10-20-2012, 06:18 AM
I bought 720 watts of panels for about $1,000 (good deal, that), eight 6-volt costco golf cart batteries for $875, an mppt charge controller for $450, and I got a 48 volt to 120 volt 2,700 watt inverter free. I'm not done yet as I have to mount the panels still and it is surprising how much electrical wire and fittings costs. I must have spent $2,500 so far but I have bought most of what I need. The rest will be labor.
Using the mppt charge controller I can put the panels in series so they will always put out something to charge the batteries even on somewhat cloudy days. I think that this is important in an environment like Puna where sun intensity varies so much compared to places like the desert southwest. The batteries will be put in series (48 volts) since that is what the inverter requires. Whether in parallel or series, the batteries will store the same energy. The advantage of higher voltage is that you can use smaller gauge wire as well as less risk of uneven battery discharge and the consequent shortening of battery life that causes.
I didn't do any elaborate calculation of how much battery capacity to install. I had an inverter that required 48 volts. If I need more battery capacity in the future I think I will buy eight larger batteries rather than 16 golf cart batteries. One question I have is whether I got enough pv capacity. I will adapt my usage to pv capacity. It is a good exercise to mentally slap yourself whenever you say "I use x kwh per day now". Unless you are made of money, what you use now while hooked up to the grid is literally not relevant. You won't be heating water with electricity off grid, or if you do, you're officially crazy and you should just burn $20 bills for heat. It is what you can stand to live with or without off grid that is relevant.
Using the mppt charge controller I can put the panels in series so they will always put out something to charge the batteries even on somewhat cloudy days. I think that this is important in an environment like Puna where sun intensity varies so much compared to places like the desert southwest. The batteries will be put in series (48 volts) since that is what the inverter requires. Whether in parallel or series, the batteries will store the same energy. The advantage of higher voltage is that you can use smaller gauge wire as well as less risk of uneven battery discharge and the consequent shortening of battery life that causes.
I didn't do any elaborate calculation of how much battery capacity to install. I had an inverter that required 48 volts. If I need more battery capacity in the future I think I will buy eight larger batteries rather than 16 golf cart batteries. One question I have is whether I got enough pv capacity. I will adapt my usage to pv capacity. It is a good exercise to mentally slap yourself whenever you say "I use x kwh per day now". Unless you are made of money, what you use now while hooked up to the grid is literally not relevant. You won't be heating water with electricity off grid, or if you do, you're officially crazy and you should just burn $20 bills for heat. It is what you can stand to live with or without off grid that is relevant.