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Selection of Circuits for your Backup Breaker Panel


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We had solar photovoltaic with a battery bank installed in our house when it was built several years ago. We remain on the grid, so when all is working and the batteries are fully charged, on sunny days our grid meter runs backward.

One day during construction, the electricians asked us which circuits we wanted connected to the secondary breaker panel, i.e., which of our many circuits we wanted to be powered by the battery bank when we lost grid power.  We hadn’t thought of that before and had to ponder it with some urgency. This note is to let you get pondering now if you foresee the question coming up for you.

A key question is why you’re getting the solar with a battery bank.  On the one hand, you may be getting solar to hold down on electricity costs and also getting the battery bank to carry you over the typical and infrequent outages lasting a few minutes to a few hours.  On the other hand, your more important consideration may be to be able to survive power outages of days or weeks with only moderate discomfort. In the first case your battery bank will be needed for a relatively short time so you can allow more current to be drawn from it, i.e., put more circuits in your secondary panel.  In the second case, your need will extend for a much longer time, so the power drawn around the clock should not exceed the solar panel’s ability to recharge the bank, remembering that recharge occurs during those limited and somewhat unpredictable hours when the sun is out. The latter case requires much more selectivity in assigning fewer circuits to the secondary panel.

We tried to compromise. We picked circuits according to the first assumption, short duration outages, but we then marked those circuits that we would want to keep on if the outage were extended.  Our hope was that we would know the circumstances accompanying the outage; if we judged it could be an extended one, we would go to the breaker box and switch off the unmarked circuits. We haven’t tried this yet.

There are two problems with this. First is remembering to do it and being able to figure out how we intended to do it. I’ll assume I can get by this one. Second is that a circuit which has something on it that we want to have available in an extended outage will often have something else on it that is an unwanted use of power. So as an outage begins we need to go around the house and, for each still active circuit, switch off or unplug anything on it that we don’t want to leave consuming power; examples could be a second frig, a TV on standby, my electric razor with a nightlight, etc.

In retrospect, I wish we had at least considered the feasibility of addressing this problem while deciding in detail how to lay out all the electric circuits in the house.  It probably would have added to the wiring cost, so doing it would depend on the depth of our concern about an extended grid outage. Not an easy decision, but one better made ahead of time than after all the house wiring has been installed.

 


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