Ask a Solar Engineer - Backup Systems (Part 3)

This is the third in a series of posts on back-up for solar power systems. Read on, and learn more about how you can turn your public utility into your solar back-up with a grid-tied system.

Grid-Tied Systems

Where utility power is available, a grid-tied solar power system can supply most of the energy needed, using the power from the utility as a back-up, in place of batteries. Because these grid-connected systems are much less expensive to purchase and operate than battery back-up systems--and because the owners can effectively sell power back to the utilities--these systems are the most popular.

Public Utilities as Back-up

One benefit of the grid-tied system is that owners can both buy and sell electricity each month. This is because electricity generated by the solar energy system can be used on-site, or fed through a meter into the utility grid. California's net metering law, for example, provides that all utilities must allow customers with solar electric systems rated up to 1.5 MW to interconnect with the local utility grid, and receive retail value for the electricity produced. With a grid-tied system, the utility serves as a back-up to the solar power system, similar to the way in which batteries do in stand-alone (off-grid) systems.

Time of Use Billing

The other benefit of a grid-tied power system is the Time of Use billing structure, which is measured with special meters that calculate how much and when electricity is used. These rate structures make grid-tied systems even more economical.

For example, PG&E's Time Of Use rate would allow you to run your electric meter backwards at up to $0.44 per kWh during summer weekday afternoons (when your solar energy system generates its maximum amount of power), and purchase electricity at all other times at $0.12 per kWh! This Time of Use billing method effectively improves the economics of your solar power electric system by 15 to 40%.

Hopefully this three-part overview of backup systems will be helpful when you decided how you want to back up your solar power system.

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