Excess Solar Load

[FourPoints]

Just installed a 50W maintenance charger panel on the boat and a Victron 75/10, Even with the panel seeing some intermittent shading due to the placement I'm averaging 200-300wh per day of power out of it, and on good days over 400wh. which is far more than I need to maintain the battery SOC when I'm away from the boat. That means I'm throwing away on average 150wh per day of unneeded power, and some days over 250wh if the batteries were already topped off the day before and it's very sunny the next day.

The Victron has a load output which I can activate when the voltage is higher than a set point (though surprisingly it doesn't have a trigger for on float), I've thought about wiring in an exhaust fan to help cool the boat, or a USB charger to allow charging of the boat tablet, or somehow dumping that power into the hot water heater to lightly "warm" the water.

I understand that dumping 150wh into my 6gal tank would only raise the temperature about 10F, certainly not enough for "hot" water, but at least perhaps it could feel "warm", or maybe enough to keep the water from getting cold for a bit longer after having been already heated by the engine. The current tank is an older seaward 6 gal tank, with an unused 110v connection (currently use engine heat only), could I even effectively heat the tank with DC power at all? I know that I am probably going to need to replace the tank in the next 1-2 years just based on age anyway, so is there another tank that would be able to heat (slowly) off of that small amount of DC power?

If warming the water tank is not an option, and I'm not thrilled about leaving a USB device connected & charging, and the only fan I currently have that vents outside is the engine compartment fan (which draws about 2a and is loud), does anyone have any other ideas for beneficially using up that extra 1-2amps of power for ~5-7 hours a day while I'm not at the boat?

 

[rgranger]

I am a big believer in adding ventilation on boats.

 

[dlochner]

What kind of panels are you using? If you are getting 200-300 wh a day from the panels you are far exceeding the rule of thumb that Nigel Calder uses for estimating power output. Calder suggests daily output will average 3 times the panel's nominal rating. For your 50 watt panel he would expect you to generate about 150 wh a day. That's consistent with my experience with our 300 watts of solar. Link to Calder's article below.

Advances in Onboard Solar - Professional BoatBuilder Magazine

Advances in onboard marine solar technology improved efficiency and lowered the cost. But they also introduced more complexity.

www.proboat.com

 

[Captain Larry-DH]

This is a problem that doesn’t need solving.

 

[High Current]

Calder suggests daily output will average 3 times the panel's nominal rating

That sounds rather conservative to me for someone who only sails in the summer months. 3-4 is the figure I used to size my own system, but it was deliberately conservative. I'm not at all surprised to see that FourPoints can get 4-6x on a good day.

Apricus Solar has a chart of average daily insolation each month for various US cities (Solar Insolation Levels - Apricus Solar Water Heater). Panels are rated based on 1000W/m^3, so you just multiply your system capacity by the number in the table to get average daily production for a flat, un-shaded collector. Even in ME, where my boat lives, the average is almost 5 from June through August.

does anyone have any other ideas for beneficially using up that extra 1-2amps of power for ~5-7 hours a day while I'm not at the boat?

What about a small dehumidifer? The one linked claims to draw 36W. Or a boat monitoring system?

When I discovered I had excess capacity in my 400W / 7kWh system (for my electric outboard), I installed an inverter and an induction cook top...

 

[Johann]

That sounds rather conservative to me for someone who only sails in the summer months. 3-4 is the figure I used to size my own system, but it was deliberately conservative. I'm not at all surprised to see that FourPoints can get 4-6x on a good day.

This is consistent with my experience as well on the gulf coast. In June, my least shaded panels get 5-6x per day, my most shaded panels get 4-5x per day.

 

[JBP-PA]

The maximum load current for a Victron 75/10 is 15A or 180W@12V , but you'll never get that with a 50W panel, so you can't realistically run an inverter to power your AC water heater. Wiring an AC heater coil to a 12V source doesn't sound like a good idea, but I am not sure.
You could, of course, power a fan easy peasy. You could try it just by getting a cheap car fan, put it near a dorade and wire it to your load output, and program the load output accordingly.

 

[JBP-PA]

On second thought, you could easily run a fan 24x7 without stressing your battery.

 

[Timm R Oday25]

What I choose to do was install a small muffin ( computer ) fan in our hatch boards . It draws air out of the cabin . They are very cheap,some what
reliable and easy to replace . Most only draw .3 to .5 amps

 

[LloydB]

Run a dehumidifier for a couple of amps a day when you are not on the boat with a timer.

 

[FourPoints]

That sounds rather conservative to me for someone who only sails in the summer months. 3-4 is the figure I used to size my own system, but it was deliberately conservative. I'm not at all surprised to see that FourPoints can get 4-6x on a good day.
...

What about a small dehumidifer? The one linked claims to draw 36W. Or a boat monitoring system?

Yes, key words there were good day vs average day. On average last week I was getting a little over 200wh sitting on anchor & swinging, but on a good day I have seen well over 300wh logged in the Victron app (best day so far was 380wh). Now that I'm back at the dock (I don't connect to shore power) and the boat isn't swinging it should do a little better than at anchor and back to about 250wh / day average.

I wasn't aware of those little peltier dehumidifiers existed. Looks like the big challenge is most of them have various smart panels to turn off / on, which may not work well with the controller powering it on and automatically starting up. I did some digging and found one for $39 that claims 38W draw, 12v input, and has a mechanical switch to turn on that I can leave connected to a 12v socket if it works well enough. It doesn't have to work very well to still be better than nothing, and if I can do it all for under $50 then that's great!