I've been toying around with this idea and I'm not sure if it's feasible. I live in an area where the power company has recently adjusted rates to make grid-tied net metering rooftop solar uneconomical (SRP in central Arizona)

Rather than go traditional grid tied with net metering, I have been thinking about, for example, running my electric water heaters (yes I have 2, and not by choice, we don't have natural gas service, and I've looked at solar thermal. PV makes more sense.) from solar directly from a sub panel, and storing the excess power in a battery instead of sending it back to the grid. The long term plan would be to migrate additional circuits to the solar/battery sub panel over time, and monitor the system adding panels and batteries to get to an optimum system.

I'd like to be able to use the grid as a backup in case with an auto transfer switch if the batteries run low. Ideally this transfer would be limited to some pre-defined spread, so that the power transfers to grid once the battery is below 20% but won't transfer back to battery until the battery reached 30% (example numbers, actual levels TBD).

To further complicate things I would want to have load shedding to prevent both water heaters from coming on at the same time, prioritizing the bathrooms over the kitchen/utility, potentially adding the heat pump (8kw) to that at some point.

Ideally this setup would use panels with micro inverters to allow for easier expansion. Otherwise I'd need to get a significantly oversized inverter for traditional DC strings to allow for expansion.

WiFi / Ethernet monitoring of all the major components is desired.

I'm having a hard time figuring out if there is off the shelf equipment that could be used for this or if it's even feasible. I would install the panels myself but have an electrician wire everything up.

I doubt the power company would like this, but since I'm not going to be sending power to their grid I don't think they have any say so.


  1. I have an old 200 amp panel that will be replaced.
  2. It's assumed that I would need to move loads to a sub panel.
  3. The power company isn't going to try to intervene.
  4. Since the water heaters are 3,500 watts, I'd want to size the initial system to maybe 2kw, since the duty cycle on them is maybe 15%, and they are on timers (off from 9PM to 7AM)

So my specific questions are:

  1. is this even possible or is it completely daft?
  2. has anyone done this or heard of someone that has?
  3. if it's doable, is there off the shelf equipment that would support it?
  4. I'm not familiar with transfer switches, how is sensitive equipment affected? All my computer equipment is on decent sine wave UPS' but what about the TV, refrigerator, etc (I've had to replace the logic board in my fridge after a power surge.)?
  5. I've no idea how to size the battery.

I'm sure there are lots of details that I'm overlooking.

closed as too broad by Tester101 Dec 8 '15 at 19:38

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  • This question seems a bit too broad for this site. I think you need to do a bit more research, and get a good solid understanding of the principles involved. Then if you have a few specific questions about single aspects of the setup, maybe we could help you with that. – Tester101 Dec 8 '15 at 19:38
  • Simpler solution: net metering. Sell any unused power to the grid, buy it back when you need it. That's off-the-shelf hardware, if your utility company supports net metering. – keshlam Dec 8 '15 at 19:52
  • 1) When you see the battery costs, you'll want to minimize electrical needs as much as possible. 2) Without natural gas, you could go with propane. Electric water heaters would be pretty crazy, after you do the math. 3) Arizona is a great place for evacuated solar tubes (hi temp, low cost heat). 4) The AZ utilities squeeze solar grid-tie folks harder than anywhere else in the US, so set expectations low. 5) If stove and/or oven are electric, consider propane for them, too. – donjuedo Dec 8 '15 at 22:26
  • I don't know about an auto transfer switch, but I know people (in NM) who've done PV+batteries with a manual switch to grid for backup: thegoodlifelab.com/2008/11/video-pv-solar-off-grid.html – littleturtle Dec 8 '15 at 22:56
  • I had to make my own auto transfer switch - couldn't find any on the market. It isn't hard - just make sure it can handle the amperage. The trick is to use a relay that is controlled by the voltage in question (in my case it is the inverter output, to switch from house power to the inverter when it comes on). The parts are easily available. eBay has solid-state relays for very good prices. – SDsolar Apr 1 '17 at 4:39

Sounds like a totally custom system that you will have to design and implement yourself.

If I lived in AZ I might be tempted to install a solar water heater tied to the existing water heaters to start with. Keeps you from having to backfeed the grid and get crossways with your utility.

Another thought is to add solar panels tied through MPPT's to secondary heating coils in the existing water heaters. Using straight DC instead of changing to AC will save you ~15% of loss through an inverter for no reason since resistive heat doesn't need AC. https://en.wikipedia.org/wiki/Maximum_power_point_tracking

With either of these methods you can use the water to store your heat / power instead of batteries or the grid.

You sound like you have thought about this for a while but make some small investments first and see if you think they are worth it before proceeding. You may be very disappointed if you are not realistic with your calculations and goals.

Happy Tuesday!

  • Thanks, some good things to look into. The issue is that Grid tied is basically out because SRP has made it un economical. Based on my rough calculations and based on 11 cents/kWh, my water heaters cost me about $60/month, conservatively. – Craig Jacobs Dec 8 '15 at 22:50
  • My inverter robs 12% of my solar power, so DC is quite a bit more efficient. Count your blessings you are not in S. Cal, where grid power is 23 cents/KWh. It is a good thing that the sun shines so much. My system was self-financed (and tracks the sun and uses an MPPT charge controller) so all the solar power is free. I am off-grid, with a custom auto-switch to take me off grid power when the inverter turns on. --> Electric heating is the worst use for solar. Solar water heating is better than electric if that is your primary goal. Storing heat in the water makes a lot of sense, for sure. – SDsolar Apr 1 '17 at 5:18

What about something simple, e.g. sticking a voltage controlled DPDT relay onto your hot water heater element and auto-shunting it between solar power and grid power?

Your water may be a little cool in the morning, but this would take a significant base load off your system, without requiring massive renovations or getting your utility involved.

  • Exactly. That is a DIY project. Mine works like a charm. I did add a UPS to the computers so they don't even notice the switchover. – SDsolar Apr 1 '17 at 4:42

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