Last time I talked about the typical solar newbie post on Facebook. Asking for help, yet not providing any useful info on what his source is or what his load is. We looked at that and found out about his needs.
Let’s take one item on the list. He’s going to have to spend some money on upgrades and may not have enough, so let’s start with something critical. I suspect he wants to go full time, but after a hurricane or other disaster, what are you worried about? The fridge! Well, honestly, I worry about the coffeemaker and the microwave, too.
Back to JUST the fridge. He can work the numbers on the other items the same way and add them in to achieve his full goal. Let’s see, he used a Kill-A-Watt meter to determine that his fridge used 350 watts 75% of the time, so 350 X 24 X .75 = 6300 watt-hours (WH). Around here, we allegedly get an equivalent of 5 hours of prime sun time each day, so divide 6300/5. That comes out to 1260 watts your panels must produce. Wow! That’s a bunch more than the tiny panel he has and we have not even factored in the other stuff on his list.
The good news is that solar panels are pretty cheap, these days, especially if you shop at Sun Electronics. You really need more than the 1260 watts on the label because there are what we call round trip losses. The power from the panels goes first to a charge controller and then to the battery. Think of your panels as a wall outlet and the charge controller is the battery charger that is charging your car after you left the headlights on. (Hard to do with modern cars, but some of us drive antiques.) The charge controller not only consumes a little power, but the type of controller can make a huge difference as to how much of the panel power actually gets to the battery. Most cheap controllers are the PWM or pulse width modulation variety. They very rapidly turn the connection on and off to limit the voltage and current to the battery. You need a lot more solar with these. The other option is the MPPT or maximum power point tracking controller. You can get around 30% more useful power from the panels AND you get a lot more options in which panels you use. Use a 50v panel to charge a 12v battery, or 3 panels in series to charge. Some of the new units can even take input voltages of around 500v, though I’m not sure I am a fan of scary voltages.
Depending on the battery chemistry–Lithium or Lead– you will get more or less of the power you stored, but you will never get as much out as you put in. We were at 1260 watts, right? Lets go to 2500 watts, just to make sure. Not every day is really sunny.
Inverters have losses, too. The one on my boat idles at 83 watts. Wow, that’s another 2kw hours per day, if I leave it running. For a cabin or shed system, try to avoid an inverter altogether. You can get fans and lights and phone chargers and such that will run on DC. Sometimes you have to have it, though. Make sure to add another panel for the idle power.
Yes, I see this is starting to get to be a bit bewildering, so I am going to cut it off. We will get to the batteries in an upcoming episode. There is more to them than just how many kilowatt-hours they need to store.
–Neal