keeping the mind busy

Here are a couple of quick examples on how to size PV arrays and battery banks.
I’m taking a class, so I could be wrong, waaay off base or just plain nuts with these
figures and calculations. I just did them to keep the mind active and practice these
concepts.

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Battery bank sizing exercise (for a cabin or off grid system)

Average AC load per day: 1,500 watt hours
Inverter efficiency: 95%
Days of autonomy needed: 2 (DOA)
Maximum allowable depth of discharge: 50% (DOD)

Battery specs:

6v 200 Amp Hour
DC system voltage 12v

Find average daily amp hours:

1500 watt hours / 95% (9.5) = 1579wh
1579wh / Dc system voltage (12v) = 131.6ah average per day

Find number of batteries:

Parallel: 131.6ah x DOA (2) / DOD (.5) = 526.4ah / 200ah = 2.6 batts

Always round up to the nearest battery, this would be 3 batteries.

Series: 12v/6v = 2 batteries in series

Total batteries needed 3 series strings of 2 batteries. 6 batts total.
2 batts in series gives us 12v
3 strings in parallel gives us 600ah (>526.4ah)

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Array sizing exercise (for an average 2,000sqft home in Tucson AZ)

(for 100% PV power, net zero energy useage and also for 50% PV power)

Average yearly KWH useage: 8,250kwh

Average daily KWH useage: = 8,250kwh / 365 = 22.6kwh

22.6kwh / 6.5 PSH (peak sun hours, 6.5 is Arizona’s averaged out for the year)
= Approximately 3.47kwAC (DC side of a PV will be a few hundred watts more)

3.47kwAC /.95 (inverter efficiency) = 3.65KWDC
3.65KWDC /.88 (soiling, wire losses etc…) = 4.14kwDC

To provide a home that uses 8,250kwh AC with a net zero energy useage from PV
Conservatively a 4.2kw system would do the job. (further derating may need to be
taken into account, especially when considering seasonal loads and possible shading)

For 50% PV energy generation (essentually 4.14kw / 2 = 2.07kw)

22.6kwh / 2 = 11.3kwh
11.3kwh / 6.5 PSH = 1.74kwAC

1.74kwAC / .95 = 1.83kwDC
1.83kwDC / .88 = 2.08kwDC

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