Dec 17

25% for the 99%… My Solar Backup System Sizing Advice for the Masses

Posted on Dec 17, 2011 under Uncategorized | Comments are off

Image: Salvatore Vuono /

In this post, we will explore what is required of a solar powered generator capable of maintaining a reasonable level of comfort for a longer power outage, perhaps lasting a week or more. To date, most of my solar backup advice has been geared toward making it comfortably through a typical short-lived power failure, and having the capability to keep a few bare essentials powered in a situation where the power will be out indefinitely.
The vast majority of power outages are relatively short events in the U.S. The average customer loses power for 214 minutes per year, in total.  Excluding major storms and hurricanes, power goes out, on average, just once in 9 months. This works out to an average power failure of 160 minutes in duration, or about 2 hours, 40 minutes.  For a power outage of this length, a simple battery backup system without solar panels would suffice, assuming that utility power returns before the batteries need recharging.
What we’re concerned with here, though, is how much power your backup system will deliver during an extended outage. In a perfect world, we would love to have our usual “limitless” supply of power, even when the power utility is down. But, at least for most us with limited means (say, “the 99%,” according to Occupy Wall Street) a solar system of this magnitude would be cost prohibitive. Remember, what we’re talking about here is a solar backup system. Unless you want to supply all of your power from the sun, and get off the grid altogether, the cost for a “full blown” system is probably not justified.

My solar backup system recommendation for “the 99%,” therefore, would be to aim for a system capable of delivering 25% of your normal power usage indefinitely, to get you through a long power outage. This will help keep the initial system cost down, while allowing for a reasonable level of comfort until normal power service is restored. According to the U.S. Energy Information Administration (E.I.A.), in 2010 the average annual electricity consumption for a U.S. residential utility customer was 11,496 kWh, which represents a monthly average of 958 kilowatt-hours (kWh). Taking 25% of these figures, a hypothetical system sized for the national average would be capable of providing 2874 kWh annually, or about 240 kWh per month. But it’s important to note that these are national averages. State-by-state average usage can vary widely, depending on what other resources are available, such as natural gas, for example. To illustrate, the Tennessee’s annual consumption was 16,716 kWh per household; Maine’s, only 6,252 kWh.

Rather than rely on national or state averages, however, it is best to understand the backup needs for your home. Simply review the usage figures from your own power bill, and divide by 4 to arrive at the 25% recommendation for backup power. This should serve as a starting point when shopping for a system that will provide a reasonable level of comfort for a sustained period, and avoid a long, “powerless” nightmare.

In the next post, we will dig a little deeper into my solar backup system specifications and expectations, as suggested here. Like I always say, “A chicken in every pot and a solar backup system on every garage.” (25% for the 99%!)

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