Dreaming of Apartment Scale Solar


California gets a lot of sun; where I live, we get about 9 months per year of continuously sunny days. In fact, it’s genuinely surprising when the weather isn’t sunny. And we’re not talking wimpy East Coast sun, either. The sun here is bright and strong, and there’s rarely any pesky clouds to interrupt it.

It’s a shame, then, that so much of the sunlight which falls on my house gets wasted. Solar, as you’d expect, is a big deal here, and lots of people make good money installing big 5kw+ units on their roofs.

My issue, though, is that I rent. My landlord probably wouldn’t be too thrilled if I started climbing around screwing things into the roof. And I’d have to stay in the same place for an awfully long time to recoup the investment involved with installing a solar system on someone else’s property.

Lately, though, I’ve been thinking about ways I could turn some of that constant California sunshine into free (or at least cheap) (or probably expensive, actually, but fun to generate) energy. Why not build a solar system that’s the right scale for an apartment?

Realistically, an apartment scale solar system probably wouldn’t save money, at least in the short term. Even in California, my average electric rate is only $0.18 per kilowatt hour, which is pretty hard to beat.

But there’s other reasons to have a solar system. In addition to sunlight, California also gets earthquakes, and they can cause extended power outages. It would be nice to have a way to generate power even without the grid. And there’s the general self-reliance aspect, which is nice too. Oh, and of course there’s the environment. That thing.

In thinking about designing an apartment scale solar system, a couple requirements come to mind. First, I would want it to operate without the grid. Net metering—where you buy power from the utility when your panels aren’t generating enough to supply your needs—is the trendy model these days. There’s even some plug in solar options (which claim UL certs but still look sketchy to me) that claim to do net metering for apartment dwellers. But these systems are required by law to shut down if there’s a power outage, so you don’t electrocute lineworkers from PGE, so they wouldn’t address the off-grid aspect.

Secondly, the system would have to be portable. It would need to be something you could move from property to property, so no fixed wiring, panels installed on the roof, etc.

Lastly, and probably most important, it would have to be seamless to use. I’m not about to go swapping out my lightbulbs for 12 volt models. And I know myself well enough to know that I’m too lazy even to have to unplug from one outlet and plug into another when the system wasn’t generating enough power. So my system would need to be automatic; it would use my existing appliances, and would run on solar when solar was available, but would switch to another power source (assuming one was available; see the off grid comments above) when the sun went down and the battery ran out.

So here’s what I came up with.

Panels and Charge Controllers

Renogy offers some really nice looking monocrystaline panels for cheap. They’re small enough to set up on your porch or deck, but efficient enough to generate some decent power output. $180 buys you a 100 watt kit, which includes the panels, cabling, and a PWM charge controller.


These are pretty easy. All I would need is a deep cycle 12 volt battery from Autozone. The RV models are designed to hold a bunch of juice, and can output lots of power over a long time without losing capacity.

A 60 amp hour battery runs about $120. Applying some basic high school physics, if my panels output 100 watts at 12 volts, that’s 8.33 amps. So a 60 amp hour battery would hold about 7.2 hours worth of California sunshine (and yes, I’m simplifying by ignoring charging losses and panel losses…sue me).


Here’s where things get interesting. Everything so far is pretty straightforward hardware. And the inverter could be too; you can get little car inverters which turn your 12v input into about 100 watts of AC output, for as little as $20. You could hook up one of these to your battery, plug in a lamp or something, and be done with it.

This is where the laziness comes into play, though. I would want my system to be automatic; more like the panels people put on their roofs than the hacked together systems Midwesterners put in their hunting cabins. So I don’t want an inverter where I have to physically unplug my appliances and plug them in elsewhere when the solar power runs out. I would want something which runs on solar when it’s there, but can move over to grid power when it’s not.

Luckily, there’s a model for this kind of operation. It’s a Uninterruptable Power Supply (UPS). People use them to power computers and TVs during intermittent outstages. A UPS has a battery, an inverter, and a connection to the wall. Under normal operation, the wall power powers the appliance plugged into the UPS, and also charges the battery. If the wall power is cut, the system switches (within a couple milliseconds) to the battery, giving your enough time to power down your computer safely, or finish your show.

My system would be like a UPS, but in reverse. It would run off the battery (charged by the Renogy panel) under normal conditions, but could instantly fail over to wall power if the battery ran out.

I’m not sure first one to have this idea. Like most things, countless people on the internet have been there first. Just type “solar UPS” into Youtube, and you’ll see what I mean. But I don’t want a solution which involves hacking a traditional desktop UPS. I want an apartment scale solar system, not a house fire. So what are the options?

Enter the APS750 inverter from Tripplite. It’s a beefy, commercial grade inverter which produces up to 2000 watts, and has lots of neat features, like filtered power output for running sensitive electronics. But most importantly, it has a UPS mode. You can plug it into wall power as well as a battery, and it will fail over to battery if the wall power is cut.

So how would I cut the wall power at will, keeping the system in a failure state by default so it can run off the battery/solar?

Control System

Enter the home automation portion of the project. I figure the best bet would be have an Arduino or similar board monitoring the voltage of my 12v battery using a simple voltage divider circuit. The Triplite inverter’s wall power plug would then go into a Powerswitch tail before connecting to the grid, and the Powerswitch tail would also go to the Arduino. The 12v input on the Tripplite would go to the battery.

When the Arduino detected that the battery voltage had reached a certain level, it would trigger the Powerswitch, cutting the grid power to the Triplite. The Triplite would think a power outage had occurred, and would instantly switch over to running off the battery (again, charged by the Renogy panel). Tada—instant solar system! When the battery got too low, the Arduino would turn the Powerswitch Tail back on, restoring grid power to the Triplite, and keeping my appliance running.

The whole thing would be very portable, as it would just be a bunch of self contained units plugged into each other; Renogy to Battery, Battery to Triplite, Triplite to Appliance and Wall. Nothing to short out or require professional installation. And there are a ton of things I could plug into it. My media center condenses a television, receiver, DVD player, Roku, and a couple floor lamps into a single plug, so it would be a great candidate.

The Economics

So, how much would it cost?

• Renogy kit: $180

• Battery: $120

• Triplite: $300

• Arduino: $30

• Powerswitch Tail: $30

All told, it would be $660 for this apartment scale solar system. Supposing it generated 100 watts of power for 8 hours per day, that would be 800 watt hours per day at 12 volts, or 66 amp hours, which translates to  717 watt hours per day at 120 volts (ignoring charging and conversion losses for now…again, sue me). That means .71 kwh per day, or $0.1278 worth of electricity saved. You would need to operate the system for 14 years, under absolutely perfect sun conditions (which don’t exist in reality, even in California) to recoup the cost.

So it wouldn’t exactly be saving a bunch of money. But there’s some good news; a system like this might be able to qualify for federal sustainable energy tax credits, which would take about 30% off the price.

And of course, the best reasons for building a system like this probably aren’t financial. A couple years back, before I was on the West coast, I experienced Hurricane Sandy and its lovely aftermath; a 4 day power outage which left my neighborhood pitch black at night. I was grateful to have an old car jumpstarter and an inverter, which I hacked into a system for powering some lights. Having the ability to go off grid is nice for situations like that. And it’s also nice just to know you have the option.

With all good things home automation related, there are a bunch of ways my apartment scale solar system could be expanded. You could always add more panels, or a larger battery, to increase the capacity. And you could find other ways to charge it, too. If your landlord is really nice, you might be able to justify a wind generator, which outputs a ton more power than a solar panel. And if not, there’s always a bicycle generator, which would let you make free power and improve your quads.

And of course, there’s a ton you could do if you were willing to go just a bit more hardcore. Swapping out your 120v lights for 12v light bulbs and then running them off your system would skip all the hassle of the inverter, and could give you some serious run time and power savings. And why stop there? Amazon all kinds of 12v appliances. Solar powered vacuum cleaner, anyone?

So that’s my idea for an apartment scale solar system. Now the big question is, should I actually build it?