Sizing Solar
By Mountain Guerilla
Entirely too often, when we start talking with the average “prepper” type, with their focus on some potential future singularity that will result in SHTF or TEOTWAWKI, we see an emphasis on trying to maintain the status quo as much as possible. This ranges from “I’m gonna stockpile enough fuel to run my BugOut Vehicle, because walking is for plebes” to “I need a 18.9 KWH stand-alone off-grid power system with PV panels and an automatic switchover diesel/propane/etc generator, because I want to run the same six televisions, four video game consoles, eighteen laptops and assorted other electronic devices, and I want to be able to leave the lights on all the time if I want.”
This is a ridiculous notion of course, and even though too many “preppers” unconsciously adhere to it, they still generally have the sense to at least verbalize, “Man, SHTF is gonna be rough,” even as they imagine themselves not suffering too much. The more I’ve studied and discussed things with experts in other areas, the more the concept of “Collapse Now and Avoid the Rush” has made sense to me. By reducing our supposed needs now, we enjoy two benefits: first, we begin to recognize how little we actually need, and thus, how much of our purchasing is a matter of being convinced by marketing that we “need” or “want” shit that we really don’t have any use for.
Doubt it? Look at the growth industry known as “self-storage.” The idea that you have so much shit that you don’t use daily, that you need a separate, off-site place to store it, is mind-boggling to me. I’ve rented self-storage units before, but only in the short term, while in the process of moving, or—in the case of the farm—while building the house.
Specifically today, we’re addressing the electrical demands of “off-grid” living during the decline. Typically, when I’ve looked at either installed package deals for off-grid PV/Solar, or I’ve read the descriptions in various books and magazine and web articles, there is this assumed notion that the purchaser is going to run what might be termed a “modern” household on the system. Of course, this includes not only lights, refrigeration, television and DVD, but also the requisite Amazon “Alexa” device, gaming consoles and Internet routers, food blenders and processors, microwave ovens, and a host of other things that I’m sure I’m overlooking, since don’t use any of it, let alone rely on it.
When people find out I’ve spent so little on building our PV system, they usually respond initially with “bullshit!” but then they want to know how I managed it.
There are two basic ways to determine how big of a system you need. The industry method is to determine what you typical daily usage is, factor in some marginal amount (10% for instance), and then multiply that by 3, in case of cloudy days or inclement weather, and then build your system around that demand.
In order to do this, you simply need to know the wattage of every electronic device you will use on the system. Some devices this is easy to determine, because it is listed on the packaging. Light bulbs for instance, almost invariably tell you exactly what their usage is.
For other items, like fans, or televisions, this requires a little bit of math. Fortunately, that math is pretty simple. If you look at the UL label on the back or side of various electric appliances and devices, it will generally tell you what the voltage is, and what the amperage draw is. By multiplying these together, you get to figure out the wattage. As an example, our bedroom fan (we don’t have A/C in the house yet), draws 0.6 amps, at 110 volts. So, 66 watts, assuming I just did my math right… Our television, on the other hand, also uses 110 volts, but draws 5 amps, so it is running 550 watts. By adding up the wattage of all of your electrical demands, you can determine what your total daily consumption should be. Then, you multiply that by a given modifier, representing a number of days, to account for the maximum expected days you might have total cloud cover, negating the panels’ ability to recharge the battery bank. It’s really pretty simple.
The problem with this, for us, was that we didn’t have any idea what our total demands might be. I might go all week without using my laptop at all, and then be on it most of the day on Sunday, prepping my articles for the blog for Monday. At other times, I might spend 5-6 hours every single day, for several months, working on a book. My kids don’t generally watch more than 30-60 minutes per day of cartoons on the DVD, but on a shitty weather day, we might let them watch a couple of movies, adding up to 3-4 hours. My wife goes through phases where she won’t watch a movie for weeks, and then she’ll go through a phase where she stays up late, watching movies until 1 or 2 in the morning, while I lay in bed reading.
Of course, that is daily life now, rather than in a post-grid environment, where we don’t have time for recreation, right? Well, not exactly. You’ve got to remember, after all, that we live this way every day. It’s not a summer vacation for us. We take care of the animals and the gardens every day, just like we will when things get worse. I spend time in the gym and on the range every day, just like we will when things get worse (We have a pretty decent Crossfit box in the backyard, as well as cross-country sprint interval tracks cleared and measured for 200, 400, 800, 1000, and 1600 yards. Anything longer than that, I map out a cross-country route, or just run the roads). The only thing that will really change much, in our daily scheduling, as things get worse, is we can reliably expect to have more people around, as various members of the clan decide it is time for them to emigrate to the farm from their homes in town. So, there might be slightly less television time for the kids, since they can be sent to play with their friends and cousins, more often than the 2-3 days per week they currently get to spend with our people.
There will be slightly less light usage, since I’m more likely to be outside after dark, providing security efforts, but even then, that will be balanced out by the demands of recharging batteries for night vision, flashlights, and etc….
(Which, in itself, is a great example of re-thinking traditional prepper concepts. When I was growing up, the conventional preparedness wisdom was, have lots of kerosene and/or white gas Coleman lanterns to see by after dark. For Night Vision, you still have people who seem convinced that “ten minutes after the lights go out, all the battery-powered equipment will be useless. My question becomes, which do you think is going to run out quicker? The amount of kerosene and white gas you can safely store in your home/garage/outbuilding/etc, or a handful of quality rechargeable lithium batteries for aircraft aluminum flashlights and battery powered lanterns? Take your time thinking about it. It took me a good decade to realize my older conviction on the matter was inherently flawed…)
Additionally, we had limited funds when we were building (we still do, to be sure!), and a lot of the appliances we already owned were not at all appropriate for an off-grid PV system. So, rather than sit down and try to pencil in hypotheticals, I sat down and determined what our absolute minimum “must-have” was, to keep my wife and kids from mutiny. Then, I looked at what I could afford to do. I realized I could probably do the above, with three days worth of reserve, based on the needs I had listed. Those boiled down to, “low wattage demand lights in each room, the ability to charge two cell phones, and two laptops, as well as the ability to run the television and DVD player, and a chest freezer (which I will come back to…).
Once I knew the wattage demands, I needed to convert those to watt-hours. To do that, you simply multiply the watt demands by the number of hours you expect each device to be used daily. So, if my television draws a hypothetical 500 watts, and it is turned on for 6 hours a day, that is 3000 watt hours, or 3 kilowatt hours (KWH). If I keep the lamp in the living room area on from around 9PM, when I go to bed, until 7AM, when I’ve finished getting out of bed, doing PT, and eating breakfast, then it is on for 10 hours a day. If the bulb draws 9 watts (it does), then I’ve used 90 watt-hours.
So, based on my total KWH usage, I can determine both the wattage of my panels needed, and the battery bank storage I need (and, in fact, to a lesser degree of importance, the size of the inverter I need. Lesser, because I simply oversized that to a 5KW inverter, so I knew I would have ample leftover load availability).
We’re far enough South that, even in the winter, most days, we’re going to get a solid 8-10 hours of sunlight on the panels. I have a 1.5KW panel array currently that, according to my charge controller, which is a 60 amp controller, produces around 58-59 amps, for 6+ hours a day, even in winter time. That means, my panels are producing more than 9KW per day. Really, they’re producing closer to 12KW, according to my half-assed record keeping efforts. Of course, that’s on sunny days. On overcast days, they produce less. I will say though, that, unless we are socked in with rain or fog, even on completely cloudy days, we’re producing at least 40-50% of those numbers.
Our usage at night is low enough that, usually by 9AM, my batteries are back at 100%, and are in a float and equalization phase, maintaining their life span.
Our battery bank started out with 6 Everlast Marine/RV deep cycle batteries from Wal-Mart. Those batteries, that no serious off-grid solar person would look seriously at, cost me $70 each. They lasted the better part of three years (and some of them are still in use in different applications). When I replaced the battery bank, I wanted something better, but I knew I couldn’t afford to buy forklift batteries or any of the other typical high end off-grid battery choices. On the recommendation of a local acquaintance who specializes in off-grid installs, I went with Duracell AGM batteries from Sam’s Club, for $170 each. Initially, I bought six of them, each of which is a nominal 105 amp hours. Multiplied by the 12.6 volts of a fully-charged 12V battery (don’t ask, because I can’t explain why the fuck a battery is called a 12V, if a 100% charge is actually 12.6 volts….), that ends up providing just short of 8KWH of storage. Of course, if you discharge below 50%, you dramatically reduce the life cycle of the batteries, so functionally, that battery bank provided a mere 4KWH of storage. In theory, that should not have been enough, since it didn’t offer any leeway for cloudy days without sun. In practice though, we found that it did. All we had to do was, on days without clouds, the kids weren’t allowed to watch anything on DVD until after the batteries had reached 100%. Since we get some charge from the panels even on cloudy days, this really ended up not making any difference in their lives at all.
Nevertheless, in the interest of keeping the system more robust, I eventually added more batteries, as I was able. This is generally frowned on in the solar world, because if one of your older batteries is weakened, it will draw down and damage the brand new batteries. What I’ve found has worked well for me is to simply make sure I tear the whole battery bank down, and test each individual battery, before adding the new batteries to the mix. So far, this has worked well for a couple of years anyway, and I don’t foresee any sort of reason why it shouldn’t continue in the future. What I have ended up with thus far is 12 of the Duracell batteries. That’s a nominal capacity of 15.8KWH, or a practical limit of 7-8KWH. My 1.5KW array, as I mentioned previously, tops that off by 9AM, even in the winter.
For us, that means that, since the panels then maintain the battery bank at a float charge of 13.75 watts, until late in the evening (around 7:30PM this time of year, and—conveniently—because of latitude, roughly the same most of the winter as well), when the house shadow finally blocks the sun from hitting the panels, we can use all the electricity we want, with no concerns.
Assuming you paid the $1/watt for panels, the panel array could be put together for $1500. The battery bank cost me $2200 (I had to pay core charge on a couple of the batteries). My inverter was $500. My charge controller was around $300. So, for less than $5K, I’ve got a system that I barely put a dent in the maximum capacity of.
How did I do that? By reducing our demand. We collapsed now, and avoided the rush.
What do we have, in our house that runs off the electrical system?
A 54 inch flatscreen television. It’s not connected to anything except the DVD player, and occasionally my wife’s laptop when she downloads a movie to watch.
A small DVD player. Seriously. It was like $30 at Wal-Mart like 4 years ago.
7-9 watt light bulbs, throughout the house. Off the top of my head, we have 10-11 in the house. At any given time, somewhere between 2 and 5 of them will be turned on. We’ve conditioned the kids to understand that they have to turn lights off when they leave a room, and they know they are not allowed to sleep with the light on.
We have four fans in the house. Three are box fans that draw 0.6-0.8 amps. One is a small reciprocating fan in our bedroom area that draws 0.4 amps. For further air conditioning, we installed a “geothermal” system that involves 100 linear feet of 4” pipe, buried 6 feet below the surface, and coming up through the floor in two different places. It works…..meh. Between it and keeping windows open, it will keep the house 20 degrees cooler than the outside temperature. That’s significant…until the outside temperature is 120F…..
We charge two laptops (occasionally. My laptop usually gets plugged in on Sunday morning, so I can write my articles for the week. If I’m working on a book, I do it during daylight hours, and then unplug the laptop for the night). We charge two cellphones, and most of the time, I forget to plug mine in until I get in the truck to go somewhere.
I run a Ninja blender daily. It’s actually got a pretty high draw, but it’s only on for literally seconds, so it really doesn’t even count, as far as I can tell. Beyond that, we’ve got Streamlight rechargeable batteries for flashlights, Yaesu radios on chargers, a couple of cheap Cobra handy-talky radios on chargers, and a AA/AAA charger with rechargeables that gets plugged in when I need to charge some.
The only other thing currently on our system is a 7 cubic foot chest freezer. It draws a pretty significant amount (I want to say like 500 watts?) when it is running, but it only runs a couple hours a day, even in summer, so it’s not that bad. The big issue with the chest freezer is the same issue with running power tools and refrigerators off inverters, and that is the start-up surge. Typically, we tell people to budget for 2-3x the running draw for the start-up surge. It doesn’t hurt the battery bank or the solar panels, since it only lasts for a second or two. What it is rough on though is your inverter. We weren’t able to run the freezer on our first inverter. That was a 2KW inverter, but it was purchased at the local AutoZone, and is used by plugging cords directly into it. The problem with them is that none of the outlets will actually tolerate a 2KW draw. Each is only good for 500 or so watts. My 5KW inverter was fine with the load, unless I turned on everything in the house at the same time.
The current 3KW inverter that I purchased to replace the 5KW one, after it was damaged by lightning a couple weeks ago (ground your shit!!!!!), tolerates it, even plugged in to the outlet, but just barely. It will actually chirp the overload alarm. That’s an easy fix, I just need to add another breaker and outlet for it to the household power wiring. I’ve brought in a cousin who is an electrician to do the household wiring, and we’re waiting for him to be able to come back out and finish that task.
The same issue will arise with refrigerators. We have been using a propane refrigerator, bought used from a RV dealer. It works, but it’s a pain-in-the-ass, because it goes through so much propane. Since, even used, it cost me $1300, and it blows through a 20# tank of propane a week, it would have actually been more cost-effective to have simply bought a standard electric refrigerator, on the 5KW inverter system….(I’m familiar with the idea of using a chest freezer with an external thermostat plugged into it. We tried it. It didn’t work worth a damn for us, just because of the inconvenience).
The real moral of this isn’t how inexpensive a solar PV system can be built. We’ve covered that before, not all that long ago. What it is about is determining what you HAVE TO HAVE to live comfortably, if not excessively. What do you really need?
We can look at this from the rule of 3s….
Three minutes without oxygen.
Three hours without shelter.
Three days without water.
Three weeks without food.
Unless you’re on oxygen for health reasons, the only thing you’re really going to need electricity for is possibly ventilation fans, to keep stale air flowing, if you’re dealing with airborne contaminants outside of some sort, whether from an CBRN threat or smoke from wildfires, etc….
Three hours without shelter usually makes people think of staying warm in cold weather. Running electrical heat on solar is a non-starter. The demands because of inefficiency are simply too much. I’m aware that some people are running mini-split heat pumps that serve as both A/C and heat, but I genuinely don’t know how they are doing the heating side without it being ridiculously expensive.
More practical is what I mentioned previously. The use of strategically placed fans, with a properly designed house for the environment, will provide ventilation adequate to stay alive, and moderately comfortable. To be sure, if you’re used to year round climate control, and keeping your home frigid with modern A/C, it’s going to take some getting used to, but people forget, already, that the first practical residential scale air conditioning in the South didn’t come about until well after World War Two. (Carrier built the first electrical A/C unit in 1902. The first residential installation of electric air conditioning occurred in Minneapolis, in 1914. The first window unit was developed in 1945.).
One of the big applications of electricity in the household is one I’m currently trying to get caught up on, and that is running water. I’d like to stick with a 12V system, but the pump needs to be located far enough from the battery bank, on the opposite side of the house, that it’s not practical because of line loss. Instead, I will probably be using a high efficiency 110V pump, since it won’t run for long periods of time anyway. My backup water system, pumping water from the pond to the storage tanks in dry weather, when the rain doesn’t refill the tanks fast enough, I can get away with a 12V pump.
If I was in a place where I could install a water tower for an elevated, gravity-fed system, I wouldn’t need the household pump, but I would build a water trailer, with a large tank, and a dedicated two battery bank, small charge controller, and its own panel.
Three weeks without food. The electrical application to this is obvious. Sure, we can—and should—can foods, dehydrate foods, salt and cure foods, etc, but the convenience of a freezer, for both long term and short term storage and preparation, cannot be denied. I don’t know that I’ve ever met a “prepper” that didn’t have a chest freezer. Too often, their stated plan when the power goes out, falls back on a gas or diesel emergency generator. That’s fine, as far as it goes, but its not a particularly resililent plan, is it? After all, if you’re prepping for anything beyond a short-term power outage, what do you do with all that meat and food when you run out of fuel? Even the prepper porn fiction regularly discusses some woebegone prepper’s wife suddenly forced to cook up everything in the freezer before it goes bad, and sharing it around the neighborhood.
Now, I’m the last motherfucker out there that is going to suggest that sharing food with neighbors is a bad idea, obviously, but…wouldn’t it be nice if you had a way to keep that food a little longer, and share it out in a more rationed approach? I think so…
(I’m also familiar with the argument that having solar panels will just make you a target for thieves and raiders. Meh. If raiders/bandits/outlaw bikers/cannibalistic San Franciscans are going to be a problem, they’re going to be targeting anyone who seems to have anything, not just people with solar panels….The defense for that is not a lack of preparedness, but rather, is projecting security outward, and interdicting them before they get close enough for it to matter. That, of course, is the entire point of Volume One of The Reluctant Partisan….
Of course, beyond just storing food, in the freezer, electricity also facilitates food production in the form of electric mixers and food processors that can be legitimate labor savers. Most of those devices don’t draw a particularly large load, and they’re generally not used very long (the exception would be “bread maker” machines. Those abortions of inventions are not practical, in my experience, off-grid.).
Beyond those, really, the big demands for the off-grid system, from our perspective, is just the ability to keep flashlight batteries, etc charged, and those don’t draw much at all.
Ultimately though, whether you’re planning on moving your entire house off-grid (I highly recommend it!), or you’re just thinking about building an emergency power system to power a few key systems like lights and freezers and maybe charge batteries for radios and flashlights, you need to come up with a way to size your system.
Sizing your PV array is determined by available sunlight hours daily (generally in the winter, since that will be your least sunny season), and the kilowatt hours of your battery bank, and total wattage of your load(s). Sizing your battery bank needs to be based on your expected maximum daily demand. If you’re going to draw 1000 watts, for six hours a day, then a 5KWH battery bank (nominally a 10KWH battery bank, remember!), is not going to be adequate. Really, for that 6KWH demand, you’d want at least an 18KWH storage capacity, but again, as I mentioned previously, that’s not necessarily realistic, and it may not be necessary.
On Batteries
Typically, when you talk to solar off-grid folks, the first thing you’ll hear about is not the PV panels, but the battery bank. Most battery banks are built from reused forklift batteries. These are massive—really MASSIVE—batteries, and are typically 6V, instead of 12V. In order to get to a 12V system, you’ll need to wire two of them together in series, and then each series can be wired together in parallel. The advantage of the forklift batteries is that they are so massive, they offer a lot more power available per battery. The disadvantage of forklift batteries is that they are so massive, you damned near need a forklift to install them. The other disadvantage of forklift batteries is that they are generally procured used, and when they are used in forklifts, they are used hard, and a lot of times have been overdrawn repeatedly, throughout their service life. That means they’re probably not going to last as long as you would hope.
A bank built of 12V batteries will require more batteries, but they are much more manageable. Additionally, the availability of maintenance-free, deep cycle batteries makes them damned near idiot-proof (evidenced by the fact that I’m managing to deal with them successfully….). I’m completely sold on the Duracell DTG31AGM batteries I am currently using. They’re only 105ah each, but they are holding up remarkably well, and they are maintenance free.
