Introduction: Portable Solar Generator
*** UPDATE to "Step 3: Solar panel & connections"
I built this system because its fun, I'm a nerd and its fun. Did I mention its fun? I've been in computer electronics for 20 years now and working with 12v comes pretty natural to me. What I don't know much about is solar power and how I can use it in a practical way.
My home office is the perfect space to test out this little generator. I’m planning to operate a 6 watt LED lamp, a 14" Dell laptop, 22” Dell LCD monitor, my home internet router, a Cisco VOIP phone and a set of Bose computer speakers. My total running wattage for these devices is: 75.
I bought my system based on guesswork and math principles BEFORE I knew my actual wattage. Don't make my mistake and buy the Kill A Watt meter FIRST so you don't have to guess on battery sizing.
Here’s what I bought:
- 100 watt WindyNation polycrystalline solar panel kit /w
- 20 amp charge controller
- 40 feet of solar cable
- 2 pair of MC4 connectors
- 1 Aluminum stand and hardware for the panel
- 35ah deep cycle AGM battery
- AGM is sealed so it's safe for indoor use and zero maintenance which appealed to me. They are pricier than flooded when compared to the per amp hour rating though. LiOn batteries are lighter and smaller but the price point is still too high for me.
- Bestek 400 watt 12v inverter
- 12v marine grade plug
- USB port and voltmeter kit Misc cabling
- Some additional MC4 connectors
- A couple pairs of crimpers
- Terminal connectors
- Wire (14awg and 8awg)
- Dewalt 22” rugged toolbox
This whole system was $410 my cost
I bought parts from Amazon.com, Home Depot and Harbor Freight. Most of it was from Amazon.com though.
Step 1: Battery Sizing & Math
UPDATE : This month I signed up for PVOL101 by https://www.solarenergy.org. I've always wanted to know more about this technology and now I'm on my way. That being said I wanted to update a little information from my original post.
In order to properly size a PV system you need to know what your loads are. In my original use case I said I wanted to run my home office (see above). The first step to properly identifying your electrical usage is with a tool like the "Kill A Watt" or something equivalent so measure your devices. I plugged all my home office gear into a power strip then connected the meter between the strip and the wall. My running wattage deviates between 68-77 watts. The first update to this section is here. Watts is a measure of energy but it has no time component to it. As you as you turn on your loads you are using a certain number of watts. To measure watt hours (or Wh as you'll see next) determine how many hours you run the load... simple right? Here's the math :
Voltage X Amperage = Wattage (<--- this will make sense in a minute so stay with me)
Its easiest to convert all your devices to watts (or use the Kill A Watt) and add up the totals. Wattage in my case was 75 (average) times the number of hours you run the devices. In my case I had a goal of eight hours or one work day on battery power.
75watts X 8 hours = 600 watts
Not knowing my actual numbers in advance I guessed wrong on my battery and undersized it, oops. I bought a 35ah deep cycle battery on a guess. So my battery's capacity is...
More math :
12v battery X 35ah total capacity = 420watts in one hour so divide 420 watts by 8 hours (my original goal) and you get 52.5 watts per hour. Knowing this I really guessed wrong since I needed 75 watts per hour. Does it start to make a little sense?
So there's another wrinkle you need to know about lead acid batteries like mine. You shouldn't discharge your lead acid deep cycle batteries more than 50% or you can damage the battery's ability to hold a charge. My 35ah battery can delivery 35 amps for one but in doing so use 100% of the battery's capacity. We just learned that's not healthy for the battery's cells. So in reality divide your battery's listed capacity by 50% (so you don't damage the battery) and your actual usable power is only 17.5ah.
More math :
17.5ah X 12v = 210watts total. 210watts / 75 watts per hour gives me only 2.8 hours usable
If you are still with me, what I should have bought was a 125ah battery. Why well lets look at the numbers :
More math :
125ah X 12v = 1500watts / 50% = 750watts / 8 hours = 93.75 watts per hour (more than I need!)
Now mine at 35ah was $65 but the one I'd need is $250 so there's a big difference. Had I bought the right battery my 100 watt panel would have needed 7.5 hours of perfect condition daylight to recharge it. Using this math I would need 1-2 more panels to keep up with my rate of discharge. So this means more math to determine how to scale up you array to fit the batteries and your load. I'm learning this stuff gets expensive VERY quickly!
Step 2: Building the Box
In my case the toolbox provided ample space for my needs. I originally thought about using an ammo can but want sure everything I bought would fit. The layout can be whatever you need it to be an your components are going to be different size than mine.
I centered the battery so the balance would be good since you carry it by a centered handle. Next I mounted the inverter and charge controller on the right side of the box. I just used some small bolts washers and nuts I had around the shop. The USB/12v/Voltmeter combo required three 15/16 holes to be drilled through the box as well. They are held on my plastic nuts from the inside.
Step 3: Solar Panel & Connections
So I've learned that you want to have 1.5x the solar panel output compared to the battery capacity. Here's and example from an earlier step. My panel is max 100 watts in perfect conditions. So in 5 hours of perfect daylight I could hope for 500 watts of power back to the batteries. Unfortunately there is loss at all points in the system so if you need 100 watts per hour you should have 150watts worth of panels. This rough estimate should compensate for the losses in other places of the system.
This panel comes with two pair of MC4 connectors and they require a special crimper to make extension cables for. My kit came with the wire but I bought extra connectors and a crimper. The crimper was $20 and in my opinion worth the cash spent for the time saved. Also the connections are rock solid. The idea behind MC4 if you don't know is to create a weather tight connection that will not disconnect on its own unless a special tool is used. I watched a few YouTube videos to learn how to make MC4 connectors.
Step 4: Grid Tie Inverter & More Panels (future)
I know I said I undersized my battery so you might be asking why in the heck would you worry about adding grid tie when you use up all your battery as it is? Well, for me this is an experiment first and I know if I replace the battery I'll solve my issue but I want to see what grid tie would look like.
My home office is only used two days a week so If I'm only using solar partially that's OK and besides if I use solar in the morning, switch to grid then back to solar after that batteries charge that's still better than nothing.
This grid tie inverter works by taking a 12v input (normally directly from a solar panel) and converting it to 120v and feeding it back into the grid. What this means is that you are pushing power back in and spinning your meter backwards. I'm planning to call my utility company before I buy this and make sure I'm not breaking any rules.
This model and many others like it have something called "islanding" built in. Islanding basically means the inverter needs to sense the grid to operate. The minute the grid fails (power loss event or blackout) inverter shuts off. This is to protect the poor lineman who is working on the grid trying to restore power for you. He/she doesn't deserve to get zapped just because we want to save some money on our power bill right? Exactly!
Step 5: Other Important Stuff to Know But Don't Get Overwhelmed
I talked about using the Kill A Watt to help size your battery. Here are some other things to remember.
- Distance matters! 12v doesn't do well long distances
- Distance from the panels to the charge controller should be as short as possible. The shorter your run the less voltage & amperage will drop
- Distance from the battery to the controller should be as little as possible for the same reason I mentioned above.
- Using an inverter should be avoided when possible. The more you can run 12v natively the longer your battery will last.
- I know this isn't possible in all cases but you would be surprised on how many appliances run on 12v. Just lookup RV stuff
- Cable size matters!
- The cable I got in my kit from the panels was 10awg
- The inverter I use has 8awg cable
- Charge controller type matters PWM vs. MPPT
- There are tons of articles out there on the differences and you'll get confused as I did but for me the PWM was fine.
- Inefficiencies are rampant in these systems
- You lose power between each step in the process.
- The more you spend on brand name gear the less loss you'll probably see
- You get what you pay for (most of the time)
- Flooded lead acid
- Cheap, reliable, time tested but off gasses explosive hydrogen
- AGM (absorbed glass mat)
- Cost more than flooded, can be mounted at any angle, reliable,
- LioN Lithium Iron Phosphate
- VERY expensive, weigh nothing compared to led acid, can be discharged to 80% instead of 50%, last MUCH longer than AGM or flooded.
- Flooded lead acid