When you design your first residential photovoltaic system, it becomes apparent very quickly that the key to an affordable system is energy conservation. To satisfy the needs of a typical American home with photovoltaics requires a system costing upwards of $25,000. To bring residential PV into reach requires careful system design and sizing.

Many books on the subject offer tables of data and worksheets for calculations. Nevertheless, the calculations are tedious and arriving at just the right combination of components requires many rounds of recalculation. A software program called Solar Sizer removes the tedium and makes designing residential PV systems downright fun. Better yet, professionals will find it robust enough for designing real world systems.

The basic user interface is a house graphic. You select components from a "catalog" and drag them into the house. Detailed performance and price information is provided for each catalog item. Once the item is in the house you can change its characteristics without affecting the catalog. This allows you to quickly get a rough estimate, but fine tune it later with the exact specifications of your equipment.

The program adds up the electrical requirements of your appliances and helps you select the proper components, such as PV modules, the inverter, controller and batteries. You see a warning if you make a selection that's outside the system's parameters or incompatible with other components. The program includes solar radiation data for 240 sites in North America and 50 around the world. You can also enter your own data.

On-screen reports summarize information on energy usage, storage and production as well as initial cost, annual costs and a life-cycle cost. You can adjust the economic assumptions that form the basis of the economic calculations. A detailed printed report describes all of the items installed, gives relevant sizing information for each component, and includes the energy and economic summary information from the on-screen report.

Solar Sizer was developed by the Center for Renewable Energy and Sustainable Technology (CREST) and Solar Energy International with funding by the U.S. Department of Energy.