figured how to calculate the time to heat
Pt = (4.2 × L × T ) ÷ 3600
K is liters
T is temp difference
the result is devidede by the kw number from @Robert.Wall
Power (kW) = Flow Rate (l/s) × Temp.Rise (°C) × 4.2
of course this assumes that everything is 100% efficient and there is no heat loss
so i would add say 10% at least to be sure
and it seems that my hosting company is closed for the weekend and maybe also for summer hols, but at least i have found a few free MQTT servers i can use, then i only miss node red
reason i want to go with hosted is that its cheap enough for me, and i can maybe get by with an esp module at the pool end