Solar cells show promise as an energy source for small devices used indoors, such as sensors, wearable electronics and other smart home components. So far, solar cells have been developed in accordance with the properties of standard conditions (STC) that are similar to sunlight. As such, existing solar cells are considered not to necessarily be appropriate for indoor use. The aim of this project is to investigate how indoor illumination determines the efficiency of kesterite Cu2Zn(Sn,Ge)S4 monograin layer solar cells. To conduct the study, three questions were formulated: 1. Under solar simulation, which of the solar cells studied has the highest efficiency? 2. Under indoor illumination, which of the solar cells studied has the highest efficiency? 3. Which source of indoor illumination results in the best performance of the solar cells studied? The author measured the I-V characteristics of solar cells with the composition of Cu2Zn(Sn1-xGex)S4, where the value x varies from 0 to 1 with a step of 0.2. Measurements were conducted under solar simulation (AM 3.0) and under the illumination of LEDs (2700 K, 4000 K, 6500 K). The data was then used to calculate the efficiency of each combination in order to analyse the effect of the light source on the performance of the solar cells studied. Results show that under solar simulation as well as under indoor illumination, highest efficiencies are reached by Cu2ZnSnS4 and Cu2Zn(Sn0.8Ge0.2)S4, with a Ge to Ge+Sn ratio of 0 and 0.2 respectively. Out of the artificial light sources, the two aforementioned solar cells performed best under the 2700 K LED, all others under the 6500 K LED. In addition to the main findings, the results indicated that unlike solar cells with a high Ge content, a solar cell with a lower ratio of Ge reaches its overall highest efficiency under solar simulation. This serves to confirm that a solar cell’s efficiency depends not solely on the material’s bandgap and the properties of incident light separately, but rather on the compatibility of the two. As a result, a solar cell deemed unsuitable for operating outdoors could be viable for indoor use.