This originated from a Facebook post I wrote, I thought maybe it would be worthwhile to place it here for folks new to vehicle (actually, bus specific) related off grid power solutions. I’ll probably expand it later I guess.
What is off-grid power?
Going off-grid for power is exactly the same as going off-grid for water, or any other liquid fueled endeavor. It allows you to not require point of use services, like water, electricity, communications, and fuel. We will talk about electrical off grid, as it relates to feasible implementation in a truck or rv, using solar energy as your power source. (because you can’t really hook up hydro power to your bus, for example)
Electrical power storage is:
Just like you have an engine that consumes so much fuel for distance, and the size of your gas tank.
Just like the water tank that you can fill up, and how effectively you use that water before refilling.
The engineering trifecta
First, I bring out the engineering trifecta: Your project can be Fast, Good, or Cheap – but you only get to pick two of those aspects. I personally lean towards good and cheap, and not fast.
Since you’ve got a month, you get Fast and Good, or Fast and Cheap.
What you’re going to end up doing here is iterating through a list of requirements, then ordering them by value. Once you’ve ordered them by value, you are going to determine the cost of the system, repeatedly, until you can no longer afford it. At that point, you draw a line through your list, and anything below the line is not implemented.
Here’s a list I’d start with:
“Hypothetical offgrid solution”
1. easy to use
4. fits on my vehicle
5. doesn’t take over all the space
6. refrigerator power
7. water pump power
8. interior lighting power
9. heating power
10. stereo/radio power
11. laptop charging
12. cell phone charging
16. air conditioner
…and so on
So the thing is, as you repeat this exercise, you’re going to find out that certain things you can really live without, and certain things you cannot. If you know how to pop things into spreadsheets, you could assign a cost estimate to each item. From there, all sorts of sorting magic can happen, where you average a “must have, maybe have, meh whatever” priority to your items, and come up with a truer priority of your items.
The reason you want to go through this exercise is because it can become expensive to change your mind later. Let’s say you decide that you want to include air conditioning as a “must have”.
Suddenly, your power collection, storage, and conversion requirements have increased 4x, and your cost has increased 10x. I have found that there is a non-linear scaling of overall power capacity to cost, when the space stays the same size (like on a bus)
Once you have a first pass at your prioritized list of requirements, you need to figure out their power draw. There are many off-grid load calculators available on the internet. Here’s one for example:
Note the numbers work in watts, watt-hours, or kilowatt-hours. This means, that for an instant amount of time, a given device requires x watts. In order to run that device for one hour, it needs x watt-hours.
You’ll find if you are realistic, that the substantial loads are your air conditioner, then a furnace, followed by the refrigerator, then the lighting. These end up being large loads due to the nature of running for long periods of time, causing long term draw.
As you go through your draw calculation, you now have a rough idea of the amount of KWh that you are consuming. This means, how many watts must you constantly collect (or generate) to support your load.
Now you’re probably getting an idea of how the KWh load can be highly variable, depending on climate and use. One of the most important things about knowing KWh load is that you need to build your system for the maximum KWh load you expect to use.
Once you know your ideal maximum, you can strengthen portions of your system to support that load, and minimize (the cost) of your system to keep costs, or size down.
For example, if you elect to only run high draw, low time appliances like toasters and hair dryers, then you only need to have enough inverting capacity (sort of expensive), and not necessarily a battery (the most expensive part) to operate it 24/7.
Unfortunately, we can’t cool down with toasters, so you’re going to start thinking about air conditioning. A small A/C unit draws at least as much as one hair dryer (1500 watts) for possibly many hours during a 24 hour period of time. This means, it is a large KWh draw, and you’ll need large reserves of storage and generating or collecting capacity to operate it effectively.
Calculating system size
Start with the battery first
If you are planning on off-grid, I recommend you always start with the battery. Think of it just like a water tank. The solar panels are your hose you use to refill this tank.
Now think of your electrical consumers as various water fountains, faucets, toilets, and lawn sprinklers. Some of them are on all day long, some of them are only in use for a few seconds. They all consume water and wash down the drain, when in use.
Your battery (water tank) only has a fill hole of a certain size, and the tank itself is only so big. Any time you try to refill it with too much flow (collected energy from panels), all that precious water spills over the tank and onto the ground, wasted.
If your flow is too small, then your tank (batteries) end up not full when the water spigot turns off for the day (when the sun sets)
Then follow up with the solar panels next
The panels are like a water hose that turns on, all by itself, for a small part of the day. You use this hose to refill your water tank, at the same exact time that it’s being drained by your various appliances (sprinklers, faucets, toilets)
The more solar panels, the more flow and volume your hose (panels) have when it’s on.
Some of the details!
There are so many tiny details and interactions between systems! For every type of module, system, or configuration, there’s another detail. You need to work through the process of requirements to limit the scope of choices to be made.
For example, where you live and the type of weather affects the solar panels.
Even the way the panels are facing the sun during the day affects their ability to collect power. Any panel that doesn’t “track” with the sun looses effectiveness. Maximum energy is collected when the light rays are exactly perpendicular to the panels. The lower the sun is in the sky, the less bright it is. These all affect how much power you can collect, and thus how much you can fill into your collection tank (your battery)
Other things: if you are running air conditioning, how energy efficient is it? This means, not just the unit itself, but how much heat soak/loss do you experience in your home/vehicle/box? If it’s highly insulated, you don’t need as large of an air conditioner, or it doesn’t need to cycle as much. If it’s really lossy? (like those lovely school bus windows) Then you’re using more power to maintain a given temperature.
Lights – more efficient, the less watt/hours.
Refrigeration – bigger the fridge, usually more power. Have kids who open the door and stare at the food? More power.
You need to learn the details of each type of system you plan on using. All the electrical equipment has various rated limitations and capabilities. As you research each of them you can start to draw conclusions about what relates to what. Sorry that I’m not expanding on this portion, but you need to read about each of the major aspects of the main systems (batteries, inverters, panels, charge controllers, air conditioning, heating, appliances) and find the commonality between them: duty cycle, watts consumption, electrical requirements.
Ok, so basically you need to repeat this exercise of calculating your drain with the supply until you can find the maximum size you can afford of both panels and batteries, and that also fit inside or on top of your vehicle without compromising the other aspects of it. Just think about it in the terms of watt-hours (or kilowatt-hours). We will talk about voltage and ac/dc in a moment.
You want to end up with a net zero or slight surplus for a given 24 hour period. Optimize for the worst environmental conditions, (winter, hot, no sun) but don’t over-optimize and artificially inflate the capability.
In a lot of ways, it’s like buying a vehicle: buy it for what you’ll be doing 80 percent of the time, not that other 20 percent.
Volts and amps, and how they affect you:
We are not talking about AC/DC conversion yet. Simply voltage and amps. These concepts apply (in general, but slightly differently) between alternating current (like household power) and direct current (like battery power)
Now that you’ve got an idea of what your watt consumption is, you need to know how system voltages affects amperage, and how that is related to watts.
Watt’s law rule summarized here:
Volts * Amps = Watts
Watts / Amps = Volts
Watts / Volts = Amps
An ampere (amp) is the VOLUME of electricity.
A voltaire (volt) is the PRESSURE the electricity is moving.
The combination of the two describes the electrical flow. Just like a water hose of different pressures and volumes.
Now that you can convert between the numbers, you need to consider the system capabilities.
For example, let’s say you have a 1000 watt power draw, for one hour – 1 KWh.
If you want to power that with 12 volts, then you need 83 amps of current.
If you want to power that with 48 volts, then you need 20 amps of current.
Parallel vs. Serial
For a given device, like a lightbulb or a battery, you can connect the circuit in parallel or series. If you connect several 12 volt batteries in series (one after the other) you add up the voltages, and their amp capacity stays the same.
If you connect several 12 volt batteries in parallel, the voltage stays the same, but the ampacity increases.
Now I introduce you to Ohm’s Law:
Ohm’s law summarized here:
Amps = Volts / Ohms
Volts = Amps * Ohms
Ohms = Volts / Amps
This translates into the real world as wire size, and the voltages that your devices operate at. The bigger the wire, the less resistance for a given distance.
This also can be seen as: the higher the voltage, the smaller the wires you need.
I’ll touch on one of the most important aspects of electrical power:
The higher voltage your system is, the smaller the wires you’ll need to carry the same power.
If you have 8x panels that output 12 volts, you can put them all together in parallel. But your panels only have so much ampacity before they catch on fire during a fault.
Now, I’m simplifying by omitting a few things in the explanation below, but things can happen that cause failures or more specifically, connections where you don’t want them, or breaks in connections where you don’t want them.
If all 8 of those 12 volt panels, connected in parallel, generates 5 amps of current, then you will have 40 amps of current at 12 volts. The size of wire required to carry 40 amps continuously at 12 volts is substantial.
If there’s a “failure” in one of those panels, and those panels are only rated at 15 amps of current carrying capacity, then the 32 amps of remaining current can permanently damage one of those panels. You’ve exceeded the safe capacity (ampacity) of the panels by wiring in that way.
Now, switch to ohm’s law for a second. The longer the cable is, the bigger it needs to be to prevent voltage lost due to resistance.
Wires come in different sizes, and the bigger the wire, the heavier it is, the more expensive it is, and the more difficult it is to work with and fit places. Solar panels “look” basically like a wire of a certain size and length to an electrical system.
Now if you connected all 8 of those 12 volts @ 5 amps panels in series, you’ll have 96 volts DC, at 5 amps. The size of wire required to carry 5 amps with minimal loss is far smaller than the first scenario.
Electrical handling equipment optimization
Your solar charge controllers which optimize power from panels into electricity that the batteries can use, and the inverter which converts direct current into alternating current, and the batteries themselves, all can be specified and configured for varying types of voltage settings. You can wire your panels for 96 volts, and your charge controller to convert 96 into 48 volts DC batteries. The inverter can take 48 volts DC and convert to 120 volts AC.
You could also do the same with a 24, or 12 volt DC battery system, and you can configure your panels from 96 volts all the way down to 12 volts.
These systems are all dependent on each other’s configuration for optimized power handling. It’s up to you to decide what your loads, capacities, and budget looks like to select an optimal power solution for your lifestyle. (this is why I said it’s complex and has significant impact on your choices)
Once you’ve invested in a particular type of set up, you may need to discard and completely re-do portions of your system. It’s difficult to balance the equation and create a system where you can just “add more later” without some sort of drawback or inefficiencies.