Home . Articles . How-to . Boat Doctor . How many solar panels do I need?

How many solar panels do I need?

2012 January 2

Dear Boat Doctor,

My wife and I love to anchor while cruising the Great Lakes. My major concern is my batteries. After one or two nights on the hook, I hold my breath when starting my engine.

Is there a simple way to determine the amount of solar panels necessary to trickle charge two batteries hooked up with the one-two-both switch? And, is this procedure simple enough for a boat owner to accomplish or does it require a marine electrician?

Jerry Davis

Portage, Michigan

Dear Jerry,

Solar charging is a very viable option, even on the Great Lakes. You won't get the output from panels that you'd get in the tropics, but they still work well.

The key to determining the capacity you need is based on your power usage. The amount of panels you can install is essentially limited by your mounting space and budget. You can't really have too much panel capacity, and every little bit helps.

Start by estimating your power usage. You can build a simple balance sheet listing the current that each device draws (in amps) and how long you intend to run it (in hours). Multiply these numbers together to get an amp-hour total per device. For example a one-amp load running for four hours, yields a four amp-hour draw. The total of all your devices will determine your total draw.

You may want to consider swapping out some incandescent bulbs for LED replacements, which draw a fraction of the amps that incandescent bulbs do. The bulbs will snap right in and the cost is less than $20 each.

A typical 75-watt panel will supply about 6 amps of power under ideal conditions. Because you're not in the tropics, I would derate the panel about 25% and estimate maybe six hours a day of peak charging time. Even at these conservative rates a single panel can still put 20 to 30 amp-hours back into your battery bank.

I also strongly recommend that you isolate your batteries into a house bank and comparatively smaller engine battery. Arranged this way you can drain your house bank to zero and still be able to start your engine. This may just entail a simple rewire or maybe adding a small engine start battery. Once you separate your banks you'll need a way to combine them for charging but this is relatively simple too. I have helped several boat owners make a change like this and the stress level reduction is dramatic. You may want to consult an electrician for some consulting but the overall job is fairly straightforward.

Adding a panel, reducing your usage as much as you can, and improving the configuration of your banks will allow you many stress-free days at anchor.