Solar and Power


This page is going to document things I've played with for portable power. I've divided it into a few sections for solar-power (panels or kits), batteries and charge regulators, power inverters, and connectors. For the most part this isn't aimed at saying one thing is better or worse than another, but rather experiences I've had with using them and what seemed to be good or limitations I ran into. Within each section, the products are sorted more or less in the order I purchased them, no special priority to one over another.

Test Setup

For testing, I positioned the solar panel as "square on" to the sun as possible on a flat sidewalk. I measured the short-circuit current and fine-tuned the panel until I found the peak reading I could get. I then recorded this as the "short circuit current". I then switched and measured the "open circuit voltage". This gave me the values most companies advertise (and multiplying them should yield the wattage advertised) though I don't think this is "that useful" of a reading since you will rarely if ever hit either condition.

After that, I connected a resistive load (car tail light bulbs and a rheostat in parallel) and connected them across the panel's output with the meter still reading voltage. I then adjusted the rheostat (and if neccesary disconnected bulbs) until the panel's voltage rose to what I considered useful (for 12V outputs, about 12.8V like a charged battery; for USB about 4.95V that would still be within spec). I did not use any regulators or charge controllers since I wanted to measure only the panels's performance and I was using a purely resistive load. After obtaining a load that resulted in a useful voltage, I switched the meter to be in series with the load and measured the current my test-load was drawing. This gave me a result "XX Current @ YY Voltage" suggesting a starting point for how big a load it can "offset" while keeping a battery charged.

Solar Panels

Harbor Freight 13 Watt Briefcase Solar Charger

Discontinued - Newer version on Harbor Freight website

Test Conditions Open Circuit Voltage Short Circuit Current Useful Power
Virginia, USA - Summer, 80 deg F, Clear Sky 5:00 PM 22.28 V 0.95 A Inconclusive (need smaller test load)

This was the first solar panel I got, it wasn't cheap at the time but it was available to me locally which meant I could get it quickly and play around. I learned it was wired in series which gave very high (almost 40V) open circuit voltage but extremely low current. I re-wired it to be in parallel so it was more useful for 12V systems. It looks like this was discontinued and replaced with a new version which, among other things, adds a voltage selector switch (a very good improvement). Mine came with a couple different ways of connecting including "alagator clamps", a female lighter plug, and a male lighter plug, and a "tool battery charger". The tool battery charger was useless since it didn't fit anything I have but part of it had a standard barrel jack which was useful. The female power socket adapter was fairly useless, most of the things I tried didn't like the high open circuit voltage. The male power socket would be useful to charge a car battery but only if the power sockets are always on. Probably the most versitile thing was the battery clamps since they can be attached to anything. I did discover without a charge regulator even the low current is capable of boiling a small battery so care should be taken for smaller batteries.

Goal Zero Nomad 7 / Guide 10 Solar Charging Kit

Maybe replaced? - Goal Zero website

Output Test Conditions Open Circuit Voltage Short Circuit Current Useful Power
unregulated Virginia, USA - Summer, 80 deg F, Clear Sky 5:00 PM 7.35 V 0.53 A Inconclusive (need smaller test load)
USB Virginia, USA - Summer, 80 deg F, Clear Sky 5:00 PM 5.00 V 0.52 A 0.47 A @ 4.95 V
12V Step-Up Virginia, USA - Summer, 80 deg F, Clear Sky 5:00 PM 14.88 V 0.51 A Inconclusive (need smaller test load)

This was the second solar panel kit I got, its absolutely the perfect size to carry around and tie on a back-pack while out in the sun. I had it in college and also took it on a few trips. It wasn't especially powerful but was quite effective at powering a MiFi or charging a cellphone in full sun. The panel has a couple different outputs -- the raw unregulated output is 6-8 volts but it also has a low-current step-up output for 12V thru an automotive female power socket adapter as well as a 5V USB charge output.

The battery pack is very nice, while the LED seems fairly useless to me its other features are quite extrordinary. The battery pack takes 4x AA NiMH batteries which are easily removable. This means not only can they be easily replaced when they wear out but also it's highly useful to recharge general purpose batteries for use in other devices (radios, flashlights, etc) as well as recharging cellphones and other devices. I know at one time they even sold an AAA tray adapter for it. It's also very nicely designed with more than one way to charge, either mini-USB or the solar input, and it's one of the only USB battery packs I have seen which can simaltaniously recharge and pass-thru power to another device. I've been able to use this feature to charge a smartphone that needed slightly more power from a slightly lower-output AC adapter by passing thru the battery pack. Overall for it's size I have been extremely happy with how this has performed, although the older unit I have doesn't put out enough power to run and charge modern smartphones/tablets while they are charging. It's possible they have implemented a new design but I don't know. Even without that feature it's still quite useful as the raw output can power some simple things such as my radio scanner (requires 9V but works ok on 7-8V) or my handheld radio (which can take 5 to 15 volts) and it's still very useful as a general purpose AA battery charger for all sorts of devices. I think it would also be an amazing little pannel to charge some of the modern fancy Lithium Ion battery banks, though I don't know how quickly it would charge them.

Goal Zero Nomad 13.5M Solar Panel

Discontinued - New version on Goal Zero website

Output Test Conditions Open Circuit Voltage Short Circuit Current Useful Power
unregulated Virginia, USA - Summer, 80 deg F, Clear Sky 5:00 PM 20.21 V 0.66 A Inconclusive (need smaller test load)
USB Virginia, USA - Summer, 80 deg F, Clear Sky 5:00 PM 5.42 V 1.33 A 0.92 A @ 4.96 V

I got this panel used a while after I got the first Goal Zero panel. I'm not quite as happy with this panel, it's difficult to get it to spread out in a way that catches the sun properly but I see they have re-designed it to have 2 larger panels instead fo 4 smaller ones. This probably resolves the "flopping around" issue I have at the cost of making the folded-up size slightly bigger. Aside from the minor annoyance of how to keep it spread out to catch sun on all 4 panels at a good angle, it seems to be a decent panel. I haven't used it a lot but it folds up very nicely and seems like it's at least as good as the Nomad 7 panel in the very limited experience I have with it. One plus, it can be daisy-chained to increase the output power so with the addition of a Goal Zero 12V extension cable I can plug the Nomad 7's 12V step-up output into this panel and get a slightly higher net 12V output current for powering or charging. I used this on a camping trip to recoup some of the power I was draining from my car battery running phone, tablet, and MiFi chargers.

Renogy 50w Panel

Renogy Store website

Test Conditions Open Circuit Voltage Short Circuit Current Useful Power
Virginia, USA - Summer, 80 deg F, Clear Sky 5:00 PM 21.72 V 2.63 A 2.54 A @ 12.8 V

I learned of Renogy from another local ham in my area, he was demonstrating some solar power stuff at a club event and suggested I look at them if I wanted to get into solar. After some checking I found they seemed to have a nice selection and decent prices (from what I could tell). I carefully selected the 50w panel to be the maximum possible output that could also fit in my window if I wanted to.

I've been pleasently surprised at the output from this panel, though it does look rather fragile since it's really designed to be mounted on a roof or other solid mounting system. I've had three positive experiences with this panel so far. The first time I put it to use was when our apartment was having electrical work done, it was not in an optimal position (we face North) but did help to extend the pair of marine batteries thru the entire work day running two aquariums, a fan, the internet, and a light. The other two times I have used it have been loaning it to people for ARRL field day (the second time along with the Renogy 100w folding panel below). Outside of that I have only taken it out for brief demonstrations.

Renogy 100w Folding Briefcase

Renogy Store website (panel only)
Renogy Store website (panel + charge controller)

Test Conditions Open Circuit Voltage Short Circuit Current Useful Power
Virginia, USA - Summer, 80 deg F, Clear Sky 5:00 PM 22.19 V 5.22 A 5.10 A @ 12.8 V

After seeing the 50w Renogy panel, the 100w panel wasn't as much of a surprise but did meet my expectations. It's essentially a pair of 50w panels connected together with a hinge (uncertain if it's parallel or series) with other hardware to make it more portable and easier to set up at an effective angle. I also got a "Y" adapter so I can connect this in parallel with my other panel which gives me quite a bit of useful power. The folding panel also comes with a very nice case to help protect it. My only wish is that it was a bit lighter (or maybe I mean smaller?) but at the same time I wouldn't call it heavy or big considering it's built with metal frames and glass panels rather its very reasonable all things considered and easy to pick up with one hand. I'm very happy with it as a semi-portable power source, especially when combined with my other panel.

Batteries & Regulators

DIY Battery + Regulator in Rubbermaid Box



This is a portable battery supply I built to go with the Harbor Freight solar panel. Originally I was planning to get a second battery and connect them in parallel to increase the capacity but that hasn't happened so far. The current battery is a 10AH sealed lead-acid battery. It includes a home-built charging regulator which is based on a linear voltage regulator circuit to prevent overcharging and limit the charge current to around 1A. For output it has a 20A molex plug from RadioShack which is switched that I could plug in a small power inverter. The grey squares are velcro so I can stick the inverter to the side. It also has a 12V DC car style socket. Recently I have also added an Anderson SB50 connector for interoperation with my other devices.

Deep-Cycle Marine Flooded Lead-Acid

For semi-portable power I have a pair of deep-cycle Marine batteries. They aren't anything fancy, we got them from WalMart when a hurricane was threatening so we would have some basic backup power at the apartment when connected to the power inverter from my car. They aren't identical but they are very similar; the combined capacity is about 200Ah. Since they are flooded lead acid type, they live outside in case they vent. Since I can't leave my solar panels out all the time (I don't want to have anyone complain since I'm in an apartment) but I keep them connected to a boat/ATV/car battery maintainer so they can't go flat. When I'm going to use them for anything in particular (such as field day) I disconnect the maintenance charger and run down the batteries with a load overnight and then recharge them with the solar panels and solar charge controller. This proves they are still holding a charge and in the case of field day allows them to qualify as solar-charged (since I ran them empty and recharged from solar).

Renogy 30A PWM Solar Charge Regulator

Discontinued - New Models on Renogy Store

I got this to go with the Renogy solar panels, picking the 30A module to leave me some room for future expansion. I've had to add some snap-on ferrite beads to the input/output wires because the PWM signal was causing noise on HF when I used it with a radio but otherwise it seems to perform reasonably well. One day it would be nice to get a MPPT controller but they are considerably more expensive and I haven't been able to justify replacing this perfectly good controller. I don't use the "load" screws, instead I just connect my load directly to the batteries so I don't have to worry about current limits with the controller.


Harbor Freight 200w Inverter

Harbor Freight website

These are very cheap both in cost and quality. I initially got this to power a netbook in the car, which it did a decent job of. A couple months after I got it the USB port stopped functioning, I traced the problem to a bad solder joint which restored it to operation. I've come across a few devices which don't like the modified sine wave (my roomate's laptop charger refuses to run on it without additional load connected and motors buzz) but it's certainly better than nothing and dirt cheap. It does work very well for lights, phoen chargers, and other things that use AC/DC adapters.

Xantrex PROwatt 600 Inverter

Xantrex PROwatt SW website

Originally I got this when I got my own car because I knew the modified sine wave (MSW) inverters have some issues and I wanted to have decent amounts of clean, reliable power since I was installing a dedicated 12V/50A accessory feed at the time. It includes a GFCI outlet which is nice in case the cord runs somewhere wet. The power does in fact seem to be a pure sine wave as much as I am able to measure, certainly far better than the cheap inverters. The only complaint I have is it is physically fairly large by comparison. I've been able to successfully start a mini-fridge from this inveter and my two marine batteries without too much difficulty. For reasons I don't understand it seems to have an easier time starting the refridgerator if there is another load (such as an incondescant light bulb) though I'm not sure exactly what the optimal load is. When I was experimenting with this, I used a 40w string of Christmas lights that were witin reach as the initial load and unplugged them as soon as the mini fridge compressor started. I'm now interested in finding a larger capacity Xantrax unit after seeing how well this 600w model performs.

Power Connectors

Anderson SB50 Connectors

Anderson Power SB50 series website

I found these connectors when I was searching for a reliable high-current connector that was polarity-safe nomater how it's connected for my car so I could disconnect the power inverter if I wanted more foot room for the back-seat passengers. I also knew I wanted it to have at least 50A of capacity so I would never need to upgrade if I wanted to move my power inverter elsewhere or install more accessories in my car. It's easier to over-build it to begin with than have to re-design it later.

Once assembled, these connectors are quite remarkable, they are keyed so it's impossible to connect them incorrectly (down to the color of the housing mattering). Once they are snapped together, they are rated to carry much more current than I will probably ever run thru them so I feel confident they have a good safety margin for any application I would use them. They also click together with a very satisfying snap and stay firmly locked together unless they are deliberately pulled apart. The only downside is they are physically larger and more expensive than some other connectors such as the PowerPole series. I still like these as my go-to for anything over 20-30A continous or wires heavier than 12AWG, and I have constructed adapters to inter-operate with people who use the PowerPole plugs for Amateur Radio events.