|20W solar panel - 15" x 20" (38 x 50cm)|
A lot of reading preceded all of this, much of it having to do with all of the variables associated with sizing a panel and battery with the varying load that define the operating characteristics of a CW or SSB rig. It became a bit frustrating since every variable is variable to a large degree.
I found that I could come up with any end result I wanted by manipulating the variables by reasonable and realistic amounts. One big variable is, What is the duty cycle of a QRP rig with known current requirements when receiving and transmitting? Well, that leads to another question: How often do you transmit? Contest or ragchew? My answer to these questions was also variable - sometimes I contest, sometimes I listen and never transmit and sometimes I lose track of time altogether in a pile-up.
Other variables are:
How many hours of sunshine are available for solar charging? How much variation in summer and winter? Partial shade or no shade? PWM or MPPT charge controller?
Never getting the same answer twice to the "What do I need?" formulas, I decided to take a different approach. About this time, Elecraft announced the KX2 and I immediately bought one since everything about it appealed. Many others snapped up the first round of these radios and soon the YahooGroups and various reflectors started filling up with reports of how much operating time people were getting with the built-in battery.
Amazingly, 5-7 hours seems to be the norm - and this is largely among SOTA and NPOTA ops who are using the rigs in a "high duty cycle" fashion and at the radio's full 10 watts out. So there were my "calculations" done for me, reduced and corroborated by others - a 2.6AH battery powers a 10-watt rig for 5 hours when used in such a way that the op is transmitting often. A NPOTA activation of my own with a KX2 and small gel cell confirmed for me just how long a battery lasts and it is a lot longer than I would have thought a 10-watt rig could be powered. Since I have no interest (or need) to take a solar panel with me while I operate QRP afield (NPOTA or otherwise), there were only two questions remaining and neither relied on formulas with blurry variables.
So the only two relevant questions are, "How long do you intend to operate" and "How long are you willing to wait for the battery to be recharged." The first question tells you what size (in mAH) battery to buy, the second tells you what size (in watts) solar panel to buy. It's as simple as that - dispense with the idea of trying to reduce all the variables into one answer and instead look at it as these two distinctly separate issues.
I very seldom operate for more than 3 hours in a single day. Usually, it's much less unless there's a sprint or a DXpedition on the air. Then I may go the full three hours. My abacus says that my KX2 will consume 1560 mAH of juice in that amount of time.
My solar panel for QRP is a small 20-watt panel that puts out 20 volts at 1 amp. It'll return that 1560 mAH's back into the battery in a little over 2 hours on a sunny afternoon using a cheap and inefficient Chinese PWM controller. I don't usually play radio on afternoons, sunny or otherwise, so we're golden - the battery is topped off for that night's radio'ing.
In this scenario, a 20-watt solar panel is overkill for a 10-watt radio...at least as I use them. The truth is, I don't use the radio every day. I could easily support my operating habits with a 5-watt panel and never turn on an AC-powered 12V supply. Saturday, Sunday and maybe one evening during the week are all I operate - say, 10 hours total. That's roughly 5000 mAH of battery capacity I use in a week. A 5-watt panel will soak up enough of those free photons falling into my backyard to put that 5000mAH's back into the battery in 12 hours (2 days) while I am at work.
Here's what I use for QRP solar:
|Despite its name, this is a PWM controller|
12V 3300mAH battery
MPPT30 charge controller
Despite having "MPPT" in the name, the MPPT30 is a PWM controller and it does produce a bit of RFI. This is a non-issue with me since I don't charge and use the battery at the same time. Here is a video of the charger and the RFI. In the video, his battery is nearly full - for a battery needing a more significant charge, the controller would be pulsing much more often and the QRM would be worse. The solution is to either buy an MPPT controller or simply don't charge and operate from the battery at the same time.
All PWM controllers will produce RFI as a by-product of how they operate - they produce square waves (pulses). Square waves are full of harmonics that extend into the RF region. Remember, the amount of RFI produced is a function of that particular controller and of how depleted the battery under charge is. The extent to which that RFI affects your rig is based on how far the controller is from your antenna (or open wire feedline) and the NB/AGC effectiveness of your receiver. As I mentioned, the best solution for me is simply to not charge the battery while I'm using it to power my radio.
Another thing to remember is that a 20-watt solar panel with a PWM controller is effectively a 15-watt panel due to the level of efficiency of PWM controllers.
If I had it to do over again, rather than the items mentioned above, I would buy this and a 4- to 7-AH gel cell. Cheap enough to go off the grid QRP for many hours and learn a bit in the process. Any upgrade you may want to make to your system can then be based on how you actually operate and, in my book, practical knowledge beats theoretical any day of the week.