There are three basic technologies used for solar heating of water. The least expensive is the unglazed flat black plastic mat like the ones sold by FAFCO shown here ([EDIT] now here). The glazed flat panel Gobi-style is about 3 times as expensive for the same area with an example from Heliodyne shown here. Evacuated tube panels are about twice as expensive per gross area as Gobi-style flat panels such as the Navitron shown here (or somewhat less efficient Silicon Solar panels here).
The maximum amount of sunlight in the summer in most locations with the panel pointed at the sun is around 1000 Watts per square meter. In the charts below I use 800 W/m2 to represent a sunny day with either less than optimal panel placement or a less than perfectly transparently clear sky. A mostly cloudy (covering the sun) or overcast day varies in its brightness, but I use 300 W/m2 as a reasonably bright overcast day. The efficiency data comes from this FAFCO PDF file, this Heliodyne PDF file and this Navitron PDF file.
[EDIT] As described here, if one uses the most efficient evacuated tube, then add 13% to the values in the evacuated tube columns. [END-EDIT]
....................... MOSTLY CLEAR AND SUNNY (800 W/m2) ............ CLOUDY OR OVERCAST (300 W/m2)
deltaT(ºC/ºF) .. flat black .. Gobi glazed .. evacuated tube ........ flat black .. Gobi glazed .. evacuated tube
. 0 / . 0 ................ 80% .......... 74% ............... 41% ........................ 80% .......... 74% ............... 41%
. 5 / . 9 ................ 68% .......... 71% ............... 40% ........................ 46% .......... 66% ............... 39%
10 / 18 ................ 55% .......... 68% ............... 40% ........................ 13% .......... 59% ............... 37%
15 / 27 ................ 43% .......... 65% ............... 39% ........................ N/A .......... 51% ............... 35%
20 / 36 ................ 30% .......... 63% ............... 38% ........................ N/A .......... 44% ............... 33%
25 / 45 ................ 18% .......... 60% ............... 37% ........................ N/A .......... 36% ............... 30%
30 / 54 .................. 5% .......... 57% ............... 36% ........................ N/A .......... 28% ............... 28%
40 / 72 ................ N/A .......... 51% ............... 35% ........................ N/A .......... 13% ............... 24%
50 / 90 ................ N/A .......... 46% ............... 33% ........................ N/A .......... N/A ............... 19%
60 / 108 .............. N/A .......... 40% ............... 31% ........................ N/A .......... N/A ............... 14%
70 / 126 .............. N/A .......... 34% ............... 29% ........................ N/A .......... N/A ................. 9%
If the Gobi glazed or evacuated tube panels use a heat exchanger to avoid having pool water through the panel, then the efficiency would be lower than shown by a factor. The chart above shows that the less expensive flat black plastic mat panel works well in sunny warm climates [EDIT] (with 2.4 MPH wind) [END-EDIT], but that the Gobi glazed flat panel performs better in cooler weather (or when there is more wind) and in overcast conditions. The evacuated tube is better for domestic hot water heating or for spas (especially in cold winter climates) because the temperature difference is greater and the volume of water is lower (so fewer panels are needed so the higher cost and lower base efficiency aren't as much of a problem).
Richard
The maximum amount of sunlight in the summer in most locations with the panel pointed at the sun is around 1000 Watts per square meter. In the charts below I use 800 W/m2 to represent a sunny day with either less than optimal panel placement or a less than perfectly transparently clear sky. A mostly cloudy (covering the sun) or overcast day varies in its brightness, but I use 300 W/m2 as a reasonably bright overcast day. The efficiency data comes from this FAFCO PDF file, this Heliodyne PDF file and this Navitron PDF file.
[EDIT] As described here, if one uses the most efficient evacuated tube, then add 13% to the values in the evacuated tube columns. [END-EDIT]
....................... MOSTLY CLEAR AND SUNNY (800 W/m2) ............ CLOUDY OR OVERCAST (300 W/m2)
deltaT(ºC/ºF) .. flat black .. Gobi glazed .. evacuated tube ........ flat black .. Gobi glazed .. evacuated tube
. 0 / . 0 ................ 80% .......... 74% ............... 41% ........................ 80% .......... 74% ............... 41%
. 5 / . 9 ................ 68% .......... 71% ............... 40% ........................ 46% .......... 66% ............... 39%
10 / 18 ................ 55% .......... 68% ............... 40% ........................ 13% .......... 59% ............... 37%
15 / 27 ................ 43% .......... 65% ............... 39% ........................ N/A .......... 51% ............... 35%
20 / 36 ................ 30% .......... 63% ............... 38% ........................ N/A .......... 44% ............... 33%
25 / 45 ................ 18% .......... 60% ............... 37% ........................ N/A .......... 36% ............... 30%
30 / 54 .................. 5% .......... 57% ............... 36% ........................ N/A .......... 28% ............... 28%
40 / 72 ................ N/A .......... 51% ............... 35% ........................ N/A .......... 13% ............... 24%
50 / 90 ................ N/A .......... 46% ............... 33% ........................ N/A .......... N/A ............... 19%
60 / 108 .............. N/A .......... 40% ............... 31% ........................ N/A .......... N/A ............... 14%
70 / 126 .............. N/A .......... 34% ............... 29% ........................ N/A .......... N/A ................. 9%
If the Gobi glazed or evacuated tube panels use a heat exchanger to avoid having pool water through the panel, then the efficiency would be lower than shown by a factor. The chart above shows that the less expensive flat black plastic mat panel works well in sunny warm climates [EDIT] (with 2.4 MPH wind) [END-EDIT], but that the Gobi glazed flat panel performs better in cooler weather (or when there is more wind) and in overcast conditions. The evacuated tube is better for domestic hot water heating or for spas (especially in cold winter climates) because the temperature difference is greater and the volume of water is lower (so fewer panels are needed so the higher cost and lower base efficiency aren't as much of a problem).
Richard