gideonzook
New Member
- Joined
- Jun 15, 2021
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How much is lost going with pwt versus mppt on solar controllers?
pwt versus mppt
- Thread starter gideonzook
- Start date
Horsefly
Solar Wizard
I assume you mean PWM rather than PWT.
If you use PWM, the voltage coming in to the charge controller needs to be pretty close to the voltage of your battery. For MPPT, the voltage can be much higher than that of the battery. So with PWM you lose the ability to use the larger panels that are built for grid-tie installations. Also, because the voltage is lower, you have to use larger wire for a given wattage of solar power.
MPPT is much more efficient than PWM, but the exact numbers vary quite a bit depending on manufacturer. An old rule of thumb I had heard was to assume 80% efficiency for PWM, and 95% efficiency for MPPT.
The downside of MPPT is that it is more expensive.
If you use PWM, the voltage coming in to the charge controller needs to be pretty close to the voltage of your battery. For MPPT, the voltage can be much higher than that of the battery. So with PWM you lose the ability to use the larger panels that are built for grid-tie installations. Also, because the voltage is lower, you have to use larger wire for a given wattage of solar power.
MPPT is much more efficient than PWM, but the exact numbers vary quite a bit depending on manufacturer. An old rule of thumb I had heard was to assume 80% efficiency for PWM, and 95% efficiency for MPPT.
The downside of MPPT is that it is more expensive.
fratermus
Solar Enthusiast
- Joined
- Mar 19, 2021
- Messages
- 509
It depends.
MPPT will, on average, make somewhat more power than PWM (let's stipulate +15% overall) but that advantage is situation-specific. It also comes at at substantial price premium.
I've tried to collect the variables involved in this how to choose article. It was written for vandwellers but might be helpful as a jumping-off point for your own research.
Example:
Generic 100W panel:
Vmp=17V
Imp = 5.9A
An MPPT will operate the panel at very nearly that voltage and current in perfect solar conditions producing very nearly 100W. Most MPPT are actually much higher
A PWM shorts the panel to the battery pulling the panel nearly down to the battery voltage. In that case, let's say you're at 14V. 14V * 5.9A = 82.6W in perfect solar conditions. Also, power decreases with battery voltage, so charging at lower battery voltages gets less power (12.7V * 5.9A = 75W), and your highest power is at absorption voltage right before the transition to constant voltage (absorption phase).
Hence the 20% estimate of the performance hit with PWM vs. MPPT.
For small projects where I might want to power a gadget or an irrigation pump, I would choose hands PWM down due to the cost. Otherwise, it's MPPT for me.
The one exception to the above is 60 cell panels on a 24V system. The penalty is smaller because their operating voltage is around 30V. However, 60 cell panels may be insufficient for reliable full charging of lead-acid and equalization charges.
Generic 100W panel:
Vmp=17V
Imp = 5.9A
An MPPT will operate the panel at very nearly that voltage and current in perfect solar conditions producing very nearly 100W. Most MPPT are actually much higher
A PWM shorts the panel to the battery pulling the panel nearly down to the battery voltage. In that case, let's say you're at 14V. 14V * 5.9A = 82.6W in perfect solar conditions. Also, power decreases with battery voltage, so charging at lower battery voltages gets less power (12.7V * 5.9A = 75W), and your highest power is at absorption voltage right before the transition to constant voltage (absorption phase).
Hence the 20% estimate of the performance hit with PWM vs. MPPT.
For small projects where I might want to power a gadget or an irrigation pump, I would choose hands PWM down due to the cost. Otherwise, it's MPPT for me.
The one exception to the above is 60 cell panels on a 24V system. The penalty is smaller because their operating voltage is around 30V. However, 60 cell panels may be insufficient for reliable full charging of lead-acid and equalization charges.
rin67630
Solar Enthusiast
A comparison at full power is biased and does not reflect reality.
Generic 100W panels with Vmp 17-18V are designed to work quite well with PWM.
MPPT has a hard time with them since the voltage difference to a full charge 14,4V battery is too tiny.
Basically, one must differentiate: what do you want to achieve?
a) Being off grid in a typical camping pattern with a 100W panel brings you in different situations:
- if you catch a shiny day your battery will be fully charged at 2 o'clock and you will be clipping and throwing energy away the rest of the time:
Then it does not matter if you have MPPT or PWM.
- if you catch a cloudy day, your 100W panel will produce about 7W max with MPPT and 6,5W max with PWM
At noon. In the morning and evening much less...
Then the own consumption of your solar charger (24h a day!) is not negligible any more, MPPT solar chargers frequently will consume the 0,5W difference for themselves. PWM has far less own consumption.
So at the bottom line, it does not matter if you have MPPT or PWM.
b) Of course, the figures are different if you have home-grade panels at a higher voltage (250W and above) and if you are feeding the grid.
Then MPTT is the way to go without discussion.
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