Cannot make sense of how to use a SunnyBoy 700U...

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williamzsmith
williamzsmith Registered Users Posts: 2
in Solar Beginners Corner #1
I figure I'd dabble with a small panel setup to grid tie into my house. So I looked at the SunnyBoy 700U, which has around a 700Watt capacity. So now I'm thinking about getting large panels, meaning 24v 230watt types. I figure oh wow, I can just get one to try it out, then I can expand to 3 panels when I feel that I want to expand.


That's great as it sounds, until reading it further that it requires a 77V minimum input. WTF? It then goes to say what a versatile unit it is that will handle from 77V up to 200V. What kinds of panels do they expect people to buy that can reach 200V and still be under 700 Watts??? 20 Watt types? That's about the only way I can see it happening.

Am I reading this incorrectly? If not, is there any grid-ties there that will work with one panel and expand for the future? When I think to expand, I am thinking using the existing panels and buying more, so I would try to get the best, which at the moment, those 230-250watt type panels seems to be the cheapest. So I wouldn't think about buy 20watt panels that are more expensive. Am I wrong to think on this lines?

I really want something like a sunny boy and not one of those ebay units just for the sake of safety.

I'm confused...
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Cannot make sense of how to use a SunnyBoy 700U...

    Welcome to the forum.

    We specialize in confusion here. Usually we dish it out, sometimes we resolve it. :p

    700 Watts is not that unusual nor is 200 Volts. It's all in the math.
    Divide 700 by 175 Watts you'll get 4 panels. Some 175 Watt panels have Vmp of 35. Four of these panels would then deliver 140 Volts. Other panels of other Wattages have different Vmp's. The idea is that the Sunny Boy has an operating range: minimum 77 Volts for it to work up to a maximum of 200 Volts.
    You could have three 235 Watt panels with a Vmp of 30. The Watts would be 705 and the Voltage would be 90.

    You're absolutely right that in terms of $ per Watt the 235's are a bargain just now. Small (under 100 Watt) panels tend to be pretty pricey per Watt.

    Good that you stayed away from the plug-n-play specials but ... You do realize that connecting a GT without the cooperation of the utility is not legal? No, they probably will never notice. Including when you start producing a surplus and they charge you for the extra because you don't have a bidirectional meter installed. Oh; the insurance company will have a fit too over such an install; perfect excuse not to pay out on any claim.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #3
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    Re: Cannot make sense of how to use a SunnyBoy 700U...

    Yep, you pretty much understand the trade-offs trying to define an array... For a 700 watt GT inverter, the "cost effective maximum" solar array would be roughly:
    • 700 watts / 0.77 nominal overall efficiency of panels+charge controller = 909 watts (more or less is fine)

    Then you have to look at Voc-cold and Vmp-hot of the solar panels you are thinking about using--and put them in series parallel to meet the input requirements of the inverter... In this case, Vmp~130 volts nominal or so, or ~3x 43 volt Vmp-STC panels in series... And you could have one, two, three, etc. strings in series.

    The other way to install a small system would be to use Micro-Inverters... These are (typically) one panel per AC inverter (at ~190 watts maximum per GT inverter).

    But, you may want to add a "gateway" (collect data from the micro gt inverters--adds cost).

    Also, most places require notification/inspection by the utility/local building inspectors (plus signed contracts with utility) to install Grid tied Solar (legally)... That makes installing a small system even more costly, and adding a panel or two at a time very difficult to justify (possibly new inspections, new contracts, etc. for every upgrade). Typically, looking at a 3-5 kW system is the lower end of "cost effective" in much of the US.

    In California, we have even other issues... Many utilities require changing from flat rate to Time of Use metering... $0.09 per kWH during off hours and upwards of $0.30 to $0.50+ per kWH during peak rates. There were people that installed small solar systems on their homes to see how well they worked, only to see the electric bills rise (such as using A/C during summer afternoons at very high tiered power rates).

    And for commercial customers, the math becomes even more complex.

    Not saying not to do what you want--But make sure you understand the costs and implications before you start laying out your $$$$.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • williamzsmith
    williamzsmith Registered Users Posts: 2
    #4
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    Re: Cannot make sense of how to use a SunnyBoy 700U...

    Ok so I wasn't crazy! These grid-tie inverters are not very flexible and cannot grow. So to work with the sunny boy, it means you have to build it with 3-4 panels right away and there's really no room to grow.

    So much for wanting to start out with one to play with. I will have to look into those micro-inverters instead!

    Thanks!
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