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After several discussions recently with customers contemplating adding solar to their electrically powered vessels, there is still a lot of confusion about exactly how much power can be realistically expected from solar panels. I have an inkling that these customers are so honed in to watts and kilowatts from their dealings with propulsion that they assume that all watts are equal. But solar watts are a different animal.

The watt is a measure of power and is normally derived electrically from multiplying volts times amps (W = V x A), or amps squared times resistance (W = I² x R). So, using simple math, if we have a 5,000 watt (5 kilowatt, or 5kW) DC electric propulsion motor running on 100 volts, we would expect it to be drawing 50 amps at full load. If the same sized motor was designed to run at 50 volts then the current draw would be double that at 100 amps. For reasons of wire sizing and cost of ancillary equipment we’d want to keep the amps as low as possible, so the higher the voltage the better (except for having highly lethal voltages in damp environments). As you can see, the numbers are all very simply calculated and it’s all cut and dried, and it had better stay very dry indeed!

Solar watts are different, and I’m referring here to the wattage rating of the panel(s).

The panel wattage rating is indeed derived from measuring the volts x amps output, but at perfect conditions produced in a machine using artificial sunlight. A 100 watt solar panel will probably never ever produce 100 watts, and if it ever did it would be only for a fleeting instant. Yes, it is theoretically capable of producing 100 watts, but you would have to be on the equator at mid-day in freezing temperatures to see it! Also, that 100 watts is simply a nominal figure. Every panel should have a published Power Tolerance to let you know what to expect. A 100 watt panel with a -10/+10 tolerance could have tested anywhere in a range from 90 watts to 110 watts, and you bet your sweet sunshine that it’s more often on the low end!

houseboat solar dreamstime s 21004146What a swiz! You buy a 100 watt solar panel and then find out that it will most likely never produce 100 watts. Is this some kind of underhand sales trick?

No. No sales trick. It’s a matter of safety. When planning a solar array, we need to know what is theoretically possible of coming out of the two wires at the end so that wire sizing, fuses, and other ancillary equipment can be sized accordingly, just in case the stars all aligned and the panels all actually produced what they are rated for. Er, although if there were stars, then ...

So, could we power an electrically powered boat directly from solar? Well, let’s look at the 5kW motor example from above.

Just for giggles, let’s pretend that solar panels actually do produce what they are rated for. To produce those 5,000 watts would require a total surface area of around 300 sq ft, weigh over 700 lbs (with traditional glass panels), and need perfect conditions to do so. Not really a practical proposition on your average family puddle plodder.

OK, let’s get more realistic and practical. How much solar would we need to replenish the batteries with the power required to run the motor for, say, two hours a day? That should be doable, should it not?

At this point we should introduce some time elements into our calculations. Two hours running at 5 kW equates to 10 kW/hours. Running for two hours total during a 24-hour day results in 10 kW/hours per day. With me so far?

Because there are differences in the way different types of solar cell work at different sun angles, in different conditions, and at varying rates of efficiency, we give guidelines to help estimate daily solar yield from the various types of silicon cells. These are very generalized numbers and are for estimation purposes only, and assume a “good” solar day with a full-time load. (A solar day is a 24-hour day, including aligning stars ...)

  1. For panels with SunPower® 22.5% efficient monocrystalline cells; i.e. Solara Power M walk-on and Ultra glass panels and the Solbian SP series, multiply rated panel wattage by 4.5 for watt/hours per day.
  2. For panels with regular monocrystalline cells of 17%-18% efficiency, multiply rated panel wattage by 3.5 for watt/hours per day.
  3. For panels with polycrystalline cells, multiply rated panel wattage by 2.5 for watt/hours per day.

Using those factors, we can estimate, and it is only a wild estimate, how many panels we might need to provide that 10 kW/hours per day for the daily two-hour, 5 kW motor operation. For example;

  1. If we were to use 140 watt Solara or Solbian panels with SunPower® cells we would require; 10,000/(140 x 4.5) = 16 panels.
  2. If we were to use the same rated panel size of 140 watts, but with regular monocrystalline cells we would need; 10,000/(140 x 3.5) = 20 panels.
  3. If we use 140 watt panels with polycrystalline cells we’d have to use; 10,000/(140 x 2.5) = 29 panels.

So, three different types of solar panel, with three differing types of silicon cells but the same rated wattage, will produce significantly different daily watt/hour yields over a 24-hour solar day. There are big differences in construction between the various marine solar panels available, and also major cost disparity, but basically, you get what you pay for.

Electrical watts - Simple. Solar watts - So confusing!

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