For me personally, understanding power has finally clicked. It took a little time, but I could never (and still can't!) fully get my head around the 'water analogy' for understanding power. I was always left wondering if the water tank was the Volts or if the pipe was the Watts?! Should I have a bigger hose or a smaller tank?! Do I need a better nozzle?!
I've heard others say the same so hopefully I'm not alone in being left a little #confused. I thought there must be an easier way to understand power using something that everyone understands...money!
The 'Solar Money' Analogy
(Note: I will say this analogy is a work in progress and may need some refining over time!)
In real life we earn money, store money and spend money and our currency in the USA is dollars ($) - hopefully you earn more than you spend so you will have some money stored in the bank for a rainy day.
Earn Store Spend
In boat life it's the same deal but your solar money, or currency, used aboard is Watts - you earn Watts, store Watts and spend Watts.
Earn - You earn Watts aboard from the sun via the solar panels on your boat (your day job) but you could also earn some Watts from other sources like plugging into shore power, running the engine and charging via the alternator, maybe a wind generator or a hydro generator (call these your side hustle!).
Store - You store your Watts aboard in a piggy bank which is your 12V battery bank (for most boaters but increasingly we are seeing more 24V or 48V battery banks).
Spend - You spend Watts from your battery bank by powering all of your items aboard your boat. That could be 12V loads like all your lights, fridge, navigation equipment or 110V loads like a TV or coffee maker that you have to run via your inverter.
In boat life, as in real life, hopefully you earn more Watts than you spend so you have some Watts left in the battery bank for a rainy day (literally!)
In summary here are the key players so far in our solar money analogy:
Earn - solar panels - generate solar money
Store - battery bank - where you keep your solar money
Spend - everything that uses power aboard - how you spend your solar money
OK hopefully you are still with me on this solar money analogy?! Let's take a closer look at how you earn, store and spend your solar money (Watts)
Earn (part 1)
Aside from shore power, hydro and wind etc we're going to focus on the solar panels aboard. For example, with a 100W solar panel you earn 100Watts in one hour, known as 100Watt hours (or you will see it written as 100Wh). Now here's the key point with earning solar money via your solar panel, it doesn't perform as rated (eg. 100W) at all hours during the day (and clearly it doesn't work overnight when the sun has set).
In fact, maybe solar panels are similar to us working in real life? It takes a few hours in the morning for us to get up to speed, you work at rated output (or maybe a little higher!) for a few hours in the middle of the day, and then you slow down in the afternoon and call it a day around sunset.
Solar panels are much the same, they wake up in the morning and slowly increase their output until they perform as rated for a few hours and then slow down in the afternoon. As an over-simplified example:
Morning - 1.5hrs at 50% rated output
Lunchtime - 2hrs at 100% rated output
Afternoon - 1.5hrs at 50% rated output
Across the course of a day, you are getting 5hrs at 100% rated output, or 5 sun hours. So back to our example of 100W panel you are getting100W x 5hr = 500Watt hours (500Wh). We often refer to this as your 'daily yield'.
The number of sun hours you get on your panels during the course of a day will vary. You may get more, or less, but 5 sun hours is a good average to work with.
These are just some of the factors that will affect the number of sun hours:
Latitude of your boat - are you down south In Saint Lucia enjoying tropics or up north in Maine? This will affect the angle of the sun hitting the solar panels. Lower the angle the less power.
Time of year - is it summer or winter? Again, this will affect the angle of the sun hitting the solar panels and also how long the sun is up in the sky - longer days in summer means more solar power.
Aspect - for a house in the northern hemisphere you may have your panels on a south facing roof, which will perform better than panels on a north facing roof. For the majority of boats, the panels are placed flat but maybe your dock faces north, or you are swinging around on a mooring? The aspect of the panels to the sun will change.
Earn (part 2)
The second part to earning solar money is getting paid. How do you want to receive those 500Watts your solar panels have worked hard all day to earn? If Watts is our currency, what denominations do you want to get paid in?
An easy money analogy would be if I owe you $500, would you prefer I give you 1x $500 bill or 500x $1bills to put in your wallet? You're still getting $500 but maybe it's easier to handle a single $500 bill and easier to store that single $500 bill. Otherwise you will need a pretty fat wallet for all those $1 bills and it's more likely you might drop a few bills during the transaction (resistance!)
If I owe you $500, would you prefer I give you 1x $500 bill or 500x $1bills to put in your wallet?
Dollars = # of bills x size of bills
# of bills = Dollars / size of bills
size of bills = dollars / # of bills
Similar with solar panels would you prefer 1A at 500V or 500A at 1V, either way you are still getting 500W:
500 Watts = currency
500 Amps = how many of these bills you have
1 Volts = (size) denomination of your bills
Watts = Amps x Volts
Amps = Watts / Volts
Volts = Watts / Amps
This is an extreme example when in reality solar panels sit somewhere in the middle. So, a 100W panel is in fact rated at 5.9A and 17.1V, so it's going to give you 5.9 x 17.1V "bills" to put in the bank.
Store - OK so now we have to store the Watts in our battery bank BUT your 'piggy bank' has a certain size hole and will only accept certain size denominations (voltages). For example a 12V nominal battery bank will only accept say coins of 13-15Volts so if you tried to send the money directly from the solar panel to the bank it won't "fit" as it's too large.
You first have to go through Mr. Blue the money changer, aka your MPPT controller. Mr. Blue (the Victron MPPT controller) will take the higher voltages from the panel and reduce it down (buck) to the size the battery bank wants at the time and deposit as many as he can. Mr. Blue is always talking to the solar panel to see what high voltages it can give and always talking to the battery to see what lower voltages it can accept. The added bonus is that an MPPT controller will take the difference and turn it into "extra" amps.
MPPT charge controller - solar money exchanger/converter AKA Mr. Blue
Hopefully you are still with me? Let's take an example and work it through from solar panels to battery.
Let's say we have 1x170W SunPower E-flex panel. Each hour my 170W panel produces 170W, that's like saying my 170W panel just gave me $29.4 bills and 5.84 of them.
I want to deposit my 170Watts (5.84x $29.4 bills) into my Piggy Bank (12V battery) but it doesn't accept such large bills. Trying to stuff a large wadge of bills in the small piggy bank hole isn't going to work, some bills will drop on the ground (resistance) and is not very efficient. The bank only accepts say $13.5 bills (the charge voltage of my 12V battery). So, I have to go to the solar money exchanger (my MPPT charge controller) I can give it my 170Watts (5.84x $29.4 bills) and in exchange it gives me 170Watts (but as12.5x $13.5 bills) to give to the Piggy bank (12V battery)
Summary
In summary of my solar money analogy, your currency is Watts. Watts are what you are earn (from the sun via the solar panels) and deposit in your bank (the Bank of 12Volt!). You are also spending Watts from your bank account by powering all of your items whether that be 12V loads like lights or 110V items like a TV via your inverter.
In short here are the key players:
W - Watts = currency
V - Volts = denomination of your bills
A - Amps = how many of these bills you have
solar panels - generate solar money
MPPT charge controller - solar money exchanger
Battery Bank - where you store your solar money
As i mentioned in the beginning this is still a work in progress but hopefully it helps you to better understand power aboard your boat and how the solar money transfers from the sun, to your solar panels via your MPPT controller to your battery bank.
Feel free to email us with any questions info@sunpoweredyachts.com
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