To measure solar irradiation and photovoltaic array output energy a measuring accuracy cannot be guaranteed unless the data sampling interval is appropriately selected. From this viewpoint, actual irradiance has been measured by comparatively high speed sampling of 1-4 s for 44 months and the daily errors of the numerical integral have been estimated for various step sizes. Approximation formulae of the error versus the step size have been statistically obtained as well as their probability density function covering +/-4 sigma. Finally, a nomograph is presented to decide an appropriate sampling interval. A concluding example shows that the deviating component of the error exceeding +/-1% can happen once for every 1 month or 6 months if the step size is selected as 105 or 65.5 s, in each of which the total error becomes -0.0485 +/- 1% or -0.0336 +/-1% including an average error component according to the data measured at Tsukuba.