This question is asked quite a bit and there is a great deal of confusion. I’m writing this in direct response to a redditor who is asking the question. I’ll later expand upon this article.
Lets get right into it!
Duty cycle, what the F@#$ is that?!
Duty cycle is how long, on average, a device will be active and running. To put things into perspective my fridge has a duty cycle of around 20%-50%. My AC unit has a duty cycle of between 30% and 100%. The duty cycle will vary because the climate varies. The hotter it is, the higher the duty cycle is. The “duty” is like when a cop is “on duty” they are active, working and doing their thing.
For the remainder of this, we will assume the fridge has a duty cycle of 30% and the AC unit of 40%.
Watt the hell Amp I gonna do with all these different rating numbers?!
So this gets kind of confusing as there are a few parts to break down. There is Amps, Amp-hours, Watts, and Watt-hours. One is a unit of power, the other of energy.
The Watts and Amps are like how fast you drive (say 60Mph). Watt-Hours and Amp-Hours are like how far you have driven (180 Miles today — I was driving for 3 hours)
So why do we have two measurements, Amps and Watts? Well, they actually mean different things. The above isn’t specifically correct, but it is close enough for most non-technical people. 🙂
Watts are a complete and universal way to measure power or energy. Amps are only half of the equation. The other part of the equation is volts. Since we are sticking with car anaologies. Amps Wats and Volts break down like this.
Watts are the total number of cars that passed through per second. Volts are how fast each car is going, and amps is how many lanes of cars in total. This also seguways into another point. If we only have a 3 lane highway, but we are pushing 5 lanes of cars through it — they will keep bumping into each other. This gets everybody all excited and heated!
If want to make sure you have enough “lanes” open for all those “cars” to get through when you open the road. In this case that has to do with how thick the actual wire is. There are diagrams. There is one such “ampacity chart” at https://www.powerstream.com/Wire_Size.htm
In that chart you can see that the 10AWG is only good for 15Amps. Notice it doesn’t say how many watts. For this we don’t really care how fast the cars are going here. We only care about how much amps. That decides if the drivers are playing bumper cars or not.
Now that some of the prerequeistes are over, I’m going to jump in with some math.
First, we want to calculate our power usage.
I know that I need about 14 hours of battery power to get to the point the next day where I have decent sun. Here are some numbers to play with:
Air conditioner 450W 30% Duty cycle
Fridge 250Wh a night
12Volt LED light strip, 2Amps. Expected usage 1.5 hours max.
Step 1: Lets get everybody on the same page.
We need some universal number to get everybody on the same page. We are also most interested in what capacity we need so we will simply go straight for the jugular — Watt-hours!
- Air conditioner 450*14*0.3 (450 Watts times 14 hours with a 30% duty cycle)
- Fridge — currently in Watthours. YAY!
- LED Lights: 12*2*1.5 (12volts, 2 Amps, up to 1.5 hours)
Grand total of 2176Wh. Also, 2.176kWh.
I’m going to go really quick here tonight. I’ll expand upon this later if people generally like this guide and I find the time.
2.176kWh, I need a battery that can support that. Lithium batteries are sold by their actual usable capacities, lead batteries are not. For lead you will need to take off 50% or 20% of the capacity depending on the type. for AGM we get to use about 80% of the battery. I’m going to do the math for lithium. You can always adjust for AGM if that is what you end up using.
If you don’t know any better you may be tempted to look at the Battleborn battery and see, gee its like $950 and its 100Ah. So that means it must be 1,200Wh. In the case of lithium, thats true. In the case of AGM, a battery sold as 100Ah will be damaged if you take it below 20%, so its actually around 960.
From the math you need 2 Lithium or 3AGM batteries… or… do you?
All of this math is assuming losses are built in, but what if they are not? Suddenly we take a hit for every bit of copper we run. Remember when we were choosing AWG, those ampacity charts? If the total volume of cars going through is high, but there are not enough lanes we will end up with heated drivers. Whats worse, the more accidents that happen the more accidents will happen! Its like if you were driving down the freeway and had to dodge doors and tires and such. You would likely hit several cars in rush hour traffic!
So, to try to sum this up as I’m tired and I feel like I’m rambling, we have our total capacity, and the batteries we need. Important thigns to note that I’ll cover during a rewrite.
- If you have a lead battery, you need to ensure you get a full battery several times a week to prevent damage — lithiums don’t care about this.
- You cant get power from nothing. You will need a way to charge the batteries. If you go solar only and the sun doesn’t shine your stuff doesn’t turn on!
- To compensate for the above, you may want to get 2 or 3 times your needs in battery storage to ensure you can deal with bad sun days. — Like all of last few weeks since I’ve got my new battery bank — not even kidding.
- You cant use more power then you can generate. Make sure you have enough solar, or generator power to get those batteries back up to charged!
- Make sure you size your wires correctly — keep those wires calm, and those traveling on it nice, calm and cool 🙂