System Design for Dock Lights

[SunEagle]

Well the project is finished and working well! I ended up making a post on reddit with a link to the Imgur album here: https://www.reddit.com/r/SolarDIY/co...wer_on_a_dock/

Looks like you did well sizing and pricing your system. Nice job. Keep us informed on how it preforms.

[Datech]

Hey SunEagle, I was wondering if you could refresh my memory on why you said the following:
"Next to size your panel you need to use 1/10th the battery system Ah rating which would be 20 amps."

What's the math behind the 10% figure? From what I remember you should hope for 4 hours of sunlight a day and therefore multiply the sizing by 4 to get the panel wattage. I feel like I looked up your reasoning when you suggested it but I can't remember it now. I also couldn't figure out how to Direct Message you, hence the zombie post Here.

[SunEagle]

Hey SunEagle, I was wondering if you could refresh my memory on why you said the following:
"Next to size your panel you need to use 1/10th the battery system Ah rating which would be 20 amps."

What's the math behind the 10% figure? From what I remember you should hope for 4 hours of sunlight a day and therefore multiply the sizing by 4 to get the panel wattage. I feel like I looked up your reasoning when you suggested it but I can't remember it now. I also couldn't figure out how to Direct Message you, hence the zombie post Here.

To size your pv wattage you first need to be able to have enough amps to charge your battery system. For a Flooded Lead Acid type (FLA) the amount of charging amps should be between C/8 and C/12 where C = your battery Ah rating. C/10 is the sweet spot which calculates to 10% of your battery system Ah. If you use an MPPT type CC then you can multiple the battery voltage x the amount of charging amps needed to get an approximate wattage of your solar array. That is half the formula.

Now to make sure you have enough time in the day to replace what you have used in your battery system will depend on where you live and the lowest value insolation hours of the year. So if you use 1000 watt hours in a day your pv system needs to put that back and about 15% more due to losses.

So for example if you need 1000wh a day and you have say 2 hours of use full sunlight then you can calculate the panel wattage. ie. 1000wh / 2hr * 1.15 = 575 watts.

To see if that panel wattage gets you the amount of charging amps then the approximate wattage, which with an MPPT CC, will yield 575w / 12volt ~ 48 amps of charging. That should be able to support a 480Ah 12volt battery system.