Sizing my DC Electrical System

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Sizing my DC Electrical System

Postby ThinkLibertarian » Mon Oct 31, 2016 10:10 pm

To figure out how much solar I need, I have to estimate how much power I will consume on a daily basis. This is measured in Watts. If Watts are not specified on the device label, then I used the formula Watts = Volts * Amps, since this Volts and Amps are always there somewhere.

IPad Charger: 10W
IPhone Charger: 5W
LED light: 3W
Roof Vent Fan: 3 to 50W, depending on speed setting. I'll use the average of 26W.
Total: 44W

How many hours will I be using the devices? Let's say I am using both chargers and one light continually for 10 hours a day, or 180 Watt/hours per day. I do not know how often I will use the Roof vent fan, so I am guessing here: 3 hours on a medium setting so 26x3=78Wh. 180+76=256Wh of electrical usage per day.

I used a calculator I found at https://www.renogy.com/calculators#solar-size

The renogy calculator wants Watts and Hours entered seperately, so I divided 256Wh by 44W and rounded up to 6 hours.

I plugged these numbers into a solar sizing calculator, assuming a PWM controller and 5 hours of usable daylight per day and got these numbers:

Recommended Solar Panel Size: 66W
Recommended Battery Size: 44Ah (Amp hours)

I will need to run AC appliances on occasion, so I will need an inverter.

The devices I need to run occasionally are:
Airbrush compressor (240W)
notebook computer charger (80W)
my printer (10W)

The compressor is the most power hungry, so I will use that for my calculations.

Induction motors such as air conditioners, refrigerators, freezers, microwave ovens, and pumps (like my Compressor!) may have a start up surge of 3 to 7 times the continuous rating. Let's assume the worst, and my 240W compressor will surge 7x to 1680 Watts!

AC inverters are rated in watts, with a surge rating of double their base. So if my compressor surges to 1680 watts, I will need a minimum of an 840W inverter. That is not a standard size, so I rounded up to 1000W.

Some electrical devices are sensitive to power fluctuations, like computers and printers. So a "pure sine wave" inverter is reccomended to provide smooth power.

Let's assume I will run the compressor for an hour a day: 240W for 1 hour. I know that inverters are not 100% efficient, so let's round that up to 300W, and assume the extra 60W is converted to heat and wasted. Plugging these numbers into the renogy calculator results in:

Recommended Solar Panel Size: 75W
Recommended Battery Size: 50Ah

Combining the two, I used the larger of the recommended solar panel sizes (75W) and rounded it up to 100W, the smallest commonly available panel size. I added the battery sizes together to get 94Ah, again rounding up to a more common size: 100Ah.

So, by my calculations, I will need:
A 100W solar panel kit
A 100Ah battery (at 12V)
A 1000W pure sine wave inverter

Please let me know if my calculations are flawed. I hope my example helps others.

Rich
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Re: Sizing my DC Electrical System

Postby MtnDon » Mon Oct 31, 2016 11:01 pm

Question... I read your topic but I only had a quick look at the renogy link. Looking at the battery capacity that was generated raises the question of what depth of discharge are the results based on? Fifty percent is generally looked at as the most you want to use on a regular basis, to maintain a decent lifespan of the battery. So looking at the first battery suggestion of 44 Ah, is that 44 Ah the amount calculated to use, or has that had a correction factor applied? My gut feeling is they are using 50%, but I wonder if you found an actual value on their site?

Another factor sometimes used is how many days of autonomy do you want? If the next day is cloudy and produces little power do you want sufficient battery capacity to cover a second days use or will you run a generator or plug in someplace?

And also are you a warm weather camper? Or a cold weather camper. Lead acid batteries lose capacity as the temperature falls below 80F. At 40 F a lead acid battery loses about 18% of its rated Ah.
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Re: Sizing my DC Electrical System

Postby troubleScottie » Tue Nov 01, 2016 2:44 am

Just looking at your compressor 240W or 2A @ 120VAC or 20A @ 12VDC. Assume an 80% efficient inverter = 25A or 25AH per hour. That assumes no power surge (read more amps per time ). Although short in time, they could be a big draw. So still more power usage.

Your battery is 100AH, looking at most a 50% discharge: you have 50AH before you must recharge. Your compressor is using half of your total if run 1 hour per day. So with only the compressor, you only have 2 days power at most.

As you stated, motor (and resistive eg heater ) are really big power draws. When camping without shore power, one generally wants to avoid them.

If you add the other devices ( fan more time, and the lights less ), you are going to get close to using all your power in 1 day. You will have to watch your printer, a draw while on and not printing. If you are really using all the electronics, you have still more draw -- your estimate 4A per hour ( 44W = 4A * 12VDC), easily consuming the other 25 AH.

You might have issues recharging. Can you get enough hours each day to completely recharge the battery? Can the panels be deployed in the correct direction every day? For instance, if you are traveling for part of the day, the panels are not out. Panels mounted on the TD mean poor exposure or a warm/hot TD or both. Can you leave the panels up if you are away from the TD? Can you tow vehicle recharge your battery.

Gut feeling, you have too big a draw and will run out of power in under a day.

You might want to consider a generator. A small one -- 1500 to 2000 W can run for 4 to 8 hours on a tank of gas.
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Re: Sizing my DC Electrical System

Postby Shadow Catcher » Tue Nov 01, 2016 8:22 pm

Always plan for more than you think you will need!
Consider a high voltage (meant for grid tie) panel with a MPPT controller. I have a 66 cell 185W panel that puts out 70V no load. Typically I see better than 35V sun up to sun down and in partial shade. We have a 135W flexible panel that can be out out in the sun. Part of the equation is using a battery monitor to make sure you do not get below 50% SOC.
I have a jumper cable that I can run from the TV to the trailer battery and the alternator can boost the battery quickly and the gallons per hour our Subaru is .36 at idle.
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Re: Sizing my DC Electrical System

Postby noseoil » Wed Nov 02, 2016 7:16 am

A 100 watt panel & 100 AH AGM battery would work for you. The duty cycle of an AGM battery will take it down to 20% before needing a charge (50% is for a lead-acid battery), so functionally you have enough power with a group 27 size battery. Don't overthink this one.
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Re: Sizing my DC Electrical System

Postby MtnDon » Wed Nov 02, 2016 9:14 am

AGM batteries are still lead acid batteries, just made a little differently, however they still use lead and sulfuric acid.

Depending on where you read about batteries you get different answers. Trojan, a large manufacturer of both flooded and AGM batteries provides information that shows that when discharged up to 80% their AGM battery life is still shortened substantially, just like flooded cell batteries. Lifeline, another well respected AGM manufacturer recommends 50% as a maximum discharge. I know that batteryuniversity.com states that AGM can be cycled down to 80% but that seems to fly in the face of what the two manufacturers I referred to recommends for their own products. interesting chart ... you can discharge as deeply as 90%, but replace your battery more frequently. You do the math and figure the best balance between buying batteries and the number and weight needed or wanted.

I believe that an AGM battery can handle the deeper discharge better than a flooded cell battery. I believe the AGM battery may not suffer from sulfation as quickly as their flooded brethren do. So that could be an advantage when not recharging every day or two. But sulfation occurs in a AGM because that is how the chemistry works... discharging = the production of sulfur crystals on the lead. When the crystals sit there too long they harden and will not revert.

Out there in the real world of off grid homes and batteries, AGM batteries need replacing more often than flooded cell batteries. That comes from talking with a long time friend who has over 20 years of designing and selling and installing solar off grid systems. Part of that is because AGM generally can not be placed through an equalization charge which can help reverse sulfation.
Our 6x12 deep vee nose cargo trailer camper conversion... viewtopic.php?f=42&t=58336

We have a small off grid cabin we built ourselves in the NM mountains; small PV solar system; 624 watts PV, Outback CC & inverter/charger ... http://countryplans.com/smf/index.php?topic=2335.0
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Re: Sizing my DC Electrical System

Postby ThinkLibertarian » Thu Nov 03, 2016 9:05 am

Thanks for your feedback, everyone.

I have budgeted for the 100W panel, 100Ah battery, and 1000W inverter, but I will have room to expand. The roof of my camper can handle two panels, and my battery compartment will have room for two batteries.

Thanks!
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Re: Sizing my DC Electrical System

Postby GuitarPhotog » Thu Nov 03, 2016 12:51 pm

I don't think putting the panels on the trailer roof is a good idea because it requires you to either park in the full sun, or move the trailer as the sun moves. Last summer in Idaho, my campsite was in full shade, but I could put my movable panel in the sun next to the road, 20 feet away.

My $0.02 worth

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Re: Sizing my DC Electrical System

Postby MtnDon » Thu Nov 03, 2016 1:20 pm

If you have selected that 1000 watt inverter because you hope to run high wattage loads you may be disappointed. 1000 watts AC will mean you'd be drawing around 100 amps from a 12 volt system. That is taking into account some loss in efficiency going from DC to AC. That discharge rate is getting close to the maximum for most normal batteries. OTOH if the 1000 watt size was selected to allow for a surge that only lasts a few seconds, as in starting some types of motors, that could be a good choice. Lead acid batteries have a fairly high internal resistance which limits the practical rate of discharge. AGM fare a little better because of their lower internal resistance. There are some special purpose lead acid batteries that can supply up to 3 times the battery amp hour rating, but only for a few seconds. Lithium, LiFePo4 batteries can easily supply discharge rates upto 3C, but cost much more.
Our 6x12 deep vee nose cargo trailer camper conversion... viewtopic.php?f=42&t=58336

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Re: Sizing my DC Electrical System

Postby noseoil » Fri Nov 04, 2016 8:28 am

My "torture test" for the 95 AH Trojan AGM battery & 2000w inverter in the galley (actually 1000w continuous) was simply to make a pot of coffee with 110v power. It's a 900 watt home-type drip coffee maker. I won't do it again, as it drew about 83 amps for 20 minutes and did pull about 25%-30% of the battery storage down. The cables to the inverter warmed up a bit, but I had a full pot of coffee & the system stood up to it without a problem. This was the worst thing I have ever done to the system, complete abuse, but I figured if it can stand up to this, a small appliance, small vacuum cleaner, or an electric drill isn't a problem.

We use a small inverter in the cabin (200w) for the TV & sound system, which works out very well. Less parasitic drag on the battery & it runs well for a movie or two.

Just saying that a system needs to work, but a storage battery isn't your best bet for large power loads. A generator or shore power is still preferable when bigger loads are required, but it's nice to know there's power available in a worst case scenario if you need it.
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