Of Volts, Amps, Watts, and Batteries

Its been some time since Ive updated this. In that time Ive sidetracked into solar power. WOW, what a learning experience! I will try to keep the "politics" of solar out of this and just stick with my experiences. I will however say this much. To anybody who thinks solar is the be all end all of alternative energy, I challenge you to build a small system and live with it long term! That being said, on to the facts and experiences.

Electricity Terms

Before I begin all this, I will do a brief explanation of the terms of electrical measurement and flow. Electricity is very much like water. The electrical equivalent of a molecule of water is the electron. All an electrical system does is move electrons around in varying pressures and quantities through the wires which can be thought of as pipes to water. All of the following is GREATLY simplified just to give an idea of what we are talking about.

Voltage - Everyone knows the word, few knows what it means. Voltage is the "pressure" pushing against the electrons. Its hydraulic equivalent would be pounds force, or Bar.

Amps - These are the electrons. You can have a whole bucket of amps, but if there is no pressure they dont do anything.

Watts - This is the real number you need to pay the most attention to. This is the system POWER or power potential. Often it is expressed in Kilowatts (watts x 1000) for the purpose of residential power. There are also a couple related terms you may come across in AC power. These are "VA" or volt/amp, and "KVA" or Kilovolt/amp. Setting aside AC power theory, simply think of them as Watts.

Watts are the equivalent of Amps x Volts. The same amount of power is produced by 120 volts at 10 amps, as is produced by 240 volts at 5 amps - 1200 watts. So, the next big question might be, if its watts that matter, why do we even pay attention to volts and amps? The reason for this is that wire sizes are based solely on Amps (the insulation is based on volts). Lets look at this a moment from our batteries point of view.

For the sake of discussion, we will consider a 100% efficient conversion through our inverter (which is FAR from 100% efficient in real life!). Below is a "wire ampacity chart". This is the safe wire size for a given amperage flowing through the wire. As long as the amperage for a given wire stays below what is shown here there is no danger of the wire overheating due to resistance.

Using our previous load example of 120 volts at 10 amps moving 1200 watts we first look at the minimum wire size for the AC side. This shows us the minimum safe wire size is 14 gauge. This is pretty much the standard size for American household wiring (although some run 12 gauge to lower the power lost in moving through the wire).

Now we look at the DC side of things. We know we need to move 1200 watts (at least!). We will assume a 24 volt battery pack first. To size our wires we need to know how many amps will flow through the wires from the battery to the inverter. For this we divide 1200 (watts) by 24 (volts). The result of this is 50 amps. If we go strictly by the chart, we will need an absolute minimum wire size of 6 gauge wire to the inverter. Assuming for a moment that our battery pack is 12 volts we do the math and find we will be conducting 100 amps. The minimum safe wire size to the inverter is now 1 gauge wire.

An important note to all of this is wire distance. The longer the wire run the bigger the wire needs to be to compensate for resistance in the wire. This is only important when the batteries are far from the inverter, or the inverter is far from the circuit. This is only the basics. We need to look at the system as a whole.

Wiring and circuit protection

We are all familiar with circuit breakers and fuses. What many fail to realize is that those are there to protect the wires and not the devices attached to them. Because of this, we need to size everything to the potential of how much power the inverter is capable of. In my case I have a 24 volt input 120 volt output 2500 watt inverter. Wires and circuit protection should be based on that. I chose a 2500 watt inverter because it is only capable of supplying 20 amps at 120 volts AC. This is the average amperage of a single branch of a household circuit. Going back to our chart, this means we are still safe with our 14 gauge wiring.

The REAL issue comes on the DC side. To make those 20 amps on the AC side we need to draw 104 amps at 24 volts from the batteries. Because of this we look at our chart to find that we need 1 gauge wire minimum along with a fuse or circuit breaker around 100 amps. I can tell you first hand that the wires to connect to your batteries that come with the inverter are of a laughable size at best and downright dangerous at worst.

DC system size selection

A question many usually have is what DC voltage to use. Using the above example, simply from a wiring size I would say anything above 1000 watts or so should be 24 volts DC. At 1000 watts DC, we will be moving 83 amps. This keeps us in the 3 gauge side (still manageable). However if we built our 2500 watt system at 12 volts DC we would need to size for 208 amps! This means a minimum of 3/0 copper line! My suggestion, ANY off grid system should be 24 volts DC or higher. Any vehicle mounted 12 volt (by necessity) system should be MAXIMUM 1000 watts.

Batteries

The care and feeding of batteries is a whole subject in and of itself. Its not something I am going to get deep into here. I will simply add a few notes on. All I say here will be in very general terms just to get an idea of how much battery might be acceptable.

First of all, STAY AWAY FROM AUTOMOTIVE STARTING BATTERIES! There are plenty of sources out there that will explain why I say this. Secondly, the most "expensive" batteries are "cheap" batteries. What I mean by this is that you will replace used or cheap Walmart deep cycle" batteries multiple times before you will if you simply buy new good batteries from the beginning.

Do not buy flooded lead acid to use inside unless your space is well ventilated. Ignoring this runs the risk of burning down your house as these can vent hydrogen gas during charging. If you dont want to experience the last moments of the Hindenburg, just dont do it! Also, no matter what "voltage" your battery is, it has the potential to injure or even kill you if you disrespect it! Treat your batteries with the same respect you would a full gasoline tank.

Educate yourself on batteries through Google. What I write here will be simply the bare basics. To get a rough idea of what I needed for capacity, I first looked at my loads. In my case I have only a refrigerator rated at 1.1 Kilowatts per day. I wanted capacity to go 3 days without a proper charge while not drawing the batteries below 1/2 charge (going lower can dramatically lower the life expectancy of the battery). 

With this in mind I needed (1.1 * 3) * 2 Kilowatt hours of battery. There, I threw a curve with kilowatt hours. I do this because thats how to figure the capacity of the battery. They are rated in amp hours. To get Kilowatt hours multiply voltage and amp hours. This means my BARE MINIMUM requirement is 6.6 kilowatt hours of battery.

A quick search showed this https://www.optimabatteries.com/en-us/bluetop-dual-purpose-deep-cycle-and-starting . We will look at the largest capacity available. In this example it is the "Bluetop D31M". Taking the Amp hour rating and multiplying by the voltage we come up with 900 watt hours each. This means we need 7 of them to meet our criteria. Since I am running a 24 volt system I need 8 (4 rows wired in series, with each 2 battery combination wired parallel. This will give me (under ideal conditions) 7.2 kilowatt hours. Lastly using their pricing I come out to a cost of $2648.00

Summary

All of the above is simply a rough overview under ideal conditions. So far I have found my system to be about 50% efficient. Since it was for experiment and development, I went used on the batteries. I know they are not optimum and of the 10 I bought at $100 apiece I have lost 2 so far to dead cells over about a 2 year period. They are 1.1 KW each (12 volt 94 ah) giving me currently 8.8 Kw/hr.

These are charged by 3 300 watt solar panels fixed facing south. We had an optimum day today sunshine wise. With this the panels produced 1.42 Kw/hr. This gives me about 300 watts to charge batteries above what the load is taking. Considering inefficiencies in batteries, charging and inverter, this is about what I need just to run the fridge for 1 day.

All in all, you be the judge. I have ~$3500.00 invested. Just replacing the batteries will wipe out any energy savings. The only benefit I see in solar is for an off the grid system where no options are available. For me though that is what its all about. As for solar being a viable alternative, I say live with it like I have and see for yourself. The "economics" simply dont make any sense unless you're the power company and have government mandating panels on your roof (for essentially free) and grid inter-tie inverters (which by the way dont work during a "blackout").