Originally Posted by ralphmacey
I notice that larger panels may have higher voltages, is that an issue.
It still depends. Like everything on a boat you can go from extremely simple to vastly complex.
I run 40 watts of top up solar
with no controller. Alternately You can have a system with MPPT
controller that offer extensive power management for larger arrays.
Maybe if you give us a hint what you are trying to do? The basic info is pretty simple.
- take everything you plan to use on the boat and determine how many watts each item draws. If the item only shows amps the formula is volts X amps = watts. Then multiply each item by how long you plan to use it each day. Example - 40 watt light X 5 hours = 200 watts or 200 / 12 volts = 16 amps. For reference an average cruising boat consumes between 80-100 amps a day. 100 X 12 volts = 1,200 watts. There are lots of ways to reduce consumption
(batteries) - the second step is to determine how long between charging
you expect to go. You can get rain for a few days, for example. The second consideration is that a basic rule
of thumb is you do not want to discharge your batteries below 50% of their rating. So 210 a/h of battery
would last one day with no charging
. 650 a/h would go three days.
Charging - most people use the engine alternator
as the primary charging unit. It is very inefficient but relatively quick. In our case you would run the engine
every day for a couple of hours - assuming a 60a alternator
Solar - the amount of usable sun per day varies with lattitude. I reckon here we get about 6-7 hours usable but I have seen 5 hours as a rule
of thumb also. To generate 1,200 watts in 5 hours you need 1,200 / 5 hours to be free from the engine or 240 watts of panels. However, this covers one day consumption and requires 5 hours of sun every day. Two 175 watt panels would have the potential to deliver 145 amps in 5 hours and would seem a decent margin for our hypothetical 100 amp boat.