A Brief Exploration of Power Demands

To start with this, I’m going to delve into our power requirements of one simple subset of the whole system: Computers and internet access.

I have not used anything to determine how much power everything uses yet, so I am going based on the specs for each device and the peak use. Once we can handle the peak use with room to spare, then the average workload should be easy.

For a breakdown, we have two laptops, but before we move in, we will be upgrading to MacBooks (likely an Air and a Pro) that use USB-C power supplies. We also have two tablets, an Apple iPad and an Amazon Kindle Fire 10″. Then we have both of our iPhone 7’s. For backups, servers, and other functions, we will eventually have several Raspberry Pi’s and hopefully a more powerful computer for the server, all powered over DC. There are several hard drives as well, in addition to some backups. And to connect it all together, there will be a very large switch (likely AC-powered), a wifi router, and the hotspot that connects to the internet. 

MacBooks

I’ve already found USB-C DC outlets, but I have no idea if it can charge a MacBook of any flavor. Since the MacBook Air has a 30watt charger, I’m assuming that it requires only 2.5amps at 12volts. The 15″ MacBook Pro has a 87watt charger, which would be 7.25amps at 12volts. The wattage for the Pro is more than the wattage of its battery, while the Air’s charger is a little more than half its battery. I assume that means a Pro should charge in one hour, while the Air would take two hours. 

Is that accurate? I have no idea. I don’t have enough information to figure out exactly. Even if we charge each of them for three hours a day, which is pretty high even if we are using it all the time, that comes out to 30amp-Hours. I think that is high, but that’s okay.

Tablets

The iPad should require about 6.5aH to fully recharge. The Fire tech specs say it should charge in about 5 hours and comes with a 2.4amp/5volt charger. That means 12aH. That also means my calculations already seem very, very wrong. Though actually after reading this article, maybe I’m not so far off!

iPhones

The battery for an iPhone 7 is just under 2aH, so 4aH for both of ours since we just charge at night usually.

Raspberry Pi’s

And here is where it gets complicated. Eventually, I’ll have a million Pi’s, all over the place doing little obscure and specific things. Probably not, but well, I’ll still have plenty. I’ve detailed the whole Raspberry Pi server setup, but that may change at any time as things progress. Eventually, I’ll probably settle on about 8, but we’ll round that up to ten just in case. If they are all running at full power (which will not be happening, then they would all draw 2.5amps per hour. So that is 25aH, and with all of them running 24 hours, we come to 600aH. That seems really high.

Then I realized that there is change from 12volt to 5volt. So the amps also change. 600watt-Hours at 5volts becomes only 50aH at 12volts. Much better in that regard.

Hard Drives

I’m not sure if I need to, but I have all the hard drives running on a Y-Splitter to get their own power separately from the Pi they are attached to. There initially will be four hard drives, but at some point I’ll also add a backup system. For the four hard drives to have enough power to run, they draw only 1amp per hour at 5volts. That comes out to 10aH. 

When we get the Networked Attached Storage system set up, it will draw much more power probably, but that is down the road. 

Router and Switch

The router simply cannot be run on DC power yet. At least, I don’t think it can. It actually has a small power brick, so it is possible, but I’ll have to figure out if there is a cable I can use and what it needs to be set to. We are using a Pepwave SOHO Mk3, and the specs page says it can run on 10-24v DC. So if we can somehow figure out how to plug it into the 12volt system, then it should draw 26watts, which would come out to just over 2amps an hour, or 52aH a day. 

I am not sure what switch we will be getting, but since the SOHO only has five ethernet ports, we will need something with at least 16, if not 24. It might be overkill, but I’d like to make it happen. While I’d prefer 24 for eventual expansion, the 24-port Netgear SOHO switch only comes with an AC cord and has a builtin transformer. The 16-port though has a DC input, so we can manage to connect that to the DC power system. At 12volts and 1amp, its 288aH a day. That’s pretty high, but we can work with it.

Hotspot

The hotspot already runs on DC through a small USB power adapter. It draws only 1amp at 5volts though, since it is USB. Total amp hours of 12volt then are 10aH. 

Adding it all up!

Then if we add everything together, we come up with…. 463aH. And that is just for our electronics to keep online and do business. No TVs, no game systems, no making food or running a fan. Nearly 500aH. 

Assuming we get some sort of lead-acid battery and not lithium, that means we need at least 1000aH worth of storage. With 12v 100aH batteries coming in at around $200 a piece, that means about $2,000 worth of batteries, just to run these electronics. 

However, over half of that is from the switch alone. I have no idea why it would be so high, but perhaps there is a way to drop that power significantly. Most of our devices can already run on Wifi, so if we switch over to that, that drops our power demand for this to a mere 175aH. Double that for lead-acid again and we come to 350aH. 

At that target, we can instead use 4-100aH 12volt batteries and have more than enough for everything, plus some room to spare. That drops the price down to $800 for the batteries, which while still a lot of money, is much more easy to handle.

Things to consider

We have to figure out what all needs to be connected with ethernet. I’m not sure if anything really does, and our Pepwave has Wifi AC, so it is pretty fast already. If we can get good wifi throughout the house, then we really don’t need to worry about ethernet anyway. I’ll still probably run it just in case, but limit the devices connected over ethernet just to the things that need to be. The router has 5 ethernet ports on its own, which may be enough for the start as it is.