I have 4 x 440w solar panels on a RV. The middle two are flat, the panels on each end slope away at about 20 degrees, Should I just use one MPPT a 150/85 or will I get a better return from 4 MPPTs 100/20 ?
What is your system voltage? 12v?
You would technically get better yeild from individual tracking. Will also allow partial shading etc.
RV system is 12V. But I think a 24V house battery will be better. So I will use MPPTs to charge 24V battery.
For the 24v system then some 75/15s would be fine, one for each panel.
Basic maths is 440÷24 = 18A.
If your battery voltage is 24v, your efficiency will be better, cables might be smaller (or losses will be less for a given cable), but you will need to be careful of one rule; To start up the MPPT controller, it requires [battery voltage] + 5v, and to keep the mppt working, it needs to maintain [battery voltage] + 1v.
So if you drop below these, the mppt controller will stop charging - just like the way it behaves at night.
Your 440v panels should be around 38v to 45v Voc or there abouts, if they are typical modern panels, so even one panel will meet the BV+5v rule most of the time, but its still true that 2 panels in series on a gloomy day will “start” the controller earlier than 2 panels in parallel.
What really matters is the magnitude of these issues relative to each other, and my feeling after watching a lot of panel and MPPT combos on a lot of gloomy days (in parts of NZ, an overcast sky is very very common), that the difference in startup time makes very little difference. Even if the startup time is many minutes earlier, you are multiplying minutes by almost no current to get almost no power, so I would ignore it and look at the other issues.
These are;
- the higher the voltage the lower the cable losses (for a fixed amount of power) (true for batteries, and panels)
- panels in parallel will handle shading better than panels in series
So my advice would be;
- use a 24v house battery
- put the 2 x flat panels in parallel, on a 100/20
- put the 2 x angled panels, even though they are angled differently, on a 100/20
This gives you 2 mppt controllers - if one has a fault (very rare), you drop to half power input vs zero if you had only one big controller.
4 controllers would be nice, but would the gain in captured energy (by being more accurate in the power point tracking) exceed the running cost of twice as many controllers? Lets see;
2 x 100/20 = 16mA running cost ( 2 x 8mA from the datasheet), x 24h, = ~400mAh for the extra MPPTs.
The panels need to pull in an additional 57mA over a 7h day to cover this. 57mA is about 33wh for the day, so ~2% of your power on the worst day, 0.25% on a good day (your 4 x 400w panels should pull in about 4x2.4kwh on a good day, 4x0.8kwh on an overcast day, 4x0.4kwh on a bad day (this comes from using the rule that good days=nominal panel wattage x 6 in watt-hours, overcast = nominal x 2, rain all day = nominal).
So will 1 panel per controller do better? Probably. But will it do more than 2% better than 2 panels per controller on a bad day - because on a good day we probably don’t care - you will be awash in power. Its the bad days that we are optimising for.
This is where my experience runs out, but my gut feeling is that 4 controllers will outperform 2 controllers even when their running cost is taken into account, but not by much, and probably not enough to justify the additional cost of 2 controllers, the additional cable (4 runs instead of 2 or 1), the additional risk of leaks due to additional or larger penetrations (4 x twincore cables leaves a lot of spandrels, and having 8 individual glands seems a bit over the top.
My math above might be wrong - its the end of a long day.
I think putting the two centre panels (both face the same sky) in series makes sense to me. (The panels have a Voc of 39volts). No matter how overcast it is the MPPT will start early and keep going. What puzzles me is how to configure the other two panels that will face 40 deg away from each other. If I parallel them through one or two MPPTs they may never start as their sensed voltage (from the series panels already charging) may not exceed the +5 volts needed. ??