Formulas for Estimating Renewable Energy Installations

Everyone always wants to know the same initial questions: How much will it cost me? And the simple answer is, its not that simple. There is no one size fits all solar installation size. Every home has different requirements because of the electrical usage of the home. Here are a few formulas that will tell you how much of a PV installation you need to take care of your power requirements.

[Photovolaic (PV) array size (watts)] x [solar radiation (hours/day)] x [system efficiency] = [system output (wat-hours/day)]

Off-Grid Solar
[Average daily elctric usage (watt-hours/day) / [solar radiation (hours/day)] / [55% off-grid system efficiency] = [PV watts required]
Ballpark Estimate:
[PV array size (W)] x 3 - [Output (Wh/day)]
[Output (Wh/day)] x 1/3 = [PV array size (W)

On-Grid Solar
[Average daily electric useage (kilowatt-hours/day)] / [solar radiation (hours/day)] / [70% on-grid system efficiency] = [PV kilowatts required]

Ballpark estimate
[PV array size (kW)] x 4 = [Output (kWh/day)]
[Output (kWh/day)] x .25 = [PV array size (kW)]

Battery Bank Sizing
[kWh/day] x [3-5 days of storage] x 3 = [kWh size for battery bank]

Charge Controller Sizing
[PV short-circuit current amps] x 1.56 = (Total amp size)

Fusing/Breakers Sizing
[Short circuit current amps] x 1.56 = [Fuse/breaker amp size]

1 kW = 75 sq. ft. of PV panels
1 MW (system rating of PV energy powers 130 homes at the U.S. average of 31kWh/day (220 homes in California at 18kWh/day average)
1 MW (system rating) of wind energy powers 250 average U.S. homes (450 homes in California)