Sizing a Residential Solar Electric System: Part Science, Part Art

Sizing a solar electric power system involves a fair amount of science but is also reliant upon subjective inputs on the part of the solar technician and you, the residential owner. 

Some of the fundamental drivers of a solar sizing/pricing model are covered in solar basics.  As a quick recap, they include environmental (amount of solar hours at your home), location (orientation of your home), equipment (performance of individual solar components) and electricity usage (from your utility bill).  Most of these inputs can be determined with a high degree of specificity from a number of different reliable data sources.

The more subjective aspect of sizing a system comes with determining how much of your total electrical demand you want covered by the solar array and the expected changes in future electrical demand.  Some factors that will change electrical demand include the following:

• General usage habits; are you at home all day and when are you running air conditioning/heating and other household appliances.
• Age of appliances and lights; how old are your appliances and lights?  Are there local utility incentives to switch out old equipment?
• Planning on owning an electric vehicle that will be charged at home?
• Will there be an increase/decrease in number of people living in your home? 

The solar technician’s task is to balance the answers to these questions with the impact of switching your utility tariff schedules (after you install solar) from a fixed schedule to a time-of-use or TOU schedule with net-metering; where different rates apply throughout the day and where customers receive credit for excess electricity generated and fed into the utility grid. 

TOU rate schedules have significantly lower rates in the evening and overnight and much higher rates during the day.  The idea is for your solar electric system to generate electricity and credits during the middle of the day (when solar energy from the sun is greatest), and you adjust your electrical usage (to the extent possible) to the evening when you will be charged at a much lower rate.  You therefore generate at a high utility rate and consume at a low rate. 

To get the most out of your solar electric system – give some though to the issues raised above and ask your solar technician to run a number of different scenarios through the sizing/cost model.  That way you can take as much uncertainty out of the planning process as possible.