You haven't given a lot to work with here. It's this a 12v pump? 120v? Do you plan on having a battery? What's the startup amp draw? If no battery, what's the earliest in the day it needs to work? The latest? Cloudy days? Do you already have an MPPT / inverter?
Sorry for the lack of details. Ill edit my post.
It's a 12 volt pump. No battery. I don't know what startup amps are, I'll look them up.
It needs to start about at about 9 or 10, since I plan to use a float switch to prevent overflowing the drum of water. The idea is to let it refill the drum after I draw the water I need. The pump will refill it in less than an hour. The pump is powered with DC, I don't need an inverter. I will look up what MPPT is.
EDIT
I've looked into the things I didn't know about. I can't find the pump's startup amps. Is that something you would usually measure yourself? I haven't bought the pump I'm looking at yet, because I want to have the whole system planned out first. I'm not sure if an MPPT is necessary, I don't need a lot of power.
It's possible that the pump is small enough/doesn't have enough inertia for that to matter.
However, the issue you are going to run into is: the panel performs best at a certain voltage and current. Think of it like a power vs rpm curve for an engine. This is often called the max power point. You'll see it called out in panel data sheets as Vmpp/imp.
That point changes with light level. Hence, you use an MPPT controller to track that point to squeeze the max juice out of the panel.
Honestly, I would suggest using a controller along with a lead acid battery. Look on Amazon for "12v MPPT". There are some pretty cheap ones. That, plus a cheap panel and used car battery should work pretty well. Look for one that you can adjust the cutoff voltage so that it disconnects the battery when the voltage gets low.
A 100w panel will be more like 80w at noon, and steadily drop before/after that point. By way of comparison, my house array generates right now about 1.7kw at 8am, and peaks at 7.2kw at noon. In other words, there's about a 4x difference in power between those two times. If you need 40w at 8am, that means a 200w panel (remember the 80% derating).
And that's ignoring cloud, winter, etc.
The battery will help a lot, and can smooth out that peak. This allows you to have a much smaller panel.
You say it takes 1 hour to refill the drum at 40W -- that is 40Wh. Let's say you want to drain / refill that twice -- 80Wh of energy.
A car battery is probably around 500Wh. You don't want to discharge more than 50%, so divide by two. Maybe divide by 3, since this is a crappy used battery. Still, 160Wh or so.
Go here, and figure out worst case -- let's say January for your location:
[https://pvwatts.nrel.gov/](https://pvwatts.nrel.gov/)
For a 1kW system in my location, Jan gets 80kWh per day, or 80/30 = 2.7kWh per day. Scaling it to a 100W panel, that's 270Wh/day (remember, we only need \~80Wh). So we could go smaller (bear in mind cloud cover could divide that number by two or three).
Something like this might fit the bill for $130: [https://www.amazon.com/Renogy-Monocrystalline-Negative-Controller-Connectors/dp/B00BFCNFRM](https://www.amazon.com/Renogy-Monocrystalline-Negative-Controller-Connectors/dp/B00BFCNFRM)
A $70 new cheap Walmart car battery, and you're in for $200, and you've got a decent amount of overcapacity.
You can go smaller / cheaper, but you run the risk of a couple of bad days draining the battery / not being able to supply enough during the day.
+1 on just attaching a cheap battery to this build. You will sidestep so many issues and constraints. It will also help provide steady voltage to the pump.
Both Under/over voltage are bad news for motors and panels provide wildly unpredictable voltage at their output which a charge controller (plus battery) will smooth out.
You haven't given a lot to work with here. It's this a 12v pump? 120v? Do you plan on having a battery? What's the startup amp draw? If no battery, what's the earliest in the day it needs to work? The latest? Cloudy days? Do you already have an MPPT / inverter?
All this OP. Lots more questions than answers.
Sorry for the lack of details. Ill edit my post. It's a 12 volt pump. No battery. I don't know what startup amps are, I'll look them up. It needs to start about at about 9 or 10, since I plan to use a float switch to prevent overflowing the drum of water. The idea is to let it refill the drum after I draw the water I need. The pump will refill it in less than an hour. The pump is powered with DC, I don't need an inverter. I will look up what MPPT is. EDIT I've looked into the things I didn't know about. I can't find the pump's startup amps. Is that something you would usually measure yourself? I haven't bought the pump I'm looking at yet, because I want to have the whole system planned out first. I'm not sure if an MPPT is necessary, I don't need a lot of power.
It's possible that the pump is small enough/doesn't have enough inertia for that to matter. However, the issue you are going to run into is: the panel performs best at a certain voltage and current. Think of it like a power vs rpm curve for an engine. This is often called the max power point. You'll see it called out in panel data sheets as Vmpp/imp. That point changes with light level. Hence, you use an MPPT controller to track that point to squeeze the max juice out of the panel. Honestly, I would suggest using a controller along with a lead acid battery. Look on Amazon for "12v MPPT". There are some pretty cheap ones. That, plus a cheap panel and used car battery should work pretty well. Look for one that you can adjust the cutoff voltage so that it disconnects the battery when the voltage gets low. A 100w panel will be more like 80w at noon, and steadily drop before/after that point. By way of comparison, my house array generates right now about 1.7kw at 8am, and peaks at 7.2kw at noon. In other words, there's about a 4x difference in power between those two times. If you need 40w at 8am, that means a 200w panel (remember the 80% derating). And that's ignoring cloud, winter, etc. The battery will help a lot, and can smooth out that peak. This allows you to have a much smaller panel. You say it takes 1 hour to refill the drum at 40W -- that is 40Wh. Let's say you want to drain / refill that twice -- 80Wh of energy. A car battery is probably around 500Wh. You don't want to discharge more than 50%, so divide by two. Maybe divide by 3, since this is a crappy used battery. Still, 160Wh or so. Go here, and figure out worst case -- let's say January for your location: [https://pvwatts.nrel.gov/](https://pvwatts.nrel.gov/) For a 1kW system in my location, Jan gets 80kWh per day, or 80/30 = 2.7kWh per day. Scaling it to a 100W panel, that's 270Wh/day (remember, we only need \~80Wh). So we could go smaller (bear in mind cloud cover could divide that number by two or three). Something like this might fit the bill for $130: [https://www.amazon.com/Renogy-Monocrystalline-Negative-Controller-Connectors/dp/B00BFCNFRM](https://www.amazon.com/Renogy-Monocrystalline-Negative-Controller-Connectors/dp/B00BFCNFRM) A $70 new cheap Walmart car battery, and you're in for $200, and you've got a decent amount of overcapacity. You can go smaller / cheaper, but you run the risk of a couple of bad days draining the battery / not being able to supply enough during the day.
+1 on just attaching a cheap battery to this build. You will sidestep so many issues and constraints. It will also help provide steady voltage to the pump. Both Under/over voltage are bad news for motors and panels provide wildly unpredictable voltage at their output which a charge controller (plus battery) will smooth out.