Looking to buy a solar panel...how many watts?

[87roadracer]

So I am looking to buy a solar panel for my Oday 272 and was wondering how many watts would I need to keep my battery charged to power up the following:

Raymarine i50 Tridata instrument
Nav lights
interior lights
NO interior motor

Would a 40 watt panel do the trick? Should I have two batteries on the boat? Kind of new to this.

Thanks all.

[SailFastTri]

If you're going to do this right, start by putting an ammeter on your main house battery bank and see how much you draw with your loads combined. Then figure in your latitude and estimated hours of full sun, and how many hours you expect to operate your loads. Also figure how many days of overcast you want to be able to handle with your batteries. At best you'll be making a WAG about your needs.

If you're in the Northeast US you'll probably never get more than 85% of rated output from your solar panels, your batteries and solar charge controller might have a charge efficiency of perhaps 80%, and you'll get 4-6 hours of peak sun, perhaps 40% of the days will be overcast, you get the idea.

If you do the cost analysis you'll find it MUCH more cost effective to save power by converting to LED lights than by generating more power and adding battery capacity. You can also use the minimum backlight necessary to save power in your Raymarine unit. You can find cheap LEDs on eBay to replace the interior bulbs (search for LED and your bulb base type, e.g. MR11 or bayonet). For your nav lights it's more costly but a safer bet to buy from the nav light makers who claim to be CG Approved). The anchor light is the most important to have as LED, as you're likely to run it more than any other nav light.

Another tip is to use an MPPT controller and either wire two panels in series or buy a single panel rated for 24v systems. The MPPT controller will do the stepdown to 12v but you'll get more power out of the system and charge for more hours per day. You'll also have less voltage drop in the wires between panels and controller if you run at higher volts. (Don't forget fuses.)

Once you put your system in place you'll also want to install a battery monitor, so you have a better indication of how well your system is maintaining battery state. Battery monitors are like a fuel gauge for your batteries.