Grid Offset vs. Sell Back

Grid-tie inverters – those without batteries – are key components of a broad range of PV systems for residential, commercial, and utility-scale applications. These systems are popular, in part, due to high reliability and low maintenance needs. At the residential and commercial levels, these systems connect at the building power panel(s), and they enable customers to locally generate electrical energy during the day. The local electric utility, building inspector, and fire department may require design drawings and calculations, applications, reviews, permits, inspections, and/or special meters before such a system can be installed and switched on.

PV array and inverter size, solar radiation, location, orientation, and various environmental and seasonal factors affect how much energy a system produces. A “small” system may generate enough energy to reduce (“offset”) consumption from the grid. A “large” PV system may generate enough energy to not only meet a customer’s electrical demand during parts of the day – effectively reducing part-time consumption to zero – but even have excess energy available to “sell back” to the utility grid.

Either way, customers realize reduced monthly energy bills as returns on their investments, utility generation station pollution is reduced due to lower energy demand, and the need to build additional utility generation stations is similarly reduced. System costs may be reduced by federal, state, local, and/or utility incentives, and Time-of-Use (TOU) rates and tariffs may affect the cost of energy bought and sold.

Going a step further, a “net zero” PV system configuration is a goal for some, where the total amount of energy generated by the grid-tied PV system over a year essentially equals the total amount of energy consumed by the home or business during the same period. In short, energy generated less energy consumed equals zero.

Note, however, that the “net zero” customer still relies on the utility grid to one degree or another for all or some power during some parts of the day (i.e., at night or during cloudy daytime periods) or times of the year (i.e., in the winter when the days are short and the sun is low), and then sells excess energy back to the grid when the sun is shining brightly. Put another way, “net zero” systems still require electric power stations.

An awkward limitation (possibly) of this architecture is that due to “anti-islanding” safety features, virtually all of these PV systems stop generating power and disconnect from the grid during brownouts or blackouts, be they local or regional. It’s frustrating for many to sit through a blackout with a costly alternative energy system on the roof literally doing nothing until at least five minutes after stable utility power is restored.

Battery-based inverters offer useful and even intriguing alternatives to the grid-offset vs. sell back discussion.

Like battery-less grid-tie inverters, battery-based off-grid inverters can be installed on a home or business to reduce utility energy consumption. Ideally, the inverter loads are powered from the PV array and the batteries are recharged during the day. These loads are powered from the battery bank at night. If the battery state-of-charge drops too low, then the inverter automatically connects to the grid to power its loads and help recharge the batteries.

A unique benefit of this configuration is that (assuming sufficient battery SOC) the batteries represent a backup energy source and will continue to power the inverter loads while the grid is down. For example, these loads might include some CFL or LED lights, an energy-efficient refrigerator, a microwave, and a cell phone charger. And, this type of architecture does not require special utility connection approval since it doesn’t sell back to the grid.

For those who want to sell excess energy to the grid and have a battery backup, battery-based grid-tie inverters are available.

Grid offset, sell back, and battery backup systems require different design strategies, component configurations, and permitting. Give Inverter Service Center a call at 800-621-1271, and we’ll be happy to discuss your goals and the possible solutions.

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