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Introducing Personal Solar Power

by Judy Swann & Laurence Clarkberg

Laurence: People tend to think of electrical power production as something that requires a huge infrastructure including smoke-belching power plants, mysterious fenced-off areas with big humming transformers, and crackling high-voltage power transmission lines hung on big steel structures towering over us like malevolent giant robots marching to the horizon. I declare that as of today, all of that big bad stuff is unnecessary. Get rid of it! Send it back to hell where it came from. Because it is perfectly possible to generate all of the electrical power you need with four inexpensive human-scale components that you can carry with you wherever you need power, upstairs and downstairs, indoors and outdoors. "Really?" you ask. "What sort of components? What do they cost? Where can I get them, and how do I hook them up together to meet all of my electrical needs?" Read on, dear reader, and all of these questions and more will be answered.

The key component you will need is a big ebike battery. If, like many readers of this blog you have an ebike with a big battery, you are already financially halfway to freedom from the grid! Ideally you need a battery large enough to supply house current (described later) and that has connectors commonly used by ebike hobbyists (also described later). These larger "52v 20ah" or batteries are often in the form of "triangle packs" designed to fit into the frame of a bicycle. They have two big wires to output power and two smaller wires that accept power from a charger. Cost can be as low as $570. If you don't already have a big ebike battery, consider these offerings by premier ebike hobbyist vendors Grin, em3ev, Luna and Electrify. These vendors can also sell you an ebike kit to go with your new battery. To use your battery for Personal Solar Power you will also need a solar panel, a solar charge controller and an inverter (a device that converts the 52v DC of an ebike battery to the 120v AC of house current). Next I'll give you some suggestions for what to buy.

Triangle pack ebike battery.

You will want a largish solar panel that can gather enough sunlight to recharge your battery in a few hours, but not so large that it is a pain to carry around and set up. A 50w panel is the size of a pizza box and could charge our triangle pack in a couple of days. A 200w panel is the size of a small door and could charge our triangle pack in a few hours. Consider getting a slightly more expensive lightweight "flexible" solar panel that you can take on camping trips (see our Human Rights Tour posts). I splurged a bit on this $330 170w panel from Grin.

Flexible solar panel.

The solar charge controller converts the erratic 20 to 30v output of the solar panel into a steady 58.8v required to charge the ebike battery. It is basically a black box about the size of a sandwich that has two wires for the solar panel input and two wires that output power to the ebike battery's charge port. It used to be very difficult to find solar charge controllers that boosted the solar panel's output voltage high enough to charge a large ebike battery. Now there are several options. Again I splurged on this particularly nice charge controller from Grin. There are less expensive charge controllers, but this $210 charge controller uses MPPT (multiple power point tracking) to capture every bit of sunshine efficiently.

MPPT charge controller.

Lastly, you will want a simple inverter that converts the 52v DC output of the ebike battery into the 120v AC house current that most appliances use. You don't need anything fancy like the big expensive closet-sized inverters that gather power from whole McMansion-sized solar arrays. You want something inexpensive and lightweight: basically a bread-loaf-size black box with terminals where you can attach an ebike battery connector on one end and a regular house current outlet on the other end. As far as I can tell, this is a novelty item only available on Amazon. It used to be very difficult to find an inverter that accepted the large voltages output by ebike batteries. A few years ago I managed to find one that weighed ten pounds and cost $500. Now lower-cost lighter-weight inverters are getting to be more common such as this 4.5 pound $180 inverter on Amazon. Note that although the name of the inverter might say "48v", check the specs. It's likely that the inverter will be able to handle up to 60v input. A "52v" ebike battery can output up to 58.8v, so an inverter that can handle 60v should work. And the name of the inverter may also say it can output "1500w" or "2000w" but the reality might be less than that. I've found that some very energy intensive appliances might not work with small inverters.

Small inverter.

And so to summarize, the cost of the four Personal Solar Power components you will need are currently approximately as follows, and getting less expensive every year:

  • 52v 20ah ebike battery: as low as $570

  • 170w solar panel: $330

  • mppt charge controller: $210

  • 2000w 48v inverter: $180

  • TOTAL: $1,290, which is less than the cost of electricity for a year from the bad guys

Judy: We look at Personal Solar Power as shifting the power paradigm not only from stinky fossil fuels to sweet-smelling solar, but also from a "fixture" model to a "portable" model. Portable makes it practical for everyone to go solar even if they don't own a home. People can unplug their appliances from their landlord's wall and into their own personal inverters. If they pay utilities, they could save hundreds of dollars a year. Since their inverters and solar panels are not a fixture, they can take this equipment with them whenever they move to a new location. And Personal Solar Power invites you to foment revolution on another front: using your ebike battery to ride an ebike and get rid of your car.

My beloved Navigator is a physicist and a bike mechanic already, but even I – a poet and a former Russian major – comprehend that a typical ebike battery can put out approximately 1800 watts of power, which is pretty close to house current. The realization of this proximity was the Navigator’s first Aha! moment on the path to our Personal Solar Power paradigm. "Where does this number 1800w come from?" you ask. To answer that question we must digress for a moment to spell out the relationship between current (the "flow" of electricity measured in amps), voltage (the "pressure" of electricity measured in volts) and power (the ability to do work measured in watts). The relationship is simply this: amps times volts equals watts. Using this formula let's compare the electricity available from an ordinary wall outlet to the electricity available from an ebike battery.

You may know that the voltage from a wall outlet is 120v. And you may know that a common lower-power circuit in a house runs at 15 amps. 120v x 15a = 1800w. And you may know that the 52v large ebike battery we recommend above can produce 58v when fully charged, and looking closely at the specs informs you that it can output at least 30a. 58v x 30a = 1740w, which is almost 1800. Aha! Here is that comparison in table form:

When you look at this table, you should be standing in the watts column and the ebike battery row and when you look up at the household current row you realize, “We can get there from here!”

So our first Aha! moment was realizing that the ebike battery/inverter combo can replace a wall outlet. The Navigator’s second Aha! moment was realizing that the solar panel/ebike battery combination can replace the power plant and the power transmission infrastructure. The sun produces about one kilowatt of power per square yard; even a not terribly conscientious person can use as little as five kilowatt hours per day. So one square yard of solar panels per person, along with about five hours of sunlight per day, can provide all the power that a person needs. (We will explore the concept of "need" in a future post.)

And what about replacing the power transmission infrastructure, both outside the house and inside the house? Outside the house you can simply run a wire from the solar panel in your yard to the battery inside. No utility poles necessary. Or even simpler you can charge the battery outside in a waterproof container and then physically bring the battery inside and place it next to the appliance you want to use. Inside the house we advocate a new power transmission paradigm. In this paradigm we replace the concept of "power outlet as a fixture" to "power outlet as a portable resource". Instead of the highly inefficient current paradigm of installing wall outlets all over your house in the off chance you will want to use an appliance next to that outlet, we instead bring a portable outlet to the appliance when we want to use that appliance. With one bold stroke we have replaced dozens of expensively-installed wall outlets with one portable inverter outlet. Sensible, no?

Personal Solar Power has only recently become practical as the cost of solar panels and ebike batteries has gone down dramatically and the efficiency of solar panels has gone up. Now you can just plop that door-size 200w solar panel in the yard or hang that pizza-box-size 50w solar panel out of your window, plug in your charge controller and battery and in a few hours you are good to go. No need to spew carbons into the atmosphere. No need to cut a huge swath through the forests to get the power from the power plant to your house.

Laurence: Judy and I have already known and expressed ad nauseam that we don't need cars for transportation; ebikes are a viable alternative. These two Aha! moments triggered the realization that we also don’t need to be on-grid for most of our household power needs. We've already experimented with using our solar-charged batteries to power various low-power appliances in our apartment: the lighting in our bedroom (more about that in a future post); a small induction stovetop to do our cooking; and to charge our mobile phones and computers. In fact, I'm typing this blog post on a computer powered by a solar-charged ebike battery.

We've put some thought into how to power higher-power appliances such as refrigerators, space heaters, air conditioners, and hot-water heaters. We've determined that a combination of behavioral changes, new technology, and old technology will make it possible to meet all of our needs (including transportation!) with just a few door-sized solar panels, a few ebike batteries, and a few inverters. For example we can reduce the need for a refrigerator by fermenting some of our foods (behavioral change) and storing certain foods in a root cellar (old technology) and we can improve our fridges using vacuum insulation panels and low-power peltier effect cooling instead of a condenser (new technologies). Stay tuned. We plan to post more about our experiments in future blog posts. We are looking forward to the point where we can with confidence and a sense of celebration turn off the electrical mains to our house.

Judy: Let's go over step-by-step how to hook up your four Personal Solar Power components...

How to Charge a Battery from a Solar Panel:

To charge a battery, connect the battery’s charging port to your solar panel via the charge controller. Note that charge controllers for lead-acid batteries will not work with your lithium ebike battery. Also note that the more expensive MPPT charge controllers work better than the less expensive PWM charge controllers.

The ebike batteries we use have two connectors: a male XT90 connector for the output port and a smaller male XT60 connector for the charging port.

It was necessary for us to add corresponding female connectors to the inverter and charge controller. This required buying the connectors from Amazon and soldering them on. If you don't know how to solder, consult your nearest makerspace, such as the Ithaca Generator, to learn how.

Female XT90 connector added to the input terminals of a small inverter.

The 52v lithium-specific charge controller that we use is the Genasun GVB-8-WP shown previously. This Genasun’s electronics are embedded in epoxy so that the controller can be used in all weather. When we charge our batteries, we plug the battery’s XT60 connector into the corresponding connector on the Genasun, which we then connect to the solar panel’s MC4 connectors. Here is a sexy close-up of the male XT60 connector on the battery:

Here is the battery's male XT60 charge port connector connected to the Genasun's female XT60. Hubba hubba!

Here is the Genasun's input wires connected to the standard MC-4 connectors that most solar panels use:

Here are the solar panel, charge controller, and battery all connected together:

Running an Appliance from an Ebike Battery

To power lights or small appliances from the ebike battery, connect the battery’s XT90 output port connector to an inverter, and then plug the appliance into the inverter's standard outlet.

The male and female XT90 connectors are shaped in such a way that it would be difficult – even for me – to plug the battery into the inverter incorrectly. Match the shapes and push the connectors all the way in. Note that in the photo below, the connectors are not pushed all the way in:

Here is a stack of components ready to operate: battery (on bottom), inverter (in the middle), and an induction cooktop (on top). You can see that the cooktop’s power cord goes directly into the front of the inverter. This particular inverter shows the output voltage on the top display (121v) and the input voltage on the bottom display (49.9v).

Here’s another stack with the battery on the bottom, the inverter in the middle and a desk lamp on top, ready to illuminate:

Laurence: At this point you might remark "When I run an appliance from a wall outlet it will run forever as long as I pay the utility bill and as long as civilization doesn't collapse." And you might then be led to ask "How long will an appliance run when it's plugged into an ebike battery?" I'm glad you asked. To answer that question we need to introduce the concept of energy, measured in watt-hours. A lot of people get energy and power confused. Recall that power is the ability to do work, measured in watts, which can be calculated by multiplying amps times volts. Power is instantaneous. Energy, however, is power operating over a duration of time. In our context you can think of energy as either being produced by a solar panel and going into our battery (as it's being charged over time) or as being stored in the battery and potentially coming out of our battery to operate an appliance over time. Energy can be calculated by multiplying power by time. Conversely, if you know how much energy a battery contains, you can estimate how much power it can produce for a certain duration. The 1,000 watt-hour ebike battery we are using in our examples can produce about 1,000 watts of power for one hour. Or it can produce 500 watts of power for two hours. Or it can produce 100 watts of power for ten hours. Get the idea?

So to answer your question about how long a particular appliance will run off an ebike battery, you need to know how many watts that appliance draws. Is this information that you have at your fingertips? In the age of easy inexpensive power you probably rarely consider how much power your appliances use. But as civilization winds down and easy power becomes scarce, you will have to make more and more decisions about which appliances you want to turn on and which ones you want to leave off. I encourage you to get to know the power draw of your appliances. Let's look at the two appliances in Judy's photos. The desk lamp shown is a very inefficient tungsten incandescent bulb that draws about a 100 watts to produce about 1600 lumens of light. Our 1,000wh ebike battery could keep this bulb lit for about 10 hours. An equivalently bright LED bulb draws about 15w. Our 1,000wh ebike battery could keep the LED bulb lit for about 66.6 hours. Which bulb would you rather have illuminating your bunker after the Zombie Apocalypse?

The induction cooktop shown in the photo is remarkably efficient since it heats up the metal of a pan directly using magnetic induction. However, it draws a lot of power, up to 2,000 watts. Our 1,000wh ebike battery could run this cooktop on it's highest setting for about half an hour. In our experience this cooktop can boil a couple of cups of water in three minutes. So expect to get about 10 pots of tea from one 1,000wh charge.

Lastly, equipped with our new understanding of power and energy, let's look at the performance of some commercial products that rival the performance of the DIY Personal Solar Power setup we have outlined here. You can get a feel for what's available by googling "portable power station". You will find a slew of devices that incorporate all four components that we've discussed: a lithium battery, an inverter, an mppt charge controller and a solar panel in one convenient package intended for use as backup power or for camping. The biggest brand names include Jackery, Goal Zero, Renogy, Ecoflow, and Bluetti.

Comparable commercial product.

"Wow those sure look cool," you may be thinking. "Should I get one of those instead of trying to make my own?" And my answer is "maybe". The main advantage to making your own is that you can cut the cost dramatically using the ebike battery you may already have or want to have. Additionally, an ebike battery is designed to pack a lot of power into a small lightweight package whereas these commercial products use the heavier LifePo lithium batteries. The DIY ebike battery/inverter combo I describe here weighs about 15 pounds; the solar panel and charge controller weigh about 7 pounds. The comparable commercial product shown above weighs 84 pounds. Which would you rather be moving around your house to your various appliances? Lastly, the DIY version I advocate here is much more flexible. You can mix and match components from various vendors, and you can tune the components to your needs. For example, Judy and I plan to mount our Personal Solar Power components on a tandem trike for a cross-country trip next summer. But if you hesitate to enter the world of DIY, and you're not already an ebiker, by all means try one of these commercial products and let us know how well it works for you!


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