I would label myself as ‘Tiny Curious’ in that I have dabbled in Tiny House building, but I have yet to actually live in one myself. I have spent the last year building a tiny house with my son, and that has been one of the most fun and rewarding projects I have ever done. Since I made a choice to use rough cut lumber I have spent most of the time feeding beautiful boards that I bought for almost nothing through a planer to make them into the beautiful trim pieces they want to be. This article is about how you can take your off-grid tiny house and power it with an e-bike battery and a few solar panels.
If you’re thinking of running your off-grid home with lead-acid batteries, you really shouldn’t. Lithium packs have now reached the point where they are much cheaper, last twice as long, can drain safely down to 20% instead of just 50%, and take up far less space than Lead Acid. They are also pretty much zero maintenance, whereas Lead Acid batteries must have the water level checked about once a month and topped off with distilled water. This process is not only a pain, but you also should wear nitrile gloves and safety glasses because getting splashed with battery acid is never good for your complexion.
You can now buy several kinds of ebike batteries that store between 1 and 2 Kw of power for around $800-$1600 dollars. The one I am recommending for this article is the 2Kw Sur Ron spare battery for about $1600 here however almost any large 60v or 48v nominal ebike battery pack will work with the Outback charge controller. I don’t recommend using a battery that is less than 20Ah to power a house because it will be just too small. These 18650 batteries are very reliable and much safer than Lead Acid. In the last 14 years of being off-grid my wife has exploded 2 sets of Lead Acid batteries and it was a total disaster both times with smoke and acid everywhere. Don’t believe anyone who tells you Lead Acid is safer than a non-counterfeit quality lithium 18650 packs.
In order to run your tiny house off a lithium pack, I recommend getting something that has between 1 and 2Kw of usable power. If you don’t use power tools, shop vacs or microwaves then you should be able to get 2 days of power for every kW even if there is no usable sun at all (this is what we get). This all depends on your fridge, which is always the biggest energy hog in any off-grid home. We use a Sun-Danzer which we have to recharge with CFC every 3-4 years. I’m just not going to recommend that brand. The best manufacturer seems to be Norcold, but their chest freezer options are limited. There is also Edgestar, which I have used and can recommend, but their biggest fridge is pretty small at 2.8 Cubic feet here. Another option is to get a very efficient 120v AC chest freezer and hook it up to a thermal relay that cycles the fridge on and off based on the inside temperature like this one for $65 on Amazon. When you do this, you are committing to leaving your AC inverter on all the time, which will have a leech load on your batteries. Having a chest freezer with a thermal relay is by far the cheapest option. When you buy the freezer make sure to get one of the most efficient models they sell by looking at the yellow stickers.
Buy a charger with the battery
If you get an ebike battery, make sure you get an AC charger to go with it. If you get stuck in the winter without any solar, you can always just grab your whole battery and take it to work with you and then just charge it at work and bring it back home again. I’ve noticed that most fridges work pretty well when there is no power for a couple of hours as long as you don’t open them. Also when it comes to fridge shopping you should only consider chest type fridges and not ones that open up with a front vertical door like a normal refrigerator and spill all your cold out onto the floor. It’s an asinine design which is incredibly inefficient.
Getting an AC inverter to work with your battery pack
You will not have too much luck getting an AC inverter in the US for less than $500 that will work with the higher voltages of ebike lithium packs. Luckily you can get them dirt cheap from China pretty easily. Do a search on Aliexpress for the nominal voltage of your pack + Inverter + 120v + AC. For the Sur Ron pack you would search for “60v Inverter 120V AC” and pick one that has more than 3000W capacity and a metal case. If you want to connect a Sur Ron pack to an inverter you will also need the special connectors available here for $21.
Wiring your breaker box to your AC Inverter
It’s a little tricky wiring your breaker box to your AC inverter. I recommend that you drive a grounding spike into the earth at least 4 feet and wire it to the ground bar on your breaker box. This will allow your setup to have a true ground and will be much safer if there is a problem with the inverter or a short anywhere in your home. You don’t want to wire your inverter into the main power switch of your breaker box, the smart thing to do is get a double pole 240v breaker and wire that instead. Get a breaker that has the right amp rating to protect your inverter. If you want to make sure to draw less than 20 Amps from your house, then get a 20Amp breaker. 20 Amps is what I recommend, but if you get a beefy inverter you might be able to go to 30. Normally breakers protect the wiring ‘downstream’ of the breaker but in this instance, you are using the breaker to protect your inverter from getting overloaded. The main bus of your breaker box will easily carry well over 100 Amps, so the breaker will shut off when you have too much load on the inverter, hopefully before the inverter is damaged or overheats.
Be aware that the Chinese are notorious for over-rating the wattage capability of their inverters, so I usually try to buy double what I need (ex: if I want to pull 2500W I will buy a 5000W inverter.) When wiring your inverter you will want to wire the ground and the common to the ground bus and the hot to BOTH sides of the double pole breaker. Your tiny house, of course, will not be able to run any 240v appliances, but you will then be powering both sides of the AC power bus in your breaker with the same single phase 120v power.
If you are getting lost by what I am saying, then now is the time to stop and find someone who knows how to wire houses and to get them to do it for you (you can direct them to this article). If you wire the hot prong of the inverter to the ground, you can create a VERY dangerous problem, especially if your house is not grounded.
For most tiny houses you will need about 500-1500W of panels. More panels will ensure that you never run out of power. If you want to use 500W or less of panels then you can use this very cheap $37 charge controller from Amazon. This will work if you live in a very sunny area where there are rarely clouds. If you live in a place with crappy sun like I do in NY state you will need a more powerful charge controller. They can be pretty expensive and it makes sense to get one used if you can. Whatever charge controller you have should be able to output variable DC voltage that will support your Lithium packs. Make sure to hook the charge controller to the charge port of the battery, not the discharge port.
Outback is the only charge controller I recommend, the FlexMax FM80 is ~$460 here (80 Amps output) or the FM60 here (60 Amps output) for $400 and will work with 48v and 60v nominal Lithium pack sizes (it won’t work with 52v packs). We have used an Outback FM60 charge controller for 15 years with 1500W of panels (at 24v) and had zero issues. The FM80 should be able to support up to 4000W of panels at 48v (this limit depends on your battery voltage, higher voltage allows for more panels). There are other options out there, so shop around. There is also the Morningstar TS-MPPT-30 which is a 30 Amp charger for $382 available here which claims to support any battery voltage between 8 and 72v (including the 52v Dire Wolf). I have not used this charge controller so I can’t say how well it works, although it has decent reviews on Amazon. At 48v this 30Amp charge controller supports 1400W of panels (more if a higher voltage battery is used).
Buying and mounting solar panels
The people at Backwoods solar are the most friendly and knowledgable ones out there that I have found, so call them if you have questions. There are also 2 different kinds of panel setup, MPPT, and PVM. Your panel type must match your charge controller type. MPPT is much more efficient, although the charge controllers can be more expensive.
When buying panels make sure to mount them so they are facing directly south (actual south not magnetic south). Although you can throw together a cheap wood frame for your panels, having a nice galvanized metal frame will last so much longer and be more aesthetically appealing. Be aware that if you buy 1000W of panels, it is rare that those panels will actually produce 1000W of power, most of the time it will be much less. I would buy a larger charge controller and add panels as you need them and can afford them. Swapping out a charge controller is expensive, adding panels is pretty cheap, wholesale they tend to be about $1/watt (without shipping). I have bought cheap panels on eBay that were <$1/watt but have not had good luck with them (~50% failure rate).
Size your panels to match your storage size, you don’t want either one to be more than twice the other (ie if you have 1500W of solar panels you should probably have more than 750Wh and less than 3000Wh of storage). We are using 1kW of storage with 1500W of panels but would be much happier with 3kW of battery storage in the winter. In the summer we really have no issues. Most off-grid solar installations try to match usable battery capacity with total peak wattage of solar panels. You should not use any lighting sources other than LEDs and get a $14 Kill-A-Watt meter to make sure that nothing you buy or use has a vampire leech load when not turned on.
Other possible battery power sources for your home
Although this article was oriented towards ebike packs, you could also use lithium batteries from recycled electric cars or even power tools. Be aware that if you use power tool batteries that the voltage claimed on the packs is the peak voltage, not the nominal voltage. There is a large number of hackers who reclaim old power tool batteries and build massive Tesla style power walls out of them. I think this is awesome, but soldering directly to the batteries as many of them often do is incredibly dangerous. At the very least make sure you wear safety glasses.
If you have a tiny house, space will be at a premium. 1kW usable of good lead-acid deep cycle batteries will cost about $1400 and weigh 500lbs and take up a cubic meter of space and will need to be vented and maintained monthly. A lithium ebike battery with 1kW usable will cost about $750, be about 14lbs and take up 1/2 cubic food of space, does not have to be vented or maintained. If you’re trying to be smart, it really is no contest, Lithium batteries are the only way to go. When you want to go riding you can just unplug your house, put your battery on your ebike and go!