Earth Notes: 16WW Microgeneration Merit Order

Updated 2025-04-17.
Energy consumption merit order for grid-tied solar PV and storage at 16WW. #energy #microgen #storage #selfConsumption
Understanding where the grid-tied PV generation flows to maximise carbon savings.

grid-tied PV generation capacity ~5kWp(e), ~4.5kWp(e) G83 inverters, AC-coupled, half east-facing, half west-facing, slope ~23° in each case.

Merit order

As of there is a 'merit order' of grid-tied PV generation use, best first:

  1. Direct consumption in the house.
  2. Absorption by the Enphase AC-coupled battery up to kW/kWh capacity (~1.3/~6 as of ).
  3. Power-limited spill to grid (max ~250W).
  4. Diversion via myenergi Eddi:
    1. To the DHW cylinder up to kW/kWh capacity (~3/~6). Always takes priority over Thermino so as to minimise heat pump DHW runs when there is only marginal diversion available.
    2. To the Sunamp Thermino heat battery up to kW/kWh capacity (~3/~7).
  5. Spill to grid. G83 limit / G59 was waived by DNO, so nominally unlimited but in practice ~4kW maximum.

Also, when grid frequency drops below 49.9Hz (~6% of the time), any automated diversion/boost is suspended: script/myenergi/eddiFreqResponse-netrc.sh.

Essentially nothing is spilled to grid or diverted during the winter.

Having a lower static export margin in March and September, or a dynamic margin that is lower when (say) the heat storage level is lower, or when a pasteurisation cycle is pending, might cover more of the DHW for those shoulder months in place of grid power. But maybe spilling the extra to grid, and reimporting some (or indeed covering some from the Enphase electrical storage) and using the heat pump with better CoP, is better for the planet overall.

20250307 flows merit order in action Eddi view
20250307 flows merit order in action Enphase view
20250307 flows merit order in action Enphase view net grid only
All data in this figure is from sunny Spring day . The top graph is an Eddi view: green is flow to heat battery, rest to the home. Middle is an Enphase view with PV generation and electricity battery flows. Bottom is an Enphase view of just the residual grid import/export. There was a small net export to the grid over the day. The mid-day () snapshot spilt of ~2kW of PV generation was:
  1. ~100W to the fridge, my laptop, etc.
  2. ~1.1kW absorbed by the Enphase battery.
  3. ~400W spilled to grid.
  4. ~400W to the heat battery (DHW cylinder is lower priority given recent pasteurisation cycle).
  5. No further spill to grid as PV generation within storage power/energy limits.

Historic

When the first PV was installed the implied merit order was:

  1. Direct consumption in the house.
  2. Spill to grid.

When the first Enphase ACB (AC Battery) module was installed:

  1. Direct consumption in the house.
  2. Absorption by the Enphase AC-coupled battery up to kW/kWh capacity (~0.27/~1.1).
  3. Spill to grid.

...

2025-03-24: snapshot

The merit order as of EOD is:

  1. Direct consumption in the house.
  2. Absorption by the Enphase AC-coupled battery up to kW/kWh capacity (~1/~5).
  3. Power-limited spill to grid (max ~250W).
  4. Diversion via myenergi Eddi:
    1. To the DHW cylinder up to kW/kWh capacity (~3/~6). Always takes priority over Thermino to minimise heat pump DHW runs when only marginal diversion.
    2. To the Sunamp Thermino heat battery up to kW/kWh capacity (~3/~7).
  5. Spill to grid. G83 limit / G59 was waived by DNO, so nominally unlimited but in practice ~4kW maximum.

Also, when grid frequency drops below 49.9Hz (~6% of the time), any automated diversion/boost is suspended: script/myenergi/eddiFreqResponse-netrc.sh.