Assuming that the lens is made of typical oxide glass with a refractive index of 1.5, the lens would have reflection losses of ~4% per surface or about 8% total. This assumes no scattering or absorption losses and that the lens is not AR coated. There would be a little bit of absorption loss at the UV and infrared ends of the solar spectrum that you could calculate using published transmission spectra of the glass being used and the calculated blackbody spectrum of the sun (or a measured solar spectrum).