Matt, thank you, :bowdown:

I finally got some spare time to crunch some numbers.

My actual values for evaporation are;

17L/hour @ 20oC over 14 days with the solar system off and no allowance for rain.

and, 29L/hour @ 26oC over 22days with the solar system on and no allowance for rain.

From

Evaporation from Water Surfaces
gh =

θ X A X (Xs - X)

Where;

gh = evaporated water per hour (kg/hour) - l'm going to call it L/hour.

θ = evaporation coefficient (25 - 19v) (kg/m2/hour).

v = velocity at water surface (m/sec) - 20km/hour adverage = 5.5m/s.

A = surface area - 24m2.

X = humidity ratio of air (kg/kg) - 0.0098kg/kg @ 20oC & 50% relative humidity.

Xs = 0.015(kg/kg) @ 20oC, 0.02 @ 25oC, 0.025 @ 28oC & 0.027 @ 30oC.

Xs values are from http://www.engineeringtoolbox.com/humidity-ratio-air-d_686.html

Crunching the numbers I get gh = 16L/hour @ 20oC and 46L/hour @ 28oC.

Running the numbers through the online calculator for v= 5.5, A = 24 and the previous Xs values I get;

16L/hour @ 20oC water temp,

32L/hour @ 25oC water temp,

47L/hour @ 28oC water temp,

53L/hour @ 30oC water temp.

Given the variables I'd say that's no leak.

I haven't had a chance to run the solar system at night but will give it a go.

I had noticed water in our storm water outlet at the street and had thought it could be ground water from a leak but it had a salt reading of ~230ppm so not pool water which is up around 6000ppm.

As an added bonus I can use my evaporation values as a starting point for an auto topup system.

Does anyone have any evaporation data they wish to share?