Solar Panels to Kw's

[mikeyj]

Hola,

I'm just starting to make some enquiries into a new 4kw solar energy system install, some installers advertise the need for 16 x 250 watt panels, others list 6 to 8 x 250watt panels.

Currently I'm running a 3kw system that uses 8 x 140 watt panels, is there a minimum correlation, or a formula used to calculate the number and wattage of panels to obtain the necessary draw from that big yellow ball in the sky.

Thanks,

Mike.

[sdold]

It sounds like you might be confusing the panel ratings with the power that the battery and inverter is capable of producing. Are you referring to the size of the solar array, as is the case when sizing a grid-tied system, or do you mean the power the batteries and inverter can produce in an off-grid system?

[Sunking]

What are you talking about? 4Kw = 4000 watts. .................................................. ............................ 8 x 250 watt panels = 2000 or 2 Kw. If all you have is 8 x 140 watt panels you only have 1140 watts or 1.14 Kw

[mikeyj]

What are you talking about? 4Kw = 4000 watts. .................................................. ............................ 8 x 250 watt panels = 2000 or 2 Kw. If all you have is 8 x 140 watt panels you only have 1140 watts or 1.14 Kw

Hola,

Sorry for the confusion, my existing off grid set up is 8 x 140 watt panels, I have a 3kw invertor, an Outback MPPT charge controller and a 12 x 2v battery bank.

I was looking into a new 4kw set up as mine is nearing the end of its working days, I may be getting confused with grid tie systems which may need 16 x 250 watt panels.

Mike.

[inetdog]

Hola,

I'm just starting to make some enquiries into a new 4kw solar energy system install, some installers advertise the need for 16 x 250 watt panels, others list 6 to 8 x 250watt panels.

Currently I'm running a 3kw system that uses 8 x 140 watt panels, is there a minimum correlation, or a formula used to calculate the number and wattage of panels to obtain the necessary draw from that big yellow ball in the sky.

Thanks,

Mike.

I think that you, probably with some help from uninformed salesfolk, are confusing the energy per day in kWh with the instantaneous power in kW.
If you do not understand the difference, let us know and we can point you to some basic information.

If the weather and latitude are right a 1400 kW system can produce 4kWh per day even in winter.

PS: There are formulas, and the result of those formulas applied to your location can be found at PVWatts (http://pvwatts.nrel.gov/)

[J.P.M.]

I think that you, probably with some help from uninformed salesfolk, are confusing the energy per day in kWh with the instantaneous power in kW.
If you do not understand the difference, let us know and we can point you to some basic information.

If the weather and latitude are right a 1400 kW system can produce 4kWh per day even in winter.

PS: There are formulas, and the result of those formulas applied to your location can be found at PVWatts (http://pvwatts.nrel.gov/)

That's pretty big system with not very high efficiency.nu

[mikeyj]

Hi,

What factors may cause my Outback charge controller to suddenly drop from receiving around 24 amps on a continuously clear, sunny day to around 16 amps, it is still in MPPT bulk mode, not even close to absorbing, middle of the day, the batteries appear to be healthy, showing around 1.25 on the hydrometer. Sometimes running something like a washing machine through its cycle can kickstart the process. The panels are clean by the way. It appears to be preventing the set up from charging itself.

Cheers

Mike.

[inetdog]

Hi,

What factors may cause my Outback charge controller to suddenly drop from receiving around 24 amps on a continuously clear, sunny day to around 16 amps, it is still in MPPT bulk mode, not even close to absorbing, middle of the day, the batteries appear to be healthy, showing around 1.25 on the hydrometer. Sometimes running something like a washing machine through its cycle can kickstart the process. The panels are clean by the way. It appears to be preventing the set up from charging itself.

Cheers

Mike.

My first guess is that you have a shaded or defective section in one of your panels that is causing it to be able to deliver less current than the rest of the array. Depending on the operation or non-operation of the bypass diode for that section and the way the MPPT algorithm is doing its seeking you may be seeing the low current capability of that one panel section forcing the CC to find a local maximum on the array power curve instead of a nearby higher global maximum.
What are you seeing for array voltage, and how is your array wired (single string? all panels in parallel?)

[mikeyj]

Hi inetdog,

Thanks for the reply, I noticed that when I applied load to the system, i.e., ran the pool for only a few minutes, the amps picked back up to around the 24 mark. Soon after, it went into absorb mode, then floated. It must have been very close to absorbing in the first place. As for array voltage, I have a 24 volt set up, it was showing 67.0 volts, the PV input current was 6.7 amps, the output current was 15.3 amps. The battery voltage was 28.0 volts ( so close to absorbing). As soon as I applied some load via the pool, the array was showing 52.6 volts, the PV input current went to 12.0 amps and the output current went up to 23.6 amps ( this is a decent pull from the system on a normal day).

I'm really not too sure how the panels are wired up, the panels are free from shade as they are roof mounted on a tilted rack on an angle in between the ideal summer/winter setting.

I guess it may be that I do not fully understand what my Outback 60 is telling me.

Thanks again,

Mike.