Grid tie conversion to off-Grid

rubenruben Registered Users Posts: 3
I have successfully installed micro grid tie inverters on all my solar panels. However the govenment sanctions are quite tough for implementation, and should not exceed 20% power of the nearest transformer. I have been looking for ways to go off-grid with the same system, love the advantages of distibuted over central GTI and MPPT per panel due to incidental possible shading in crowded cities of India. I also have realtime updates per panel energy production on mobile.

I was thinking of a pure sine wave off grid stackable master inverter of different company to power on rooftop panel mounted grid tie inverters. My problem is that how do I dissapate the excess power generated, say I generate 100kw and ony 50kw is used. I have AC inverters, so what is the best scenario, do I use shunting like for windmills, high power resistors or install a load switch to divert excess energy to air conditioning or heater ? I wish to spend as little money on batteries as possible so avoiding huge battery bank. I was also thinking of an arduino custom power meter which will control a variac to disappate the excess load to maintain a zero point energy balance. I dont wish to reinvent the wheel if something already exists to resolve my problem. And hopefully this solutions works with third party micro inverters. TIA!


  • BB.BB. Super Moderators, Administrators Posts: 29,360 admin
    It does depend on how far you want to go... However, in general, it is not usually cost effective to convert from Grid Tied to Off Grid/Hybrid power... More or less, when you include batteries (and battery replacement) and extra hardware (battery chargers, AC off grid/hybrid inverters, and replacing those every ~10+ years)--Your average kWH cost can easily run in the $1.00 to $2.00+ per kWH (there are a couple people here that have gotten their kWH costs down towards $0.50 to $0.40 per kWH for full time off grid--But that is usually in a more sunny region--Assuming you are in New Jersey/North East?).

    When Grid Tie system subsides "go away" (has gone away in Hawaii and Nevada, and some other locations)--Solar becomes difficult justify. In general, off grid solar power (battery based) is (roughly) ~4x more expensive than grid power in the US.

    Off grid power makes sense for 1-10kWH per day type systems (30-300 kWH per month)--For larger systems (i.e., if you are using 50-100 kWH per day?), you are talking about some really large and expensive systems.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mvasmvas Registered Users Posts: 300 ✭✭✭
    Put excess power into an electric Hot Water Tank 
  • rubenruben Registered Users Posts: 3
    Thanks for the input, but that may not work if I am producing 2kw and the load is 1.5kw the heater may consume 1.5kw constantly so total load is 3kw, I need a variable draw of remaining 0.5kw only.
  • rubenruben Registered Users Posts: 3
    Thanks @ Bill, I am trying to make off grid affordable as grid tie. I can utilize the power generated directly and avoid red tape. I am in India with abundant sunshine, so makes good sense to adapt to our environment. I can avoid battery expenses by limiting my storage an focusing on direct consumption.
  • BB.BB. Super Moderators, Administrators Posts: 29,360 admin

    With grid tied systems--They act for all the world like a giant AC Battery bank (your system is "recharging" the grid) and you have an "agreement" with the utility company on how they charge you for feeding energy backwards into the grid (from paying you retail pricing, to wholesale pricing, to taxing you for having solar panels like Spain, to making it completely illegal).

    The reality is that you have roughly 6 hours +/- to gather solar energy and 24 hours in a day to use it. Your choices are:
    • Large array and store excess energy in a battery bank for use during off-sun hours
    • Large array and "do work" during the day (pumping water into a storage tank, run washing machine, pre-cooling water into ice and use melting ice to cool home/work off sun, etc.).
    • Smaller array to just power your loads when the sun is up.
    • Forget full solar and instead invest in energy conservation (LED lighting, newer/high efficiency heat pump systems, laptop vs desktop computers, etc.).
    Perhaps in the next few years, you will start to see high(er) performance LiFePO4 batteries (or similar) vs Lead Acid. The newer battery technologies can offer many advantages over lead acid batteries (faster charging/discharging, longer cycle life, no "aging" effects when stored at partial charging, etc.)--But right now, higher costs, more complexity (per cell balancing/protection against over/under charging) make Lithium batteries more cutting edge. There are some other battery chemistries that also may be cost competitive in the future too--But good quality lead acid deep cycle batteries are still (for the most part) the best price/performance at this time.

    If your power needs are relatively modest (1,000 to 3,300 WH per day or 1-3.3 kWH per day)--You can build an off grid system for less money that can, somewhat, provide cost effective alternative to utility power+backup genset (if you have afternoon blackouts).

    I always suggest doing paper designs of your system options first--They do not have to be 100% accurate, but can give you an idea of what the costs and design issues/options are--And see if it is even worth your money to build out such a system.

    There are ways to connect Grid Tied AC inverters to a PSW/TSW (Pure/True Sine Wave) AC inverter + battery bank--And actually feed energy "backwards" through the off grid AC inverter and charge the battery bank (obviously there are details to discuss)--But it does not really do much more than a standard off grid/hybrid AC inverter system does (forgetting the GT inverters).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mvasmvas Registered Users Posts: 300 ✭✭✭
    edited February 2017 #7
    ruben said:
    Thanks for the input, but that may not work if I am producing 2kw and the load is 1.5kw the heater may consume 1.5kw constantly so total load is 3kw, I need a variable draw of remaining 0.5kw only.
    The problem is, you have built a Grid Tied Micro-Inverter system to export all of the power generated and now you don't want it to do that.  I don't think this wheel has been invented, yet.  :)

    1) You could PWM the excess power into the Hot Water Tank, by sensing the Total Amps Exported by the Micro-Inverters.  This would allow you to control the Exported Power to always be less than X amps.

    2) There are Grid-Tie Inverters that can be programmed to limit the Exported Power, like Solar Edge = $$$.  But you have already purchased and installed Mirco-Inverters, which cannot.

    3) Connect only the correct number of Micro-Grid Inverters to the AC Grid, such that you never exceed the max KW export limit. The problem is how to utilize the AC Power from the remaining Micro-Grid Inverters? Sunny Island may be a solution. Utilizing GTI type AC Power in an Off-Grid manner has always been a problem.

  • jonrjonr Solar Expert Posts: 1,281 ✭✭✭✭
    At some point (ie, voltage level), any good micro GT inverter will back off and not feed any more power into the grid.  Ie, the excess power will be left in the solar panel.  No need to dissipate it as heat in some appliance.    So then the question is "what inverter does a good job of simulating the grid and isn't damaged by GTIs pushing the voltage up somewhat".  

    I am available for custom hardware/firmware development

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