limits, financing, programme revenue, taxes and replacement risk determine investment performance.

Backup value is also a separate household benefit, not an automatic cash saving. If reserve capacity is held for outages, reduce the energy-shifting capacity so the same stored kWh is not counted for two incompatible purposes.