Rules of Thumb, Watson?
Your battery capacity Ah should be around 4 times your daily consumption
. The theory says you don't want to go below 50% capacity and the upper 15% is difficult to charge, so you go down to about 60% and then back up to about 85%. That is 25% (85 - 60 or 90 - 65), so your battery capacity is 4 times your consumption
. Some say 3 times or 33%. I prefer to be conservative.
But how reliable is your charging? If you have a generator
, you can just burn some fuel
and you do not need to depend on the wind
nor the sun. So go for the rule
of thumb of 4. Or 3 if you are a daredevil. But your choice of charging device will also adjust your estimation of the buffer your require. And the buffer is the battery capacity.
If you go through the numbers, wind
generators just do not make the grade anymore for a typical tropical situation or tradewind RTW. Solar does. But you need to consider the sun / cloud cover and wind stats in your particular geographic location if you are anchored near Santa Claus or the penguins.
We had 340 W of panels
and then bought an extra 630 W of panels
from Sunelec for less than $1000 about a year ago. Pretty happy so far. Our total is currently 760 W because the last 210 W panel is lashed to the deck
and still unconnected. When installed, yes, there will be a total solar capacity of 970 W. The 760 W at the moment gives us about 140 - 200 Ah per day in practice.
"Let's call it 180 Ah, Watson, which fits another rule
of thumb about panel Watts and Ah produced per day. Divide your Watts by 4 should give you your average Ah produced daily. The theory is boring (W/20Vx5h = Ah/day. Divided by 20, not 17 for inefficiencies and it improves my lazy mental maths.) Just divide by 4 and accept it, Watson."
"Hmm," you interrupt me again, Watson, with your idle comments, "the other rule of thumb was also four for normal people and three for daredevils.... does that mean...?"
"Why of course, Watson, it is elementary! Simply calculate your daily Ah consumption with one of those Excel spreadsheet thingies, multiply the result by 4 and buy that quantity of battery Ah and that same quantity of Watts of solar panels!"
"What?" I hear you mumble.
"My dear Watson, I did say, and I will repeat it again, it is Watt. Not what! Or did you really mean how many?"
"Yes, I believe so, Sir."
"Ok, Watson, thank God we cleared that up. Our battery capacity is 880 Ah. The solar panels
Watts you already know."
"Ahhh!," Watson ruminates like the fat cow he resembles, "So your battery capacity is currently a bit more than your solar panel wattage but soon it will be the opposite. WTF???"
And I say, "Very observant, Watson. You are correct! Please keep your acronyms to yourself. And the explanation is simple...no...logical?...no.... in fact I do believe it is elementary, my good man."
And then I continue, "Because we have no generator and our engines are generally out of action, stopped or very poor chargers when they do agree to spin their rotors, we need to account for a day or two of clouds. Not a week, Watson, because we are in the tropics, but a day or two, yes.
"So I assume the battery Ah can drop from, let's say, about 90% to 50% in a typical poor situation, which means 40% or 352 Ah. Are you following, Watson? Move quicker with that slide rule, please. That means, realistically 3 days, because if cloud comes in we can reduce our excessive consumption by turning off the second fridge, less water
consumption for frivolous showers, dehydration avoidance, etc. So consumption of 150 Ah on day 1, and 100 Ah on day 2 and day 3 means a total of 350 Ah before we hit 50% battery capacity."
"Approximately, Watson, these figures are approximate. So far this seems ok and has worked well in the real situations of those verdant isles in the New World.
"We looked into buying
a Honda1000i or 2000i, Watson, but the money
was better spent on more batteries and solar panels
. With the difference saved, I bought you a truly excellent bottle of 28 year old whisky last Christmas
and spent the remainder on 7 year old Cuban rum
for my good self. Even if the monetary difference was not so notoriously in favour of batteries and panels, my good man, surely you must realize that the generator will only annoy us with the noise
, petrol consumption, oil
changes, smell, etc."
"But don't you think that...." he interrupts me with his ridiculous assertions.
"Yes, Watson, the petrol and oil
consumption is not much. I do agree wholeheartedly. But I would still prefer to be hunting marine animals
with a piece of glass on my face rather than proceeding with yet more maintenance
on another infernal internal combustion engine
. Those panels just sit there and produce energy. Almost a case of set and forget. For a lazy man who looks for efficiency in everything, that is great!"
"No, I am not lazy, Watson! Damn your impertinence. I am efficient!"
"Our motors, Watson?? I already told you. Yes, they provide a few amps, 6 each to be precise, but it is insignificant.
"On your knees, Watson, pray to Ra!"
"What did you say? Who is Ra??? No more interruptions, Watson! Please, just STFU and pray!"