How to subtract two queries (Watt on Y and time on X)?

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I have a PV on my balcony and a smart meter to log my power usage (total power in Watt).
I managed to calculate the Energy (kWh) via integral while power_pv is the power of the PV.

SELECT SUM("integral") AS sum
FROM (
  SELECT INTEGRAL("mean")/3600000 AS integral
  FROM (
    SELECT MEAN("value") AS mean 
    FROM "power_pv"
    WHERE "value" > 0
    GROUP BY time(3s) fill(0)
  )
)
GROUP BY time(1d)
tz('Europe/Berlin')

Now I want to subtract the Power which is not used by myself, marked with a negative score from my smart meter. The integral of power_pv - p_total (where P_total<0) should be the PV-Energy which is used by myself. To make it short: I want to subtract both queries

SELECT SUM("integral") AS sum
FROM (
  SELECT INTEGRAL("mean")/-3600000 AS integral
  FROM (
    SELECT MEAN("value") AS mean 
    FROM "P_total"
    WHERE "value" < 0
   GROUP BY time(3s) fill(0)
  )
)
GROUP BY time(1d)
tz('Europe/Berlin')

Any ideas?