diy solar

diy solar

System design advice for occasional use of solar

Cap'n Sly

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Aug 15, 2020
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I'd like to add solar panels to my boat in order to power a 450 watt, low-energy startup air conditioner for only a number of hours (4-6) during recreational trips out on the water here in South Florida (so generally sunny days, as we generally don't go out on the boat if it's not nice weather).

I can comfortably add two 77x39 panels.

If I were to get, say, two 300 watt solar panels, then I should be able to run the AC as long as it's sunny (please correct me if I'm wrong).

If I were to also add batteries for the times that, say, a cloud passes over us, is it possible for the system to automatically supplement the energy coming from the panels with battery power when it isn't producing enough watts to keep the AC running? Is that how these solar controllers and inverters work generally or does it require a specific type of setup for that use-case?

Thanks very much for any advice!
 
You are on the right track. Think of running the unit off of the battery and the inverter as the primary way to power the A/C unit. Then the solar is to keep the battery from draining. You may think it sounds the same but you need the battery power as a steady source.

One question is, is it okay to run the battery down some and let it recharge after you park the boat?

Next question is, will the A/C be running inside a close space so it has a chance to cool the space and cycle off, or is it just blowing cold air on someone but the door to the room is open all the time. If the AC has a chance to cycle on and off because the room is cool, that is less power demand and the battery can charge a little between A/C cycles.

You will not get 300 watts from the 300 watt panels. If the boat were parked so you could point the panels at the sun at the ideal angle and if there were a breeze to keep the panels cooler, you might get close to 300 at the panels, and a little less at your A/C after wiring and inverter loses. So you need a little more on the panel size or back to question #1, you might need a little more battery. If the panels, even at a less than ideal angle, can provide 60% of the required power, they can extend what any given battery can do by itself. If the A/C unit can cycle off some, that is even better. But in general the plan is sound.

There are portable battery inverter systems (solar generators) that might be the best place for you to start. They have built in battery, inverter, and solar charge controller. Some may not made to handle commercial sized panels because of the panel voltage, but if you shop carefully you can find those that do. Will has videos where he has reviewed many of these units.
 
Thanks so much for the reply!

Does that mean that I have to get a battery bank large enough to handle running the A/C on it's own entirely without panels? Or will the panels charge the batteries while the A/C is drawing energy from them? Assuming the panels are producing the entire amount of watts required to run the AC, will the panels recharge the batteries at the same rate as the AC is depleting them?

The AC will likely be cycling on and off. I'm thinking of just designing for 'redundancy', so just pretending it will run 100% of the time, even though it probably won't.

Thanks again!
 
" Assuming the panels are producing the entire amount of watts required to run the AC, will the panels recharge the batteries at the same rate as the AC is depleting them? "
Yes, that would be ideal and could happen if the panels are large enough. If any shade or bad angle happens, the battery takes up the slack.

Yes you really need a battery that can run the AC, at least for a short period of time. If the battery is too small, any drop in power and the inverter will shut down. Any drop means someone throws a towel over the edge of the panel or a cloud. A good cloud can knock your solar power WAY down.
 
Take a look at this as an example. (older video, don't judge based on this video) Something like this could/should run your A/C for 2 hours. Boost it with enough solar and it would run all day.
 
That's a great demo of why a battery is needed, hah. I did find a 195ah SLA battery for $175 at a dealer near me. Going to start with that and see how it goes.
 
That's a great demo of why a battery is needed, hah. I did find a 195ah SLA battery for $175 at a dealer near me. Going to start with that and see how it goes.
That's a great price for a 195ah battery, any idea where you can find that in Florida ?
 
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