AC or DC Powered

Most solar hot water systems require a pump and a controller, both of which require electricity.  That electricity has to come from somewhere, and there are two choices. 1) plug into a wall outlet, so-called AC power, or 2) use a PV solar panel, so-called DC power.  Most people with access to the electric grid use AC power whereas people that live "off-grid" choose DC power because they have to.  For my project, I could use either AC power or DC power, they both have their advantages and disadvantages


AC Power:  This is the simplest, least expensive, most flexible way to power the equipment needed to run solar hot water panels.  Here in New England, most people use hydronic heating, also know as forced hot water.  As a result, AC powered hydronic pumps and valves are widely available at places like Home Depot and Lowe's.  Manufacturers like Wilo and Taco make great AC pumps that are dirt cheap at less than $100.   There are also a large number of AC powered controllers.  A great review of them is listed in the article called "Under Control, Solar Water Heating" by Chuck Marken & Doug Puffer. 

So why not do AC power?  Two reasons; efficiency and blackouts.  The AC powered systems are not particularly efficient.  The components selected are not designed for efficiency and as a result, they use a far amount of electric and are plugged in 24/7 (although not all components will be running the whole time).   For most people, this would not be a big deal, but I am not most people.  I spent almost 1 year investigating and reducing my electricity consumption (use 75% less than before) and I will be damned if I am going to reverse this trend.   The other consideration is blackouts.  The pump for the solar panel must run so that the propylene glycol never cooks in the sunlight, even in a blackout.  That is a problem for AC powered systems.

DC Powered:  With DC powered, there is no consumption of grid electricity and there is no fear of cooking the propylene glycol in a blackout.  It is also just really cool (IMHO) to power the solar hot water panels with solar electricity.  Unfortunately, there is a price to pay both in dollars and complexity.

DC powered systems are simply more expensive.  The solar panels might cost $200 and may need some additional electronics like a solar controller (more on that later).  Also the pump is more expensive, perhaps $300.  There seems to be only one manufacturer of this type of pump, Laing.  It is a very cool pump and can be run from 10 watts or less, versus 100 watts for a typical AC powered pump.  Also, there are very few DC powered controllers. 

All and all, it might end up costing me $1000 more to make my system DC powered versus AC powered, and I almost certainly will never see any return on this investment based on electricity savings.  So most people will probably want to have their system AC powered.  But for the one time in my life that I might be taking a hot shower during a blackout, I think that a DC powered system is the way to go.

For my system, I purchased two 10 watt panels from someone on ebay for a little more than $100.  It may end up costing me another $100 on brackets and things to mount them as well. The panels will be connected in series to produce 24 VDC.

The image shows the two solar PV panels mounted to a 2 foot wide piece of plywood that is part of some temporary work that I put together to test things.  It is really interesting to think that these two small panels will be enough to power the entire system.

Going forward, it is going to be DC powered.  One small bonus, I don't need to hire an electrician or have electrical inspections.  24 VDC is low voltage and not going to hurt anyone or start a fire.