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I'm doing a project on designing an off-grid PV system to power a constant load at 900W at 12V, 24/day.

If I choose PV systems that are rated at 48 V or any other voltage will that mean that any layout will not be sufficient to power the load at 12V? Will it create problems as such or anything? I am using an MPP charge controller and it is backed by a battery system.

Any book recommendations are welcome as I want to improve on the field.

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  • \$\begingroup\$ It would be simpler to choose a 12 V PV system, if your needs are solely 12 V. Otherwise, you may need to provide a DC to DC buck converter capable of your power delivery needs, as well. You should write more about the circumstances -- more than one sentence, anyway. \$\endgroup\$ – jonk Mar 9 at 13:13
  • \$\begingroup\$ Much more detail needed. I assume this is for a course etc - not a real world system? What location (it matters). How many days without sun must it operate for? Is there any backup power source (alternator etc?). What sort of batteries? How far from panels to load. || IF you are using an MPPT controller then 12V or 48V system is not too important load wise - a 48V in 12V out MPPT controller is not unusual. || 900W x 24 H = 21.6 kWh/day . In many locations winter susn is 2-3 hours/day of equivalent full sun. So your PV system needs to be 8 to `2 x as many Watts as your load. ... \$\endgroup\$ – Russell McMahon Mar 9 at 14:07
  • \$\begingroup\$ eg 900W x 24 hrs = 21.6 kWh. To obtain 21.6 kWh in 3 hours PV system must store more than 21.6/3 = 7.2 kWh per hour or >= 7.2 kW panels. . Allowing for battery charge efficiency, conversion efficiency, MPPT efficiency ... you'd need 10-12 kW of PV panels. For 2 hours per day full sun equivalent (many locations in winter_) you'd need 50% more again - say 15 kW of PV panels !!!!. | That's for ONE day of 24 hr use. For say 3 sunless days you need 3x that = say 30-45 kW of panels [!!!!]. Battery also gets "interesting". MORE DETAILS PLEASE. \$\endgroup\$ – Russell McMahon Mar 9 at 14:12
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Please provide as much detail as possible.
I assume this is for a course etc - and not a real world system?

What is the location (it matters)?.
How many days without sun must it operate for?
Is there any backup power source (alternator etc?).
What sort of batteries?
How far from panels to load.

IF you are using an MPPT controller then 12V or 48V system is not too important load wise - a 48V in 12V out MPPT controller is not unusual. However, the system you need is a substantial one. As shown below you need 10 to 15 kW of panels!. At 12V that's 850 to 1625 Amps [!!!]. You do not want a 12V system. Even at 48V that's 215 - 400 Amps. An even higher voltage system may be in order.

900W x 24 H = 21.6 kWh/day.
In many locations winter sun is 2-3 hours/day of equivalent full noonday sun.
So your PV system needs to be 8 to 12 x as many Watts as your load.

eg 900W x 24 hrs = 21.6 kWh.
To obtain 21.6 kWh in 3 hours a PV system must store more than 21.6/3 = 7.2 kWh per hour or >= 7.2 kW panels.
Allowing for battery charge efficiency, conversion efficiency, MPPT efficiency ... you'd need 10-12 kW of PV panels.
For 2 hours per day full sun equivalent (many locations in winter_) you'd need 50% more again - say 15 kW of PV panels !!!!.

That's for ONE day of 24 hr use. For say 3 sunless days you need 3x that = say 30-45 kW of panels [!!!!]. Battery also gets "interesting".

MORE DETAILS PLEASE.

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  • \$\begingroup\$ first off, the location is malawi and in wintertime there is 4.32kwh/m2/day \$\endgroup\$ – Christos Efstathiou Mar 9 at 14:56
  • \$\begingroup\$ and the max days without sun is 2 day, i have derived what you guys mentioned above, my question is basically if i were to modele the PV system by an existing PV panel such as a 300W monocrystaline panel, with give Voltage an Amperes at MPP of 32.4V and 9.26A respectively, would that be able to power my DC load at 12V or it wouldn't match? \$\endgroup\$ – Christos Efstathiou Mar 9 at 14:59
  • \$\begingroup\$ i mean, if i were to directly attach the load to the PVs, then the PV system wouldn't work at MPP, so how do i go around that by using a charge controller and a battery? i considered Tesla powerwall due to high efficiency, discharge capacity etc. Does the battery also need to match such load? or will load ''demand'' and ''get'' what is needed by the batter? \$\endgroup\$ – Christos Efstathiou Mar 9 at 15:01
  • \$\begingroup\$ no there is no backup generator, its completely off-grid, so i tried modelling it according to winter thus i will also need a dumping mechanism for summer radiation levels \$\endgroup\$ – Christos Efstathiou Mar 9 at 15:03
  • \$\begingroup\$ by considering all the efficiencies together i figured i need about 45.96kWh for storage. Its just that i dont think its logicall to throw in any PV system without considering configurations to match my load. or is it possible? \$\endgroup\$ – Christos Efstathiou Mar 9 at 15:06

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