Metropolitan Water District of Southern California
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Homesolar cup
Solar Cup FAQ

Solar Panel Dimensions

Is the distance you can go off the sides of the boat measured from the gunwale or from the max width of the bulkhead?

Rule 7.2.2---Nothing may extend more than 36 cm beyond the the distance is measured from the gunwale.

Battery Boxes

Our battery does not fit in the dimension of the battery box that MWD provided. Can we make our own battery box and put it in the boat?

Yes, however - batteries, particularly propulsion batteries, store a lot of energy. When being charged they produce Hydrogen and can rupture with excessive overcharging, or develop a short circuit which may release Sulphuric acid. Sealed batteries have pressure release plugs. Other lead-acid batteries vent through filling ports. Accidentally shorting the battery by dropping a tool across the terminals can result in a fire or explosion.

MWD provides Sea Fit battery boxes. They have vents for gas on the top cover that prevent tools or other conductors from falling on the terminals and causing a short circuit, and have a secure means for lashing down the battery. These are available inexpensively in a wide variety of sizes.


On the rule sheet I didn’t see that the battery has to be sealed. I only read that it has to be lead-acid. Does that mean we could use a sealed battery?

Yes, the only restriction, besides weight (25 kg, 55 lbs section 7.12.2 of rules) is that they be lead acid. You can use sealed, gel-cell, etc. If you use regular deep-charge lead acid batteries, with filling ports, you will see the state of charge using a hydrometer. These are available at most boating stores.

Testing Thrust

We would like to test the thrust of our trolling motor with our boat. Do you know where we can do it?

Yes, in practically any large swimming pool or place where there is a dock. You can make arrangements through Metropolitan to test at Cal Poly Pomona.

Vertical Propeller Shaft

Can they have a vertical shaft on the propeller?


We have located a couple of commercially made solar panels but don't know what the rated output is. How can we find out? How will it be tested?

The term that is often used is "One Sun". This means that on a perfectly clear day, on the longest day of the year, this would be the theoretical maximum value. We use a calibrated solar cell and compare the output from it to its one sun value. By using this ratio, one can take the output of a panel, and determine its theoretical one sun value.

Referring to rule 7.4, what does this mean "Batteries" may not have a one sun output greater than 320 watts?

Batteries can be charged only from the inspected energy conversion devices. This means you can't have extra panels on shore, used for charging.  The energy conversion devices (commonly your solar array) may not have a one sun output greater than 320 watts. "One Sun" is the theoretical maximum sun, which strikes the earth, at the equator on the longest day of the year.

Why do we need 8 batteries?

Metropolitan recommends 2 sets of 2 batteries each for the endurance race and the sprint race. There may not be adequate time between races to recharge batteries for each event.

Can fiberglass be used on the outside of the hull?

Yes. Fiberglass may add weight which could affect performance. There is also a maximum weight restriction to consider. Reinforce in spots if you must but try to keep the hull light.

What is the function of the controller and who makes them?

The motor controller regulates the current to the motor/s and thereby enables you to have a throttle. Consult the motor supplier for the most appropriate controller for the motor or motors used.

Are there any restrictions on what motor we can use?

While there is no rule directly specifying the horsepower or weight of the motor:

  • The horsepower is regulated indirectly by the maximum nominal voltage (24v) and the required fuse (350A)

  • The weight is regulated indirectly by the maximum weight of the boat


When wiring our boat, do we have to use special wiring? What gauge wire? What about terminals?

Copper-based wiring is recommended :

  • Regular copper (do not use aluminum) wire for houses or buildings (what you buy at home improvement or similar places) typically does not have a large number of individual strands which means it is not as flexible and may vibrate or shake loose. Automotive battery cable or welding cable are more highly stranded which means it is more flexible. Welding cable is the most flexible.

  •  National Electric Code does not have a "Solar Boat Section" addressing wire gauge . The sprint competition demands the highest current. Metropolitan recommends wire of at least 2 gauge which should be available from home improvement stores, auto parts resellers or welding supply shops.The smaller the numbers the larger the wire gauge size which can handle more current.

  • Clamping type car battery terminals can fit most wire gauges. The wire can work loose over time. Other terminals types are better and come in all types of varieties for bolts etc. but typically need more sophisticated equipment to "crimp" or prepare them for usage.


The Metropolitan Water District of Southern California is a cooperative of 26 cities and water agencies serving 18 million people in six counties. The district imports water from the Colorado River and Northern California to supplement local supplies, and helps its members to develop increased water conservation, recycling, storage, and other resource-management programs.

Page updated: November 28, 2012