Using a metaphor to explain technology

Someone recently asked me about a backup solar power generator system they saw advertised for less than $2000. The large print emphasized that it had an 1800 watt inverter, along with 90 watt solar cells and a battery pack. They asked me what that all meant, and whether the system was worth buying for their rural home. I explained the system with the familiar metaphor of a bathtub and its piping.

The storage battery is like a bathtub full of energy. In the example shown above, it has a capacity of 900 watt-hours (a 12 volt, 75 amp-hour battery pack). The system includes an AC charger for the battery, so when the power goes off the tub is full.

While the AC power to our house is off, we can drain power from the battery (empty the tub) at some rate, and turn it into 120-volt AC power via the inverter. At night, if we kept 75 watts of lighting (five 15W compact fluorescents) plugged in for six hours we’d have drained off half of the battery capacity. After 12 hours in the dark, we’d have completely emptied the battery. (At the rated inverter power of 1800 watts, it only would take 1/2 hour for us to empty it).

The 90 watt solar cell panel is like a faucet for slowly refilling the tub during the day. That solar panel could keep up with any load of 90 watts or less, so if we started with a full battery it would stay charged until darkness fell. (With no load it would take 10 hours to recharge an empty battery).

This bathtub metaphor lets us think simply about how the battery capacity (tub size), solar panel size (faucet) and load (drain) determine system behavior. Whether this backup system makes sense for a home depends on the loads it will see over an entire day. We need to start from a careful load analysis.

Even an efficient full-size refrigerator could consume more than 1000 watt-hours per day. So, we probably would need either a smaller refrigerator, or a bigger generator system. But, for the first day of an outage the refrigerator might stay fairly cold even without being plugged in.

When we compare that backup system with a serious 1000 watt system for continuous use off-grid as shown above, we can see why the ad for that backup system emphasized the inverter size. That inverter really is the least expensive part of the system. An off-grid system could cost about $16,000 including an engine generator for use on cloudy days.

For continuous use, the solar panels have to be able to supply the average power being drained. That is why a realistic system is specified based on their output (1000 watts) rather than the inverter capacity (2500 watts). Also, note how much larger the battery is (15,000 watt-hours versus just 900 watt-hours).

The bathtub metaphor is useful for thinking about how a backup power system works. Reality is more complicated. Solar panels typically don’t produce their rated output, and there are energy losses both in charging the battery, and converting the output to AC. You can read lots more details in Home Power magazine, or on their web site.