When Greater Manchester Community Renewables (GMCR) was formed, we chose to start out with rooftop solar. 

We live in a large urban area, so there were many more opportunities for rooftop solar than ground mounted solar or wind, and the technology was more straightforward to deliver than micro-hydro power.

When kicking off a new project, full of energy and enthusiasm, you want to find partners to work with, who will host your panels and use the electricity you generate. But before you approach them you do need to have some idea of what you’re going to offer them. What do they get in return for sharing their roof space with you? Usually, cheaper electricity and carbon savings. But how much?

To work that out you need to know how much your solar array will generate and an approximation of what your expected costs will be.  

How many solar panels will fit on a roof

When our project started we used to measure roofs with the ruler on Google Earth and work out how many panels would fit on, look at MCS datasets to work out how efficient they would be at that pitch and orientation, and finally how much the array would generate. Fortunately, technology has moved on since then and there are now websites which make this process a lot easier! 

OpenSolar is a free online tool you can use to assess what size of solar array is suitable for a building based on its roof space and its electricity demand.  You can choose from a range of different panel makes and models, and then you just ‘drag and drop’ to design your solar array.  Not only that, it will also estimate how much of the solar electricity the building will use and how much the system could cost to install.

A couple of tips:

• Solar panels can be installed on flat or pitched roofs. If you choose the 3D view in OpenSolar, it will automatically align the panels to fit the roof pitch and orientation.
• Keep an eye out for trees and other buildings that might cause shading problems.
• It’s more expensive to install solar panels on slate roofs than on tiled roofs, flat roofs or corrugated metal roofs. 
• The default electricity profile on OpenSolar is for a domestic property.  For more accurate modelling, you can enter monthly or annual electricity usage data and configure the demand profile – for example, schools tend to use lots of electricity during the week and less at evenings and weekends when they’re closed.

We always work closely with our installer, who will propose a design for the array based on their experience.  Even at the desktop stage, they have an eye for whether a particular roof will be suitable.  They provide estimates for generation, onsite electricity usage (based on site data) and up to date market pricing. We rely on their projections for our final modelling.

However, OpenSolar is a great tool to use as a rough guide to help work out what our proposition is going to be.