Solar radiation - use global or direct

I am a student designing a solar water heating system for households in a rural village in India. I am assessing the solar radiation falling on the location.

I know that direct radiation is the solar radiation on a surface held perpendicular to the sun’s rays, having travelled in a straight line directly from the sun to the Earth without experiencing absorption, reflection or scattering. Global radiation is the total radiation falling on a surface and is the sum of the direct, diffuse and albedo radiations.

Should I use the global or direct solar radiation values for setting the criteria for the performance of my system.?

I would be grateful for any help,

Harry

Without knowing the correct answer, I'd use the solar radiation, mostly because the effects of diffuse and albedo radiations are insignificant in comparison. You might also have to adjust for the time of year and time of day, unless your collector can adjust its elevation and azimuth to track the sun. All in all, I guess it depends on how many decimal points you're carrying in all your calculations...