Too much power – what to do? Free hot water

Screen Shot 2015-04-23 at 3.14.48 PMFor most of the year my batteries are fully charged around 11a in the morning. This means that I’ve more power than I know what to do with for the entire afternoon. My 6.5kW array will produce and waste at least 25kW on a sunny day.

This got me thinking – there has to be a way to use the excess power.

Until now my domestic hot water (DHW) was delivered by an propane powered Eternal Instant water heater. But as of today I’m using the extra power to pre-heat 50 gal of DHW in a separate electrical water heater tank before it enters into the Eternal water heater.  I’m using a $500 highly insulated old school 120v / 2,000W mechanical thermostat water heater from AO Smith.

Worth it? The average temperature in the water tanks from the well is around 55F and the output from the Eternal is 125F. Since it is a highly insulated tank (R24 value) the water should stay relatively hot all day and night. The Eternal warer heater will only need to heat the water from 90-95F to 125F – which is over 50% reduction in energy use for the DHW alone.

Here is how it works: The new system uses the Outback Mate 3 Diversion Load Program for the AUX port to enable a relay that will turn power on to the hot water tank and pre-heath the water to 110F. The Diversion Load Program will automatically turn on when the charge controllers enter into float mode (i.e. the batteries are full) and turn off again once the CCs go back to Bulk (or Silent at night).

The math: This system – including the tank, plumber and electrical will probably cost me $2,000 in materials and installation which is very inexpensive in contrast to a solar domestic hot water system ($10k) and hopefully easier to manage and maintain. The anticipated savings in propane is $300/year so an ROI in just 7 years is not bad.

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