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  • 1.  How much can you realistically power from solar panels?

    Posted 01-11-2022 01:21 PM
    We are looking at a Sun Power 19.8kW system with a 26kW battery back up. How much could we realistically power at night with this system. We have two regular heat pumps with out heating strips and one mini split heat pump without heat strips. Can't get actual numbers from sales person on what we can run at night.

    Barry Rosenberg
    British Car Service

  • 2.  RE: How much can you realistically power from solar panels?

    Posted 01-12-2022 02:04 PM
    Dear Barry, It's more complicated than the limited information you have provided.

    The simple answer is, it depends. It depends on where you are. It depends on the irradiance (insolation) that is received at the solar panels each day which varies daily depending on the time of year and cloud cover for each day. It depends on the angle of the solar panels and the azimuth of the sun. It depends on whether all of the panels are on the same plane or not. It depends on the efficiency of your inverter(s). And that is just the easy part that determines the likely amount of solar production you will get. (A guess based on historical data collected by reliable sources over the years like NREL)

    Once you know the approximate production for a given time (day or month) you ten have to determine whether the inverter/battery combination has enough power to handle the surge of your critical loads and enough stored energy to run your loads for how long. 26kWhs of battery is not very much, figuring even a 3-ton AC can pull 3kW per hour or more. To run all of those you would need much much more storage. Also you would need extremely powerful inverters running in a "stacked" configuration to handle the potential surge. This is just the tip of the iceberg, as far as determining an accurate answer to your question

    Paul Farren
    The Energy Store

  • 3.  RE: How much can you realistically power from solar panels?

    Chapter Leader
    Posted 01-13-2022 12:55 PM
    your 20/26 sys is a good start try it and see if u might want to double storagebafter first year as storage costs decline

  • 4.  RE: How much can you realistically power from solar panels?

    Posted 01-13-2022 11:56 AM
    I have a smaller, but similar set up, and asked the same questions for 2 years before finally pulling the trigger on an install.
    I have 12.8kw across 32 panels, and will have about 20 kw between 4 batteries of battery backup (Batteries have been backordered since September, waiting to be installed any day now). I also have a 3 ton central A/C unit, and I live in South Georgia near the Florida border; 80+ degrees most of the year, with cold snaps of 30 degrees for a few days between december and march. 

    I agree with Paul, there is a lot that goes into answering this question. What I was able to do, is my power company allows me to download my usage, broken down by 15 minute intervals. I was able to look at my highest usage day each month of the year, and break down how much energy I used during the day and during the night on those days using the data from my power company. This helped me get a better understanding of my day vs night energy use, which for me turned out to be fairly consistent 60/40 split (60% of my energy consumption was during daylight, 40% was at night). I was told by my installer that's a fairly common breakdown. You really need to know this information to be able to determine how much battery capacity you need. Mine was available online, but one of the techs was also able to send it to me in a huge spreadsheet. Capacity alone isn't the issue though.

    I wish I had my batteries to give you a better answer, BUT, I will say with my 12.8kw of panels I am overproducing electricity by a lot. Now, I went through and changed EVERY single light in my house, including microwave, fridge, exterior perimeter lights...EVERY light is now LED in my house. I bought a $2,000 heat-pump water heater that I program to only turn on twice a day, which has been a HUGE energy saver (was using 15kw daily just on my hot water heater, now I use about 1.5kw daily). I bought a better, wifi thermostat I can control and program better to not waste energy during the day. I also installed energy monitors on almost my entire panel box and can monitor real time and record energy by each circuit. 

    I'm averaging using about 10-15kw during the day to power my house, which is significantly less than I have ever used. My inverter is capped right now at allowing my panels to only produce about 9.5kw at any time (when my batteries are installed it will allow the full use of the 12kw I'm told). We did not change our lifestyle, just changed 1 major appliance to be much more energy friendly and we have been slightly more mindful about our energy consumption: We try to wash/dry clothes during the day, cook a little earlier so we are on solar power. But, nothing major. If I need to cook or wash clothes at night, so be it. I'm only monitoring my consumption during the day when my inverter is active right now (I really should use my energy monitors to get a better idea of night use, but i havent yet), but I'm over- producing 45-60kw that I am selling back to the grid daily, on top of powering the other 10-15kw my home needs throughout the day all through solar. Even if I consume 20kw of electricity from the grid at night, I am still way ahead (My co-op buys energy back at 1/2 the price it sells, so I need to over-produce 2x the energy I consume at night to break even). I probably only use 10kw at night, meaning i would only need to sell back 20kw each day to break about even, and I'm doing way more. Even on cloudy days I'm able to over-produce by 20kw+. I use 20kw because that is my battery size, so kind of what I am eyeballing as my target for making sure i stay under each night, and making sure i can over-produce each day to re-charge my batteries when I get them.

    This has actually changed my entire mindset about how I will use my batteries when they are installed. My plan was similar to yours, use the batteries at night until they are depleted and only use the grid as a last resort, then re-charge all day instead of selling back to the grid. That has been my plan for 2 years while I did research. Now however, I think I am going to keep my batteries as a back-up, and only use them a few times a month to move energy through them...Assuming when the temperatures rise and we are using our central A/C more I'm able to continue to over-produce enough to cover night time usage. I was told that during the winter would be my least cost effective time of year with my panels, and when I may still have an electric bill...but I'm absolutely killing it right now with over-production. If this keeps on trend, I'll have no problem over-producing when the days get longer and i'll have no reason to use my batteries every night.

    I'll update in the next week or 2 when I get my batteries installed. The biggest problem with batteries is running too much at 1 time: stove, central AC, water heater... You will need 2 inverters to be able to run it all at 1 time off batteries. Example, I believe my solark 12k inverter allows 9kw continuous power from battery. BUT, if you're just using the grid at night and selling back during the day, that isn't an issue. The problem isn't battery capacity, it's the inverter. You would need 2 inverters to run more than 9kw at any 1 time, and at that point you're going to be depleting your batteries very quickly. It's just easier (I think, this will be tested in the next few weeks for me) to over-produce and sell back during the day, and only worry about the batteries if the grid goes down or SHTF scenario. But, you would have to make sure you can sell back 19.8kw at a time. My co-op is limited to selling back 10kw at a time, so 19kw of panels would be a waste of money. I'm not even certain my 12.8kw wasn't a waste. I'll know once i get my batteries if I can actually tap into all 12kw at any given time like my inverter says.

    Hopefully that makes sense.

    Robert Edwards

  • 5.  RE: How much can you realistically power from solar panels?

    Posted 01-14-2022 11:21 AM
    I also have a Rheem heat pump water heater. I just set it to heatpump ONLY mode. It only uses 400 watts when running. Best of all, during the summer I use the exhaust to cool a rec room in my basement. In winter I dump exhaust out a sill plate vent. I have an 8KW array with a "bonus" array of 4 extra panels on the garage. I live OFF GRID. All energy star appliances. 4flat screen tvs, lots of phantom loads, but it all adds up to only 350 to 500 watts load OVERNIGHT.  I have microinverters on the roof and 2 Magnum MSPAE4448 battery inverters. I tried AGM batteries and that was a total waste. Switched to YIY LiFePO4 straight from china for HALF the going price of anybody in the states, even at dealer pricing. Started with 3  20KWh banks. Just added a 4th. Typically use 20KW overnight in the winter, a little less in the summer, but we've not run the AC after dark. That's why I just added the 4th 20KWh bank. This will more than handle the mini-split in the Master BR overnight during the summer, and still keep total discharge around 30% With the micros on the roof added to the 2 magnum battery inverters, I have access to almost 16KW when the sun is out, so that's when we do laundry (my house is ALL ELECTRIC!) and bake stuff in the oven. The LiFePo4 batteries are AMAZING!! They never heat up, charging OR discharging. And that's sometimes over 110A (DC). Plus, there's ZERO maintenance!  With a typical daily discharge of only 30% I'm expecting these batteries to last well beyond 10 years... those extra 4 panels (about 1000 real watts) are for grey/rainy days. As long as I can generate enough power to run the house during the day, I can get a 2nd "overnight" out of a full charge before the generator needs to be called into play. I'm lucky I live in South Carolina where we get mostly sun and 1 day of grey at a time...

    Joe Utasi
    PV Solar Consultant
    Cinci Home Solar, Keowee Home Solar, DIY Solar Helper

  • 6.  RE: How much can you realistically power from solar panels?

    Posted 01-18-2022 05:48 AM
    Joe, if I am reading correctly, that YIY battery has a built in inverter? I assume since you are using multiple units you can tie them all in together?
    What was the price of that 20kwh battery system?

    Robert Edwards

  • 7.  RE: How much can you realistically power from solar panels?

    Posted 01-19-2022 10:28 AM
    Yes, location is everything. I did a fun calc once for storage needed for off grid. 500KWH was in the ball park. But that did not address the peak loading issues with simultaneous EV loadings or heavy inductive loads like an electric welder, motors, etc.. I am not a fan of off grid because it does not maximize the full power of the panels capability over its life, which we really need to get GW under control.

    william fitch

  • 8.  RE: How much can you realistically power from solar panels?

    ASES Award Winner
    Posted 01-19-2022 10:15 AM
    I have two buildings in Arlington, VA totally self-powered - my home and also a small 2-story office building.
    The first key action was to maximize energy efficiency - R-38 insulation (R-50 2nd floor ceilings),
      thermal barrier paintin attic, double-pane argon-filled windows with low-e coatings,  Energy Star++
       appliances and office machines, LEDs and solar daylighting. - economic and cuts the energy load
       over 30%.
    For my house I also have solar water heating and a direct-exchange geothermal heat pump which cuts
       the remaining energy load by 50%+.  And then I have a 6 kW system tield to a large AGM battery bank.
       So 100% energy coverage.
    For my office building, I have PV, small wind turbine, and a 2kW hydrogen PEM fuel cell tield to a web-
      enabled LiON battery bank. Also 100 % coverage.
    Now during the hot summer if I need to do long term welding, I will take in some grid power so as not
       not to dip into my battery storage during high HVAC-draw days -- but that i once evey other year for a day
       or two.
    Happy to send pictures or more detail by e-mail (solarsklar@aol.com). I hold weekly tours primarily for
      engineers, architects, university professos and students and international. visitors to the Washington, DC
      area. And of course I bring my students from the three different classes I teach at The George 
      Washington University (GWU)
    Best to all,  Scott

    Scott Sklar, President, The Stella Group, Ltd., E-mail:  solarsklar@aol.com.   Website:  www.TheStellaGroupLtd.com

    Scott Sklar
    President & Adj Professor
    The Stella Group, Ltd, GWU EEMI
    Arlington, VA

  • 9.  RE: How much can you realistically power from solar panels?

    Posted 01-19-2022 10:30 AM
    Edited by william fitch 01-19-2022 10:33 AM
    It starts with the climate, then your average and peak loads.... Its not like buying a microwave.

    william fitch