Sizing solar panels - Ford Transit USA Forum
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post #1 of 6 (permalink) Old 01-02-2019, 12:20:PM Thread Starter
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Sizing solar panels

I read of a rule of thumb that solar watts on roof should equal amp hours in battery. So too then a vendor stated that solar watts should be 1.5 - 2 times your battery amp hours (150-200 watts for a 100 Ah battery).

What is the reason for this rule? Is it a strict rule? Ready to purchase…. Can I use the LG 300 watt panel (described below) with Victron MPPT 100/30 charge controller to charge my 100 amp hour Battleborn lithium battery.

NOTE: Don’t want to go to small on the solar panel because I may want to add another 100 amp hour battery.


LG Model N1C-A3, 300 Watt Black Frame MonoX™ NeON Solar Panel


Watts (STC) 300 W
Max Power Voltage (VMPP) 32.0 V
Max Power Current (IMPP) 9.40 A
Open Circuit Voltage (VOC) 39.8 V
Short Circuit Current (ISC) 9.98 A
Max System Voltage DC 1000 V

Thanks a million!

Dave in Wisconsin

2018 silver T-150 cargo van, 130" wheelbase, midroof, 3.7L V6 engine, limited slip axel, privacy windows all-around, cruise control, reduced height floor brake and standard alternator.

*** After one month sold the 2018 shown above with 35 miles on it (long story)***

Now ordered a 2019 on Nov 1, 2018. Same options, but got Magnetic Metallic this time.
Anticipated arrival: March 2019. I hope to keep this one.
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post #2 of 6 (permalink) Old 01-02-2019, 12:30:PM
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Solar panels are rarely 100% efficient throughout the day. Add in potential for clouds, shaded parking, etc. and it can cut into your ability to recharge during the day.

Granted, this will also depend upon your use of power, and making an estimate of how much you may need to recover daily. Folks running AGM or other LA batteries likely won't be planning to use more than 30% of their charge. Someone running Lithium Polymer batteries might have plans to use more. All of this will figure into the overall picture.

I went with double the Solar Watts, compared to AmpHours on the batteries. Mostly because the build is all electric and will be used for cooking, heating water, as well as charging devices, lighting, etc.

Make a list of your loads, an estimate of the duration of time they will be used, and that should be a good start to determining how your needs can be used to work out how much solar to buy.

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post #3 of 6 (permalink) Old 01-02-2019, 12:59:PM Thread Starter
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Originally Posted by Travlin View Post
Solar panels are rarely 100% efficient throughout the day. Add in potential for clouds, shaded parking, etc. and it can cut into your ability to recharge during the day.
Thanks travelin. I, too, am all electric. But since I have never done this, I have no idea of use at this time. Just don't want to have to add more solar later.

Cost between 200 watt panel and 300 watt panel is not an issue.

My question is: Can I damage anything with 300 watts charging a 100 Ah battery?

2018 silver T-150 cargo van, 130" wheelbase, midroof, 3.7L V6 engine, limited slip axel, privacy windows all-around, cruise control, reduced height floor brake and standard alternator.

*** After one month sold the 2018 shown above with 35 miles on it (long story)***

Now ordered a 2019 on Nov 1, 2018. Same options, but got Magnetic Metallic this time.
Anticipated arrival: March 2019. I hope to keep this one.
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post #4 of 6 (permalink) Old 01-02-2019, 01:18:PM
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'' Can I damage anything with 300 watts charging a 100 Ah battery''


Short answer, No.
As your battery reaches full State of Charge (SOC) your charge controller will float the power coming in from your panel. some controllers have a load connection that could be utilized to power lights for example, so you are not just wasting that free power.
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2015 LR T350 XLT 3.7 Wagon 3.73 rear end with LS. 200 ah house battery, 320 Solar, 300 watt PSW and 1,100 watt MSW inverters, 140 amp VSR, 250 amp alternator, 45 amp smart charge converter A/C D/C distribution box, shore power option, 30 amp MPPT CC. and still have stuff to add.
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post #5 of 6 (permalink) Old 01-02-2019, 01:43:PM
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That ratio is generally meant to insure you have enough solar watts to actually bulk charge a given Ah bank.
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post #6 of 6 (permalink) Old 01-02-2019, 01:52:PM
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Like @makenmend indicates, just because power is available doesn't mean it will be used. The load is what determines current flow, not the source.

As for working up some idea of daily power usage, here's some guidelines.

Things you know:
Power rating in Amps or Watts for your system loads. Like induction cooktop, water heater, fridge, lights, etc.
Ohm's Law and other ways of power calculation (easy to find with a search if you aren't familiar) Essentially Watts is Volts times Amps (and can be twisted around to solve other ways), and Amp Hours is constant draw of Amps over 60 minutes.

From there you can guestimate (technical term) how many minutes of use you might have for each of your loads on a given day. Like, cooktop at 50% power for 20 minutes (2 ten minute meals), heating water for "X" minutes to provide "Y" number of showers, running lights for "X" minutes after dark (inside or outside) and any other loads you might have.

Once you have these numbers you can work out Amp Hours used. For example, the cooktop. Let's say it is a 1500 Watt cooktop and you'll be using it at 800 Watts for 20 minutes.

800W / 12V = 66.6 Amps for 1/3 of an hour (20 minutes) the result shows how this will use 22.2 Amp Hours that will need to be recovered. (66.6 X .333 = 22.2)

It will take a 200 Watt Solar panel operating at 50% efficiency roughly 3 hours to recharge for that loss. (100W / 12V = 8.33 Amps; 22.2 Amp Hours / 8.33 Amps = 2.67 Hours)

There will be losses in inversion, and variables in solar production that aren't easily calculated so adding in a little "fudge factor" here and there should get your calculations in the ballpark for selecting the components.

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Last edited by Travlin; 01-02-2019 at 02:42:PM.
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