Limit the inverter's AC output when import prices are negative.

[AndrewColes]

Is your feature request related to a problem? Please describe.
When import prices are negative and export prices are low, having solar generation can cost money.

One workaround is to consider soft-limiting the inverter's rating to 0% (or some other small %), when import revenue would exceed any potential export revenue.

Describe alternatives you've considered

In GivTCP, active_power_rate (from 0 to 100%) sets the AC rating of the inverter.

I've considered using some HomeAssistant automations to change active_power_rate, if the import price is negative and it works out cheaper that way.

But, this would very much be a fudge. The best plan may have been different, had predbat known the cost implications of the different options, given what would then be done to active_power_rate.

Describe the solution you'd like

predbat to consider changing active_power_rate when costs are negative both at planning time (working out costs), and when sending commands to the inverter (also setting active_power_rate).

For string inverters + AC coupled batteries, this is quite easy. For a given time slot:

• Import rate >= 0? Change nothing -- calculate costs at planning time in the same way, send the same commands to the inverter (i.e. keep active_power_rate at 100%).
• Else:
  • Estimate the cost assuming the string inverter's active_power_rate is 100% (i.e. what is done now)
  • Estimate import revenue assuming the string inverter's active_power_rate is 0%.
  • At planning time, use the cheaper of these two costs. Then, if the latter is cheaper, when sending commands to the inverter(s), also set the string inverter's active_power_rate to 0% instead of 100%.

For hybrid inverters, it's more interesting. active_power_rate limits both DC->AC, and AC->DC. So for charge slots, limiting active_power_rate limits the AC charge rate.

Something like this would super-cautiously avoid that issue:

• Import rate >= 0? Change nothing -- calculate costs at planning time in the same way, send the same commands to the inverter (i.e. keep active_power_rate at 100%).
• Else:
  • Estimate the cost assuming the inverter's active_power_rate is 100% (i.e. what is done now)
  • Estimate the cost assuming active_power_rate is equal to the battery charge rate, allowing for conversion efficiency.
  • At planning time, use the cheaper of these two costs. Then, if the latter is cheaper, when sending commands to the inverter, also restrict active_power_rate.

The snag is if there's e.g. 4.5kW of sun, and the battery is taking 3kW, the remaining 1.5kW can still be inverted into AC -- as active_power_rate is limited to 3kW ish. This is the price paid for guaranteeing sufficient AC to charge the battery, whatever happens.

Being more aggressive, active_power_rate could be restricted to the battery charge rate minus the 10% forecast PV generation. Better in terms of import revenue, but with a risk of under-charging the battery if the PV forecast was way off.

Additional context
I appreciate time is finite, and negative import prices are only of interest to folk on Agile. I don't mind having a crack at this, with a few pointers

[gcoan]

Great analysis of the need and potential solutions, thanks!

It might be interesting to work out some real world examples of how much this might actually contribute.

Agile rates only tend to be negative when it's sunny and windy, or its windy and very low demand (e.g. weekend).

Today was windy and the during the peak of today the rates went down to -7p

but those negative rates coincided with peak solar generation

in my case, up to 8.5kW forecast.

Assuming sufficient space in the battery (and Predbat is not great at preserving battery space, I have to limit the morning charge rate through an automation), my two inverters can charge at a maximum of about 4.8kWh together, so would be exporting about 3.7kWh minus house load.

3.7x12=44.4p earned from export, or more if the batteries are not charging at full rate

4.8x7=33.6p earned from negative rate agile import if the solar was turned off.

So not worth it for me in this example.

If your inverter charge rate is high compared to your solar array size then it would be more profitable, but my gut reaction is that it'd be marginal.

But interesting to see different numbers compared

[AndrewColes]

Maybe it's an Agile Import + Export problem! Export prices were seriously low yesterday - here's a chunk of my history:

Simple example for a single time slot:

• During the freeze export at 10:30, roughly half the generation was used by the house and the rest exported, for 0.46p of profit.
• OTOH setting active_power_rate to 0% would instead have imported 1.09kWh from the grid, for 5.5p profit.

Over the entire negative-priced period, the cost was -39p. With no exports, and all the house load from the grid instead of some being covered by PV, the cost would have been -95p.

[gcoan]

Fair point about agile export rates. Since I have a large solar array relative to my battery size I'm always on Fixed export in the summer.