iceblitz avatar image
iceblitz asked

Can I use a 400W solar panel on the 75/15 SmartSolar controller?

I have a 75/15 SmartSolar MPPT controller, and a 120W panel. This is not sufficient, and I want to maximise the charging power on mornings and evenings.

So I was wondering if I could install a 400W solar panel on the MPPT despite the controllers max wattage rating of 220W?

IF I can do this, where the heck does the excess power go? Surely it can't be dissipated as heat, because there is not enough cooling on the MPPT to remove 180W of waste heat. I hope someone can explain this to a dummy like me :D


MPPT ControllersMPPT SmartSolarsolar sizing
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1 Answer
dansonamission avatar image
dansonamission answered ·

There is no excess power, the MPPT will pull power from the panel as required, panels don't push power out.

The 75/15 means, a maximum PV voltage of 75 and max amps output at battery voltage of 15. As long as your PV voltage is less than the maximum of 75v for this MPPT you'll be ok.

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iceblitz avatar image iceblitz commented ·
Thank you for your answer. But what do you mean the solar panel dont "push power out". To me it seems like it absolutely does.

If I connect a solar panel directly to a battery, it will charge it, as there is both current and voltage coming out of the panels terminals.

That means that the controller has to cut or reduce power somehow, to my understanding. If the controller can cut or reduce power without getting hot, it has to apply resistance (ohms law). How is it doing this?

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kevgermany avatar image kevgermany ♦♦ iceblitz commented ·

It's easier to think of it as if the MPPT behaves like a light bulb. If you plug a 100W light bulb into the mains, it draws 100W, even though the mains can supply many kW. Plug in a 50W bulb and only 50W is used.

Same for the MPPT. Even though there may be 400W available from the panel on a sunny day, the MPPT will only draw what it can use. The remaining capacity of the panel is simply not used. Goes nowhere. Although the panels push, they don't force. Big difference.

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Fideri avatar image Fideri kevgermany ♦♦ commented ·
With nothing connected, my panels produce zero current as measured with a multimeter. But when a light bulb is connected, they produce just enough to light the bulb.

From my understanding, the panels "push" current into batteries provided the batteries "accept" the current. No acceptance of current, no push.

When I connect my MPPT with no battery, I measure zero current from the panels (well, there is a little current to run the charger). When I connect a battery, I measure lots of current.


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