Just when I think I got it...

[Amy@altE]

Since the appliances aren't purchased yet, and you can't test them with a Kill-a-watt meter, go to the EnergyStar web site to see the energy consumption of the models considered. If they aren't EnergyStar rated, I recommend picking one that is. Although the appliance will likely cost more, it will save money by requiring a smaller solar system. It'll tell you kWh a year, divide by 365 to get daily use. It should be more accurate than guessing, but obviously not as good as measuring. https://www.energystar.gov/productfinder/

[monicaj]

Thanks so much SunEagle for your direction in this. You've given me the exact information I needed.

[SunEagle]

Using the voltage or watts that are listed on an appliance could be misleading to what it actually uses in a day or week. I would suggest getting a low cost device called Kill A Watt. It allows you to measure the watts used by an appliance over a period of time. If you collect data for 48 hours you can get a pretty close approximation of how many watt hours that appliance will use in a 24 hour period. The same goes for lights, fans or cloths washer that plug into a receptacle.

Just remember that some days there will be no sun so you can't recharge the batteries. If the sun isn't out for two days your battery system starts to be critically drained which shortens its life. All good off grid systems include a generator that can be run to supply power for your appliances when the batteries are low as well as allow you to recharge the batteries.

Final thought would be how the battery system is built. If it is going to be a 12volt system then I would suggest getting low voltage batteries 2, 4 or 6v rated at the Ah needed and wire them in series to create a 12volt system. Most golf cart batteries are 6volt but at most only 232Ah. To get the 500Ah needed you have to wire batteries in parallel which increases the chance of not charging or discharging them equally. Trojan makes high Ah rated batteries that cost more but last longer then most other deep cycle or semi deep cycle (golf cart) batteries.

[monicaj]

Thanks for your responses. I used an excel spreadsheet calculator to try to determine his kwh (kwh right?) and I'm going to see if this post will let me upload it... not sure if you can open it if you don't have excel. Wouldn't let me upload.

The calculator said he would use 1130 kwh a day. I know that the calculator is only as good as the figures entered in... for instance, you can insert a number for the quantity of devices you'll be using, the volts of it, the watts of it, how many hours a day you'll be using it, and the number of days you'll have it on.

The spreadsheet had a list of appliances and had the figures already in there, and I know that not all freezers are the same wattage, they aren't all the same. But for example, the spreadsheet had a 10 cu. ft. deep freezer, 117 volts, 125 watts, 7 days a week. We inserted a 3 for how many hours a day we thought the compressor might run, total. BUT WE WERE UNSURE, were we supposed to enter 24 hours, or just how much time the compressor might use? -. then the formula on the spreadseet said that the deep freezer would need 375 kwh a day.

10 cu.ft. Deep Freezer

1

117

125

3

7

375.0

After adding all our info, the spreadsheet would transfer our total kwh to the next page where it would calculate batteries needed, how many days of storage etc. But it was still confusing to me, and didn't help with the cc or the inverter.

He's going to be very frugal on his energy usage (he's basically camping right now, so even having lights for an hour at night and a 12v fan for 3 hours will be a step up). But who knows how accurate the spreadsheet is... is he screwed if he wants to have a freezer and washer run off this system? He hasn't really bought any appliances yet so we didn't test the wattages.

I appreciate your suggestions on the charge controller and inverter. I'm going to get those.

[SunEagle]

He needs to step back and do a loads list. Assuming he already has what he wants to power, I suggest he gets a Kill-a-watt meter and measures the usage over a few days. Then go to an off-grid calculator to see what size system he needs. He will soon find that he cannot run what he wants off only 2 solar panels.

But to answer the original question, since you are already planning on using an MPPT charge controller, you can wire the panels either way, series or parallel, although series would be better, because you will have higher voltage and lower current travelling on the wire from the panels to the charge controller, resulting in needing smaller wire and achieving lower voltage drop.

1+. I agree with the series wiring. The OP just needs to pay attention and does not exceed the max DC input voltage for that CC.

[Amy@altE]

He needs to step back and do a loads list. Assuming he already has what he wants to power, I suggest he gets a Kill-a-watt meter and measures the usage over a few days. Then go to an off-grid calculator to see what size system he needs. He will soon find that he cannot run what he wants off only 2 solar panels.

But to answer the original question, since you are already planning on using an MPPT charge controller, you can wire the panels either way, series or parallel, although series would be better, because you will have higher voltage and lower current travelling on the wire from the panels to the charge controller, resulting in needing smaller wire and achieving lower voltage drop.

[SunEagle]

I would say that 2 of those 280w panels will require an MPPT CC rated at or above 45amps (2 x 280w = 560w / 12v = 47amps) similar to the Morningstar TS-MPPT 45A

With 45 charging amps the battery system should be rated between 360Ah to 540Ah. So if he installs a 500Ah 12v system it should safely provide about 1200watt hours daily (~ 20% DOD) and that 560 watts of pv should be able to recharge the battery usage.

Now the question is how many watt hours will his loads (lights, fans, fridge, washer, etc.) use a day. I can tell you that you can forget the washer and any large fridge. Also that wattage of the lights and fans need to be low if you plan on running them for long.

For an inverter you need to go with a pure sine wave type around 600 to 800watt. Going larger will end up draining your batteries faster.

[monicaj]

I lose it. I feel like I almost grasp it, then poof it's all gone.

I'm not trying to do anything huge here. I have a friend who needs some power. I have 2 of these solar panels. I don't know if I need to wire them in series or parallel. All I know is that I'll need an mppt controller if he's going to use a 12v battery bank.

He's not going to be running much. He hopes he can run a washing machine, and small freezer or fridge. Everything else will be minor... 12v lights, 12v fans, etc.

He's a tad more clueless than me and he says he wants to have 2 systems in case one goes down. So that would mean each solar panel having its own charge controller and both controllers feeding into the same battery bank. Do you guys see any benefit for him to do it this way?

I'm going to try to talk him into just hooking these 2 panels together for one system, and then later if he picks up panels that are different than these, then he can make another array. Am I right? What can make the system fail would be the charge controller right, so couldn't he just have a spare controller instead of having 2 sets?

I like the idea of having a monitor showing what the panels are producing and where the batteries are, etc. Don't charge controllers that monitor these cost a bit more, right?, so maybe better to just buy a separate gadget to monitor things?

I'm so lost. Will you please suggest to me which charge controller and inverter he should get, and how many amps he needs in his battery bank? He's probably going to go with golf cart batteries... my solar panels.jpg