The enthusiasm of both the client and the solar vendor can obscure (yes, that’s a pun! – no apologies) several very important facts about solar energy, the equipment and its functionality. Read on to let the sun shine!
Evaluating Solar Potential
1) Time of use vs. time of sun
You will save the most on the factory’s electric bill if all the solar energy produced is consumed by the building. Simple? Well, what about weekends? Second & third shifts? Seasonal busy periods that are not the summer? Enter the concept of Net Metering. In Connecticut for example, the amount produced by a solar (photovoltaic or PV) system is tracked and subtracted from all electricity used every month. The balance left over is paid at the wholesale rate. In Massachusetts, the net metering occurs every quarter. Here’s a graph of solar production vs. use over a year in Seattle. It very clearly demonstrates abundant production of energy in the summer, and larger use of energy in the winter than is produced by the solar PV system.
Why does this matter? The more energy you can consume “behind the meter” – that is, using it as soon as it’s produced – the better your return on investment. The wholesale electricity rate can be half of what you pay for your “all-in” rate. Wholesale rate is the rate at which all of your unused solar energy will be credited back to you by the power utility.
2) Not all panels are created equal
In comparing three bids for a solar system, always ask for the product cut sheets on the panels, racks and inverters. In particular, look at the panel dimensions, and watts / panel. Go for high power density (the most watts / square foot.) You get more power out of your roof, or get the same amount of power with a smaller footprint, leaving more of your roof open for other needs. Notably there are also differences in quality, warranty and performance degradation over time, and I, of course, prefer a locally-sourced product.
3) Grid–down & batteries
More than 60% of people who own solar panels believe they can use them for emergency power if the grid goes down. Nope. If this is of interest to you, tell the installer, and they will need to install a grid-interactive inverter, instead of a grid-tied inverter. The grid-tied inverter (standard) requires a frequency signal from the utility’s power grid to function. This is a safety measure to prevent line workers from electrocution while repairing downed lines. Grid-interactive inverters (SMA Sunny-Island, Outback Power, AMES Power, Magnum inverters, to name a few – and this is not a product endorsement) disconnect from the grid, but keep power flowing through the inverter to internal (behind the power meter) systems. A critical part of this may be to have a battery storage system to absorb the extra electricity if it is not used at the moment. Sizing batteries, choosing the type of battery, and selecting connected loads are all beyond the scope of this article. A well-qualified solar installer should be able to help you calculate everything if grid-down usage is a desired option.
