Estimating Solar Panel Size for Boats

Posted by Drew Frye at 09:22AM - Comments: (16)

Solar panels should be raised off the mounting surface to allow for cooling.

The starting point for a successful solar panel installation is quantifying your requirements. We present here a simple analysis based on the test boat used for our recent report on choosing and installing a solar panel. Some values are from experience, and others are accepted rules of thumb. For more details on choosing and installing a solar panel, see the March 2018 issue of Practical Sailor online. 

Energy Balance

Look up the current draw of each piece of equipment (confirm with panel ammeter if available) and estimate the number of hours operated. Record the number and capacity of your batteries, recognizing that you cannot draw below 50% charge without shortening their life, and that you will seldom charge past 85% while away from the dock—as a result, only 35% of nameplate capacity is really useable. Finally, total your charging sources, including engine, wind, and solar. For solar, take the rated wattage x 5 hours/12 = amp-hours while on passage and wattage x 7 hours/12 = amp-hours while at anchor (sails do not shade and the boom can be rigged out to the side). This is far below the rated capacity—sailors in the tropics will do better, and sailors farther north or sailing in the winter more poorly—but this is an accepted starting point.

Water intrusion can ruin a solar panel. Poor sealing at the base is common.

 Estimating Panel Output 

Full Sun—Panel square to the sun


Full Sun—Panel at 45° angle to sun


Light overcast


Heavy overcast

10-20 %

How many days can you manage with poor generation? Are you willing to economize during a long cloudy stretch? Will you recharge at a marina or by running the engine periodically? Long-term cruisers appriciate an abundance of power, while the occasional cruisers may be satisfied with less.

Saving Power

Every AH (amp-hour) consumed has a real cost in weight, panels, and dollars. If you can reduce consumption by 50 AH/day you will save a battery (the useable capacity), a 120 watt panel, and perhaps a mounting arch. The cost savings might be $500 and 150 pounds for just a few bulbs.

  • Lighting. Switch from incandescent to LED and fluorescent lighting, starting with the lights you use most. We use LEDs and fluorescent for the anchor, salon, and cockpit lights, but since we seldom run at night, we left the running and steaming lights alone. Likewise, the deck light and many task lights remain halogen or incandescent; they are not used enough to matter.
  • Go to bed at night and get up with the sun. Big savings in juice and more time to play.
  • The gas solenoid is a big user for us; it runs the propane fridge and cabin heater, so it is on for long hours, but we can turn it off at night or go without refrigeration now and then.
  • Fans. Run them on low speed and watch the hours. A wind scoop doesn’t use power.
  • Instruments. Do you actually need GPS and other instruments full-time on passage? Twenty years ago they didn’t even exist. Balance the sails to minimize the load on the autopilot.

Power Usage Table

A simple Excel table can help you track power requirements and estimated solar panel output.

Drew Frye is a frequent contributor to Practical Sailor. He blogs at 

Comments (16)

If it needs to be on, I suggest changing your gas monitor and solenoid to better ones. The BEP Marine uses 2 levels of power. One to trip the valve and a much lesser current to hold the valve. 600-gdl
bepmarine sa296comp-12v

Posted by: B.Ill | June 14, 2019 5:58 AM    Report this comment

In fact we do have semi-flexible panels in testing. They are typically based on the same cells as rigid panels but without the support of glass covers and rigid frames, making them vulnerable to cracking unless mounted to something rigid. They are best flexed only one time--when they are installed--and not beyond the manufacture recommended minimum radius. Repeated flexing, such as on a slack binmini or worse, across a bow, has proven to reduce panel life by causing invisible micro-fractures.

That said, in the proper circumstances, they are very lightweight, add no windage, and seem to be holding up well. They have a place. some boats can logically use both rigid and semi-flexible panels to maximize capacity.

True flexible panels, on the other hand, offer very low power density at too high a cost to be of interest.

Posted by: Drew Frye | August 29, 2018 7:28 AM    Report this comment

Why didn't you use flexible solar panel?

Posted by: Marsun | May 9, 2018 6:28 AM    Report this comment

I made a similar spreadsheet, but with a bit more detail. It lets you look at your power ins and outs over the course of a day so you can estimate your deepest stored power deficit and make some choices about when to run certain loads.

Posted by: Thommango | April 1, 2018 9:29 AM    Report this comment

Power consumption varies according to ambient temperature, water temperature, how often it is opened, and how much food is exchanged. In the summer, 2 amps might be typical, totaling 48 Amp-hours each day. Depending on the weather, it will require a 200 watt panel just to reliably power this appliance. Refrigeration is one of the more power-hungry systems aboard.

Posted by: Drew Frye | March 31, 2018 9:53 AM    Report this comment

Looks like heavy battery banks, and lots of solar wings!

Posted by: stonecipher | March 29, 2018 10:24 PM    Report this comment

I would like to be able to understand how much juice an appliance or fixture actually burns before purchasing it. I was looking at some freezers on west marine and saw 1 that had Amperage .8-2.5 Amps at 12 Volts DC is that .8 to 2.5 amps per hr? And wow! been looking for "just a" freezer, and they are tiny and insanely expensive! Anybody have any insight on energy efficient freezers at about 8 cft?

Posted by: stonecipher | March 29, 2018 10:13 PM    Report this comment

Modern chart plotters draw .8-1.3 amps. A major user of power if left on 24-7.

Posted by: Donnybrook | March 23, 2018 12:59 PM    Report this comment

If a gas solenoid is used to power heat it is on all day and night. In a warm climate the fan load would be greater. Amperage is measured or taken from manuals; the user needs to insert what is true for his boat. Seeing the numbers may encourage him to install LED lighting or turn the fans off when he leaves room.

Charging voltage was used as 12 volts as a reader simplification. The actual system may use either MPPT or PWM chargers (the merits of MPPT are discussed in other articles) and the voltage will be different. Battery type will affect charging voltage and acceptance rate. Yes, you often can reach 100% SOC, but not always, not in gray weather or with winter sun angles.

Shading, temperature effects on both batteries and panels, battery age, panel age, wire gauge and run, sun angles, clouds, and differences between charge controller types all make a difference. We used a 5 hour charging time as a means of correcting for many inefficiencies and approximations. Our intention was to provide a simple, conservative means for a sailor not well versed in charging to get a rough estimate his battery and charging requirements, nothing more. As he becomes educated, the estimate can be improved, but there will always be guesswork when it comes to weather, shading while sailing, and battery condition, so we should be conservative.

Posted by: Drew Frye | March 23, 2018 10:32 AM    Report this comment

Is the assumption that the gas solenoid is ON enabling the flow of propane 24 hours a day? We only have the gas solenoid energized when using the stove...seems like you could save quite a bit of energy by only turning it on when you need it.

Posted by: Maddox | March 22, 2018 12:37 PM    Report this comment

In preparation for my 2016 roundtrip to Hawaii I expanded upon the worksheet provided by the "Pacific Cup 2012 Sample Energy Budget". There I listed every electrical draw item and estimated hour usage, then added in my solar charging to estimate how often I might have to diesel charge. I see no way to share that worksheet in this comments section but encourage all venturing offshore to do so as it proved reasonably accurate.

~ ~ _/) ~ ~ MJH

Posted by: MJH | March 22, 2018 11:45 AM    Report this comment

It is sublimely refreshing to read an piece that so strongly pushes the energy budget. Well done!! But I have the following technical points to make:

a) to get the real amperage out of a solar panel at nominal 12 vdc, the wattage is divided by the panel's Voltage at Pmax (VMP) NOT 12 (then multiply by hours of the day). VMP values are found on the data plate of the solar panel but average 17.5 to 18.5 for a 12v panel. This article's projection of power generation per panel is misleading;

b) this article does not mention in passing that having a good MPPT charge controller is critical to getting the rated power out of a solar panel. All other non MPPT charge controllers leave about 25% of a solar panel's capacity unused;

c) propane refrigerator/freezers for boats are just simply too dangerous for the average user to have aboard. To even mention them in a mainstream article would suggest support for their use. Today's DC electric refrigerant systems use about 1/3 the power of equipment of even 10 years ago. So with sufficient renewable solar/wind/battery capacity, which has never been cheaper or more efficient, finitely fueled and dangerous propane cooling on boats should simply not ever be considered or mentioned; and

d) there is a new kind of lead acid battery that totally frees up most of a battery's rated capacity for use. These use carbon on the cathode to inhibit sulphation, increase cycle life and have a greater charge acceptance rate. These batteries can run at partial states of charge (PSOC) without long term damage. Perfect for most "small" boats and seasonal cruisers. Unlike Lithium Ion (Li) batteries, they don't blow up or catch fire. The new carbon battery means greater charge/discharge flexibility and they are ultimately cheaper because they won't be damaged by chronic under charging.

Posted by: Whiskerstay | March 22, 2018 11:14 AM    Report this comment

Good article. I used to design solar powered sites in remote locations and used a lot of complicated factors like solar insolation, latitude and shortest days without full sun by city. However, I am not sure where the 85% maximum charge factor was derived. When we used charge controllers we could get near 100% of available charge and we had a 3 year replacement cycle. The new lithium cells change this whole equation as do super capacitor storage.

If wind was sufficiently consistent above 10 kts we used wind chargers and actually had to dump charge (shunt). With the price of solar panels I would add as many panels as you have space for since you can never have too much AH on a cruising sailboat.

Posted by: Inspiration | March 22, 2018 11:13 AM    Report this comment

A couple of points
1. Even minor shadowing from rigging, not just sails and boom can cut pv power output considerably.
2. Getting full (STC) rated output with properly tilted panels, unless you sail in a cold, high altitude location, is unlikely. As sun hits the panels, they warm considerably from the 25degreeC STC reference temp., which reduces panel voltage and output. Some panels spec Normal Operating Cell Temperature values, which will be closer to the real world output. 70-75% of STC would be a reasonable approximation for NOCT values in most sailing environments for properly tilted panels. Marine haze, salt spray, etc could reduce this further.

Posted by: %2B1 for yellow - example 2 | March 22, 2018 11:12 AM    Report this comment


1. "Record the number and capacity of your batteries, recognizing that you cannot draw below 50% charge without shortening their life, and that you will seldom charge past 85% while away from the dock--as a result, only 35% of nameplate capacity is really useable."

A) This is true only for standard FLA batteries. Some AGMS and LiFePO4's can be discharged lower with less detriment.

B) This is true only for reliance on engine powered charging systems due to the reduced bank acceptance rate as it charges. The solar charging system, should be sized to recharge the batteries to or near 100% for a full sun day. While it is true solar charging system output may be lower after high noon (without solar panel aiming) it is still usable output and may exceed the acceptance rate of the near charged battery to bring it up to near 100% over the next 9 hours (mid -summer) of charge time. If it is not possible for the solar charging system to raise the bank SOC to near 100% on a sunny day, use other charging sources to quickly get the bank up in the morning to a state where the solar charging system can.

3. Many of the device load (Amp) ratings are way too low.

4. 220 A solar (Watts nameplate) makes no sense. (PS, "Watts" is with upper case "W", and Ah or Ahr or A-hr is with upper case A only.) Charging A-hr per day calc doesn't appear to be A x hrs.

5. "amp-hours" should be "Amp-hours".

6. To determine solar panel charging contribution it is better to calculate W-hrs / day (full sun), divide by panel ocV (which is never 12 Vdc) to derive A-hrs / day.

Posted by: Oh for | March 22, 2018 11:08 AM    Report this comment

A link to the provided Excel table could have been really useful as a starting point. But excellent article otherwise - thank you.

Posted by: Ragtrade | March 22, 2018 10:56 AM    Report this comment

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