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Michael Welch and Joe Schwartz wrote interestingly related articles in HP126 about carbon offsets and whether batteries are needed for RE systems. When we reduce our burning of fossil fuels, carbon and other emissions go down. Examples include running our vehicles and yard tools less (using less fuel), and adjusting our thermostats (using less oil, LP, or natural gas in our furnaces). These are situations in which we know absolutely that we are polluting less.

But it's not as certain when we conserve with electricity. My household cut its annual electricity usage by one-third from 1999 to 2006. Did my Kentucky electric utility burn less coal because I used less? Maybe not. The utility actually added new power plants in that time period, so emissions went up. Local usage and demand increased far more than ours went down. This is common. Nowadays, most utilities with occasionally underused generation capacity also sell unsold electricity to other utilities. Once a power plant is built, it's likely used as much as possible, regardless of who gets the electricity. People near the power plant get the pollution, regardless of how much energy they use (or don't use).

So we cannot ascertain that emissions go down if we individually use less conventional electricity. Only our responsibility goes down. This distinction is important in my region since the closest city (Cincinnati) is the tenth-most particulate-polluted city in the United States. If we really want less pollution locally, we need to burn less coal. Offsets don't achieve that, even if they encourage local grid-tied solar electricity. This is because Cincinnati's electric utility primarily uses natural gas generators to meet peak afternoon summertime electric demand—when PV systems put the most electricity on the grid. Large coal-fired base-load power plants run constantly, mostly unaffected by peak loads or PV energy. Grid-tied PV systems in my region reduce more demand for natural gas than coal.

Natural gas generators can be powered down and back up in as little as 30 minutes, while large coal-fired power plants need up to 10 hours to power down and back up. Since all combustion generators emit more pollution per kWh when they are cycled down, and since the EPA fines utilities when emissions exceed allowable levels per power plant output, don't expect local utilities to cycle down power plants when a few consumers lower

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their loads slightly for short durations. You need large amounts of longer-term load reduction to shut down coal power plants!

When Joe Schwartz wrote that batteries aren't necessary for a PV system, he meant that you can avoid the hassles and inefficiencies of batteries by installing a cheaper, batteryless grid-tied PV system. But that means using the local electric grid for backup. In my region, every time a cloud shades a grid-tied PV array or electricity is used at night, fossil-fueled generators supply the electricity. Fossil-fueled power plants have carbon emissions, but batteries do not. If you lower demand for natural gas during the summer afternoon but need coal-fired electricity at night or during winter, you're not achieving full carbon offset for your net-metered solar kWh, because coal-fired kilowatt-hours have 80% more carbon emissions than natural gas kWh. If you just want less responsibility for emissions generally, use and demand less fossil fuel energy. If you want lower emissions locally, reduce fossil fuel burning locally. Even if you want to absorb carbon dioxide with trees, plant locally. When you run out of reductions to implement at your own house or business, look elsewhere in your community. There's probably a gold mine of reduction opportunities close by. Think globally, but act locally and verify.

John F. Robbins • Morning View, Kentucky

"You need large amounts of longer-term load reduction to shut down coal power plants!"

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