The power supply needs to be highly reliable but also should be as economical as possible. Both requirements look like to be mutually exclusive, so there is a necessity for optimisation: Ensuring the desired degree of availability while keeping the investment and operation cost low.

This sizing and optimisation process has been done by a tool called TALCO which stands for technical and least cost optimisation. It works not only on investment cost but uses lifetime cost (comprising initial investment, operation, maintenance and replacement). Simulation time is up to 20 years of continuous operation. The tool developed by Fraunhofer ISE consists of detailed models for each power supply component and uses meteorological data for the desired location. Typically 20-30 parameters are varied during the genetic optimisation process and the design target is the minimum life-time cost for the system for a given period.

For the location of Madrid, assuming an interest rate of 5 % the production cost for the electricity was calculated to be 1.82 EUR/kWh with optimal sizing. The system then runs with a solar fraction of 93%. Cost for fuel cell and electrolyzer hardware as well as hydrogen storage materials have not been accounted for a full degree as they are still in a prototype state and therefore their prices are not able to be used. Nonetheless electricity from hydrogen is much more expensive than from PV. In a modified set-up, commercially available hydrogen from
bottles costs 2.5 EUR/kWh including the efficiencies of both fuel cell and DC/DC-converter, still neglecting fully the investment and maintenance cost for the fuel cell.

An important results from the optimisation process are the parameters for the operation of the systems. State-of-Charge (SOC) limits for the usage of the fuel cell as well as minimum PV power requirements and time slot definitions for the use of the electrolyser are outputs from the optimisation and are the rules the EMS follows in its control strategy.