Help me choose a system

solarjsolarj Registered Users Posts: 6
Were wanting to step out into the solar world and needing some help choosing the components and setting it up. We live in Central Arkansas in the country have power from entergy and wanting to go grid tie with battery back up or? We average about 56kwh a day this average was taken from 12month utility bill. Just switched out 45 can lights to led bulbs. I know we probably will not be able to go all the way off grid cause here in the south we need our a/c! I want to do a ground mount. Have been looking at the kit from Wholesale solar . These kind of batteries Tell me why this would be wrong?

Thank you J


  • ZakarumeZakarume Solar Expert Posts: 143 ✭✭
    Re: Help me choose a system

    Cant help you much on the Solar, but for batteries check to see if there is a battery outfitters in your area. I know there is 1 in Little Rock and Rogers Ark.
    1460 Watts Solar @24v. 675 AH Battery Bank using 12 6v Trojan T-105. 1 Midnite Classic 150. 1500 Watt 24v Samlex Pure Sine Inverter
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Help me choose a system

    Welcome to the forum.

    Rule #1: determine what power you actually need.

    This means, in essence, don't guess and don't buy 'kits'.

    56kW hours per day is a lot of power. You don't need all that in an emergency situation. If you do, you need to re-evaluate your power use. What you need for back-up is usually just enough to keep the essentials running. Investing in huge battery power that will sit there doing nothing most of the time is throwing away money.

    So you need a two-prong approach here: grid-tie and back-up power.

    For the grid-tie part the first thing you want to do is get in touch with your power company and local authorities and see what is allowed. They will have limits for back-feeding and possibly time of use billing, bill credit/buy back et cetera. You can't just go buy some system and put it in and expect everything to be okay with the utility.

    For the back-up part you need to get some numbers for what you must keep running in an outage, for how long, and how often such an outage is likely to occur. Usually a small generator is the best option. Sometimes a battery-based system is called for. Rarely would it need to be whole-house. Not that some serious rewiring of the AC may be in order here.

    There's no reason at all you can't have a large amount of standard GT system and a small amount of battery-based GT system if that's what best suits your needs. But first you must accurately determine those needs. Otherwise you could have a system that is inadequate or too expensive or even both.
  • solarjsolarj Registered Users Posts: 6
    Re: Help me choose a system

    Thank you for the quick responses. I know we have net metering thru our entergy company. But living outside the city limits its kinda a free for all no inspectors. But i want to do this by the books safe for my family and everyone around. I am able to run my whole house off a 8000 watt gen. Everything except heat pump and a/c. So with the inverters is one better then the other outback vs xantrex? or is it based on wattage i know outback is 8000 watt as xanterx is 6000.

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Help me choose a system

    Personally I would say the Outback Radian is better than the Xantrex/Schneider/Conext XW series. If you read through the forum and see all the problems people have had with the XW series you'll know why I think so. In fact it appears the XW 6048 has been discontinued and replaced with a new model. Haven't heard any experiences with these yet so no telling if they fixed any of the many flaws and problems.

    You still need to evaluate your exact needs. A standard GT system will be a lot less money and give you better payback. A big bank of batteries will die whether or not they are used/maintained. If you've already got a generator and that works for your back-up needs, why invest in something else? An inverter system will not be cheaper.

    As for sizing, basically the inverter capacity reflects the maximum amount of power drawn at any one time. 8kW or even 6kW all at once is a lot of power. Most back-up can be had for less, and there is a 4kW Radian and a 4kW XW.

    Then there is the battery bank size. This reflects the amount of stored power required (i.e. Watt hours). That's Watt times hours. 100 Watts for 2 hours is 200 Watt hours, so is 200 Watts for 1 hour. Note that when you size a battery bank it is done in Amp hours, not Watt hours, because of the different system Voltages available. There's a lot of math in the conversion too as you go from AC to DC with the efficiency and tare losses.

    There are minimum sizes for the inverters as well: on 48 Volt systems it is recommended to have at least 100 Amp hours for every 1kW of inverter, so an 8kW Radian would have 800 Amp hours @ 48 Volts minimum. So both the minimum size and the stored power requirements have to be met.

    Then you get into array sizing. For batteries alone it has to be large enough to recharge the batteries. This is usually calculated with a short-cut 10% current rule-of-thumb, but needs to be checked against the amount of power the array can 'harvest' in a day as well; being able to achieve the current standard for only 1 hour won't do much good. In the case of GT the array is usually sized large enough to maximize inverter output: i.e. 8kW inverter having >8kW of PV so that the peak output can be achieved and sustained for a while. No sense having the ability to sell 8kW of power to the grid if you've only got 4kW of PV to provide anything with.

    A standard GT system is far, far simpler: PV + GTI matched up to service. Whatever the PV can produce goes to loads or the grid. No efficiency losses from storing in/taking from batteries. No batteries dying over time. No charge controller. Less money up front, better payback. But no back-up power when the grid goes down (except for certain SMA inverters which have a limited back-up capacity).
  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Re: Help me choose a system

    If you already have an 8kWatt genset. Perhaps a good start would be enough battery+inverter+AC battery charger to run your home over night/during periods of light power usage.

    I.e., run the lights, cell phone chargers, laptop computer, energy star TV, pump from cistern for water pressure, etc.

    Run the genset to pump from well to cistern (or larger pressure tank), run the fridges/freezers to keep them cool, run microwave/stove washing machine, central heat, etc. during the day.

    Later (or at the same time) add a reasonable amount of solar panels to keep the "emergency" loads going.

    My suggestion for sizing... Around 1 kWH per day for light loads (call it a 300-600 Watt AC inverter). Around 3.3 kWH per day (around 1,200 to 1,500 watt AC inverter) to run a very energy efficient home (including a full size refrigerator, clothes washer, well pump--with correct motor/setup, lights, laptop).

    Note that even a 3.3 kWH per day system--That is about a 1/2 hour of run time on a fully loaded 8 kWatt genset.

    Off grid/backup power is expensive. And batteries age whether used or not (maybe 3-5-7 year life for "golf cart" type batteries).

    For short term failures (less than ~2 weeks), a genset (or several--One smaller, one larger, for fuel efficiency) is usually a better deal. For long term outages (month+), off grid solar+gen backup may be worth the costs.

    The best bang for your buck, an off grid solar power system used 9+ months of the year, sized for your loads, is going to cost the least $$$ per kWH.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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