You have to configure your taxes and surcharges as described in the Net Price Sensor chapter: https://github.com/mampfes/ha_epex_spot?tab=readme-ov-file#1-net-market-price-sensor

According to smartENERGY, the prices reported by the API already include 20% tax

That is wrong, even though they say it on their website. The provided price from smartENERGY is without tax. It's the same as on the EPEX Spot website. You have to add a surcharge of 1,2ct and then 20% tax. That gives you the 1,44ct (1,2×1,2) surcharge that you really pay to smartENERGY.

Okay, that fixes it. I guess the ticket can be closed - might be a good idea to mention this in the documentation somewhere 😉

I already mentioned this to smartENERGY

I already mentioned this to smartENERGY

Their reply (translated): "As you can set the prices in the API yourself, they can also be shown including tax. Our description is therefore a matter of setting, depending on the configuration."

Could it be that they changed it again and the price provided by the SmartEnergy API now does include the Tax (but no absolute surcharge)?
So the only way I get the price that they claim on their website is by adding 20% surcharge and 1.44ct absolute surcharge.

Example for 2024-09-08T10:00:00+02:00:

EPEX Spot price: 0.829ct
SmartEnergy API price: 0.995ct (0.829*1.2)
SmartEnergy Website price: 2,43ct (0.829*1.2+1.44)

According to smartENERGY, the prices reported by the API already include 20% tax

That is wrong, even though they say it on their website. The provided price from smartENERGY is without tax. It's the same as on the EPEX Spot website. You have to add a surcharge of 1,2ct and then 20% tax. That gives you the 1,44ct (1,2×1,2) surcharge that you really pay to smartENERGY.

That is still true, just checked.

1. SmartEnergy API provides the correct price with 20% VAT without the surcharge of 1.44 ct. Currently, the price is calculated (spot price + 1.44) * 1.2 which is incorrect. It should be spot price *1,2 + 1.44.
2. Is there a possibility of changing all prices from MWh to kWh?
3. Is possible to have all the prices (average, highest, lowest, median) with the same price structure as the net price with VAT and surcharge?

@xiaodi-dev
1.) Thanks, I have fixed the calculation in 3.0.0-dev-2. Surcharge needs to be adapted accordingly.
2.) See latest pre-release 3.0.0-dev-2. It changes all prices to currency/kWh
3.) It is not yet planed. But seems like a reasonable addition. I might add support for it.