How do we get our typical household 120 VAC current from a 12-volt battery? Let’s take a look at a simple inverted system which provides that function.
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In the previous episode, Basic System with a Battery, there were a few limitations/problems:
- It required a voltage-matched solar module
- With a larger solar panel, it was subject to overcharging the battery once the battery was fully charged
- A higher-voltage PV module would over charge the battery
- It did not supply 120 VAC home power
With our new off-grid system configuration, we will rectify all four of those issues.
This is the forth episode in the Going Off-Grid series. To start at the beginning, go to episode 9, Going Off-Grid – Introduction.
Going Off-Grid – Simple Inverted System – Video Portion
Our New Example Configuration
1. Solar Panel
We have replaced the small 12-volt trickle charger solar panel with a 72.5 watt First Solar FS-272 monocrystalline solar panel. Its MPP voltage is 67.9 and MPP amps is 1.07. We will focus on solar panel technology in more detail later.
2. Solar Charge Controller
This is a new addition to our example configuration – Renogy Rover 20A MPPT Charge Controller.
- Maximum Power Point Tracking (MPPT)
- Nominal System Voltage: 12V/24V Auto Recognition
- Rated Battery Current: 20A
- Maximum Solar Input Voltage: 100 VDC
- Maximum Solar Input Power: 12V @ 260W; 24V @ 520W
Renogy is a solar equipment manufacturer in the sweet spot of smaller systems. This charge controller has many additional features I didn’t cover in the video. I really like their products. You can view this and other Renogy products on Amazon -> here.
We are using the same 12 volt deep cycle marine battery as in the last video. Deep cycle batteries, though using the same lead acid technology, are different from car batteries. Deep cycle batteries can be discharged more deeply and more often than a car battery. Car batteries can put out more power for shorter periods of time (cold cranking amps). We will cover batteries in more detail in a later episode.
Cobra CPI-2575 2500 Watt 12 Volt DC to 120 Volt AC Power Inverter is another addition to our example configuration. Since we didn’t have power on site while building our home, this inverter provided ALL of our power needs for months before we put in our whole house off-grid system. It ran everything from coffee machines to saws to air compressors with absolutely no problems whatsoever. You can view this and other Cobra Inverters on Amazon -> here. I’ve also been very happy with Bestek Inverters, which you can view on Amazon -> here. We will cover other types of inverters in more detail further into the series.
Instead of the 12-volt DC fan, we now have a standard household 120-volt AC power drill.
The diagram for this configuration is slightly larger than the previous system, but still fairly straight forward:
Lead Acid Batteries
I hadn’t really intended to get into battery technology, but since I’ve already opened that can of worms, here is a simple article that makes a great introduction: Battery Basics.
Here’s another non-technical, but slightly more comprehensive article: Battery Basics: A Layman’s Guide to Batteries.
We will cover batteries in more detail in a later episode.
DC or AC?
Some may be wondering why we need to bother converting from DC to AC at all. When I was planning my off-grid system over a decade ago, I had this same question. And this has been a debate for well over a century. Nikola Tesla and Thomas Edison battled over which form of current is better and safer, DC or AC?
Here are a few helpful articles on the topic:
- The War of the Currents: AC vs. DC Power
- AC/DC: The Tesla–Edison Feud
- When to Use DC Appliances and Voltage Converters
DC System Pros
- Eliminates the costs of an inverter
- Removes the potential point of failure of an inverter
- Can be more efficient by avoiding inverter power consumption
- Can be simpler than an inverted system
DC System Cons
- Assuming a 12V DC system, wire sizing can be substantial, even for shorter runs. Larger wires are very difficult to work with and very expensive. The multi-strand wires are easier to work with, but get even more expensive.
- DC switches and breakers are more expensive.
- 12V DC appliances have fewer choices, can be very expensive and not nearly as functional as AC appliances. Higher voltage (24, 36 & 48V) appliances, though more efficient than 12V appliances, are even less available than 12V appliances.
- If a house is wired for DC, selling it or getting a mortgage will be nearly impossible. You may think you’ll never sell your house, but if something happens to change that, rerunning house wiring for AC will be a HUGE project.
- You may not be able to get home insurance on a DC wired home, or insurance can refuse to pay out on a claim.
- DC systems (plugs, switches, breakers, plates, etc) are less standardized than 120 VAC systems.
Then there is the decades of experience the RV industry has in this area. An RV is the absolute ideal situation to implement 12V DC only power. Yet the RV industry has determined that inverted systems still play a crucial role in RVs.
I spent a LOT of time going down the DC house path and found it to be a dead-end. You can save yourself a lot of time, headache and money by learning from the RV industry or you can be like me and learn it the hard way.
A really good article on AC power: The Power Plant: Alternating Current
Originally, I was going to do an overview of my current off-grid home system. But certain life events compelled me to cover another topic, diverging from the Going Off-Grid series, A Cord of Three Strands is Not Easily Broken.
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And, of course, be well.