Off Grid Solar Generator - Part 1: Parts and Testing

A solar generator is a handy item to have for power outages, remote backwoods camping, an off grid cabin, or perhaps the zombie apocalypse.  There are many pre-packaged options, but where is the fun in that?  

By doing some data gathering, calculating and experimentation, I concluded my base emergency loads in the house total 2400 Wh/day, with occasional additional loads for charging batteries, running computers, etc.  Loads for camping are considerably less.  Running wattage for my furnace (assuming the gas is still flowing) is fairly low, but wattage could exceed 1200 W during the startup cycle when the ignitor is on.  With that in mind I chose a Xantrex PROsine 2.0 inverter.  


The PROsine is a 2000 watt continuous pure sine wave unit.  It has a 4500 watt surge capacity, which is much better than the 2500 watt surge you typically see with a 2000 watt gas generator.  This inverter has enough oomph to start my 2HP air compressor (6HP surge).  In addition to its inverter functions, the PROsine has a built-in 100amp, 3-stage battery charger with equalizing ability and an AC transfer relay.  I chose the 12VDC model to maintain compatibility with my vehicles and other 12V accessories.


I ran the PROsine for a few years with a pair of marine deep cycle batteries I had lying around.  Their performance was adequate, but nothing to brag about.  When the time came to upgrade batteries, I looked at my budget, then purchased four 6V golf cart batteries from Sam's Club.  Golf cart batteries tend to be good performers at a fair price for people starting out in alternative energy.  My new batteries, wired in a series/parallel configuration, provide 440Ah @ 12VDC.  With most of the budget spent on batteries, I was stuck with the original 4ga cables I used with the marine deep cycles.  With an inverter the size of the PROsine, 250 MCM is usually the recommended minimum.

Taking measurements with a digital multi-meter I discovered i was losing about 0.5VDC in the cables at 20 amps DC. This severely limits the potential of the inverter at higher power levels and starting inductive loads, such as electric motors.

Up until this point, I was running the inverter as a fancy UPS device.  With a little more money in the budget this year, it was time to add the last components to finish building a true solar generator.  Shopping around, I was surprised to discover how low solar panel prices had gotten if you were willing to buy larger panels.  

I purchased three 255watt SolarWorld mono-crystaline panels and a Outback FlexMax 80 solar charge controller.  Mono-crystaline panels have good conversion efficiency (15%-17% percent) compared to their amorphous counterpart, therefore requiring about half the square footage, a bonus when hauling panels into the field.  The FlexMax 80 is a MPPT type charge controller, which has two key advantages, 1) Maximum power point tracking allows for more power to be harvested from the panels compared to a simple Pulse Width Modulated charge controller.  Conversion efficiencies are 95%-98% compared to 60%-70% for PWM controllers.  With solar still an expensive option, getting as much power as possible from your panels is important.  2) Being a MPPT type charge controller, you can wire your solar panels in a series configuration and utilize higher transmission voltages between the panels and the charge controller, resulting in lower line losses and/or smaller wires.

Since this system was to be semi man-portable, I wanted to split the 4 batteries into two separate battery "packs".  With that in mind, I also ordered a handful of battery terminals, 1/0 cable, and Anderson SB175 connectors.  SB175 connectors are typically used as high-amperage plugs for vehicle accessories such as winches, jumper cables, portable welders and inverters.  At full running wattage, the inverter will be drawing around 200 amps @ 12V, factoring in conversion efficiencies.  Since I will be running one connector on each battery pack, my total continuous rated amperage through the plugs would be 350 amps with considerably greater surge capacity.

Before going any further, it was time to rough wire the components together and test them out before any further construction.  I wired the charge controller to the batteries and set the panels outside.

Open circuit voltage was just below the rated OcV, but this was late in the day and the sun was falling fast.  I ran the cables back to the charge controller, checked the polarity and plugged them in.  The running voltage dropped to 107.5 VDC from 130 V open circuit.

I then watched the display on the charge controller and monitored the output to the batteries.  Unfortunately with the sun going down I was losing about 7 volts/min from the solar panels.  It didn't take long to drop to 70 VDC on the input, but the system was working as it should.

With everything working it was now time to turn focus to the mounting and wiring of the components.  In Part 2 I will show how the battery packs were constructed and wired.