The installation of renewable energy sources in island grids is focused on as a large part of island power generation whose power fluctuations should be compensated. A desalination system is given consideration as a controllable load. This paper proposes a model for such a desalination system, then estimates the controllable ranges under various constraints and evaluates the capacity for power fluctuation suppression. The model calculates the pressure, flow rate, and power consumption. The parameters in the model are fitted using tests with an experimental system. Next, the controllable ranges for power consumption under constraints involving the membrane or other factors are simulated using both tests and the model. The controllable ranges with the valve opening fixed are estimated to be 27% to 43% of the rated power. Finally, in order to evaluate transient response, step and ramp response tests and photovoltaic (PV) output suppression tests are performed. Most of the fluctuations in PV output are suppressed when PV output is within the controllable range. C⃝ 2017 Wiley Periodicals, Inc. Electr Eng Jpn, 201(2): 3-16, 2017; Published online in Wiley Online Library (wileyonlinelibrary.com).