Posted: 2/22/2010 9:16:56 AM EDT
|
Here's the scoop. Wanting to put a backup PV system at our grid tied cabin. (Wind and snow can make power sketchy.) Already planning on putting in wood heat, and I have a propane grill with stove top if I had to use it.
Predicted minimum load: - 120v fridge - 1.3kwh / day - (6) 120v 60w lights - 3 hrs a day use (Could switch to 11w CFL's) - 12v 50w HAM - (Minimal use, maybe 100w / hrs max.) - Misc. - add 200w / hrs. The gear I currently have: - (4) REC 210w panels Voc: 36.1v
Vmp: 28.2v Isc: 8.1A Imp: 7.5A - Xantrex 12v modified sine inverters (XM 1800 Pro) Plus a couple 12v 1000 watt'ers. - Xantrex Trucharge 10A AC to 12v DC charger. - Honda EU2000i (Propane converted.) Other useful info: - Cabin is at 48.5 degrees latitude. - Cabin is grid tied. - Roof is southern exposed with minimal if any shade during the summer, heavy shade in winter. - Cold winters with a lot of snow. - Batteries would be exposed to freezing temps, but wouldn't get past 75-80 in summer. - Low winter use from us. (Minimized power requirements.) - PV should be within 75' of battery location. - I plan on having inverters within a couple feet of bank. (Minimize 12v wire loss.) - I have a couple 12v native appliances, but I plan on inverting to 120v for fridge, lights, etc... - I don't have anything that should draw past 1000w at any given time. Questions: - What is the "ideal" array input and battery bank capacity I should have with these needs? (Do I have too much panel?) - IF I were to put a large AGM battery bank in place (12v 1500AH) could I keep the batts healthy with this setup? I would get a larger AC to DC float charger that would be grid tied 98% of the time if that were the case. (I can get a 1500AH used setup for the same it would cost for a new 500Ah bank. Trying to justify if its worth it.) |
| i would say run secondary lights that are 12v LED to prevent inverter loss for light. as for charging, since your on grid i would use a float charger to maintain the system and the solar as a secondary system for when the power is out. also, build a battery box and insulate it and heat with a standard light bulb. it will provide enough heat in a small area thats well insulated to keep the batteries well above freezing. |
|
This is just a rough estimate which depends on one important thing. How much do you plan on relying on the generator in a long term grid down situation?
You will need a charge controller to keep the batteries charged. 840 watts of panels will produce around 3-4 kwh day depending on weather. The fridge will consume 1.3 kwh per day as you have stated. Then the other small loads. And you said this is the minimum loads. There is no way 840 watts of panels is going to properly keep a 1500 amp/hr battery bank properly charged and power your loads. One rule of thumb I read once was one watt of panels per amp/hr of battery bank in a 12v system. This seems to be pretty conservative. After running an off grid system for a couple years now I have come to the conclusion to: Over estimate loads by 1.5 Take the panels you need and double it and add another 50% Then you are in the ball park on a system that will perform to your needs pretty well. Other than that plan on using that genny to supplement to make sure the batteries get properly charged because they are a huge expense in a system. But if you are planning for the temporary short term power outage then you should be fine with what you have but you still need a charge controller. |