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If I overbuild my system for the odd peak requirement, what are the consequences beyond extra money in panels and controller? Does it hurt to have a panel up there with little load on it? My RV install will have ventilation space behind it to promote circulation and cooling, just wondering out loud about the what-ifs.
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Those focused on ROI say its important not to have too much. The rest say its practically impossible to have too much. There is
no damage to having an unloaded panel; you could over current charge batteries if the controller didn't limit things. Some grid
tie inverters specify that panel rating shouldn't exceed 120% of the inverter capacity, but it isn't that critical. Bruce Roe -
If over/under sizing produces results inconsistent with project goals, the consequences can be determined at/after startup.
Fuzzy goals lead to mixed/uncertain results.
Process knowledge and equipment potential/limitations are necessary for reasonable design.
To the degree that cost effectiveness is important, some knowledge of what sizing does to the economics of the situation is probably helpful in the planning stages.Comment
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If you've got enough panels (and the charge controller can handle it) to charge your batteries at greater than a C5 rate you could damage them from excess gassing and heat.Comment
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OK, got it. If I have a bank of 2x2 T105's @12V, my maximum charge rate is about 74 amps or 2x the C5 (37 Amps) rating of the battery.
4 of the renogy 250W panels in a good light with a good MPPT controller will be something less than 70 amps by my thumbnail calculations.
The only way I get to push that hard at the batteries is if they are way down in state, so it would take a perfect storm to get to that point.Last edited by skyking; 12-11-2016, 07:59 PM.Comment
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I am looking at the possibility of running one of the split mini heat pumps from solar/battery on the RV. From the spec sheet on the heat pump:
input (Range: Min~Max) Watt 782 (292~1087)
Typically you run a generator to use the AC on an RV, but the inverter technology on the split mini gives a glimmer of hope. If I had 4x250W panels and ran the unit at a lower setting, I think it is possible to get 6~8 hours of cooling from the combination of panels and batteries.
The cooling demand would track with the maximum panel output, for the most part.
For now it is long range planning (dreaming), but it does affect the roof design for the RV.
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So with 2 strings of 2 batteries you're running a 450 amp/hr 12v battery bank, Solar professionals usually assume that considering that the panels rarely have perfect sun to put out full power and that a charge controller isn't perfect either combined with a batteries internal resistance an average system will actually put about 77% of the panels rated power into the batteries.. Personally I think it's closer to 80% but many pros use 77% as the number when designing a system. Your panels can put out more on cold sunny days.
1000w x .77 = 770 watts is in the neighborhood of the actual power the batteries will absorb. This absorb rate will be affected by panel and battery temperatures and battery SOC.
battery watt hours looks like this 12v x 450amp/hr = 5400 watt hours
so 5400 / 770 = 7 so you'll be charging at a C/7 rate which is a tad high, especially if the batteries are hot but acceptable.
Keep in mind that if your batteries are roughly below a 85% SOC (state of charge) they will have a lot less resistance to charging so don't count on rarely throwing the full power at them.Comment
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Thanks John. This RV build is far away, at least 5 years. Just dreaming out loud, but the roofline of this trailer will be very aerodynamic in design, so the question of 2 panels vs 4 panels is significant. I will fair in the panels but the roof itself will taper to the rear. An ideal shape is a constant curve so the 65" long flat sections of the panels has to be accounted for and minimized.
I have no intention of using any of the flexible panels I have seen so far, they just don't make power and handle the heat like a proper framed flat panel. Gluing a flexible panel to a 2" thick styrofoam core roof sounds like a recipe for disaster, with the high temperatures that are possible.
I will design in air space behind the panels and my fairing work will allow circulation behind the panels, both while stationary and going down the road. I understand that heat is the enemy.
I may abandon the heat pump powering entirely, which reduces the system size down to no more than 600 watts. The aerodynamics likes 4 shorter panels but the wallet likes 2 longer panels
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