CSLB C-46 Solar ContractorPlanning and EstimatingMedium
A C-46 contractor is designing a residential grid-tied PV system for a client whose annual electricity consumption is 12,000 kWh. The local utility has a net metering policy that credits excess generation at a 1:1 rate, but only up to 100% of the previous year's consumption. The contractor estimates the system's annual production will be 1,500 kWh per kWp of installed DC capacity. To achieve a 90% offset of the client's annual consumption, what is the ideal DC array size (in kWp)?
- A8.0 kWp
- B9.6 kWp
- C10.8 kWp
- D7.2 kWp
Show answer & explanationAnswer & explanation
Correct answer: D. 7.2 kWp
To achieve a 90% offset of 12,000 kWh, the system needs to produce 12,000 kWh * 0.90 = 10,800 kWh annually. Given an estimated production of 1,500 kWh/kWp, the required DC array size is 10,800 kWh / 1,500 kWh/kWp = 7.2 kWp.
Why the other options are wrong
- A. This would result in 12,000 kWh annual production, achieving a 100% offset, which might be overkill if the client only requested 90% and the goal is to optimize based on that.
- B. This calculation would be for a 120% offset (14,400 kWh / 1,500 kWh/kWp), exceeding the desired offset and potentially net metering limits.
- C. This calculation would be for a 135% offset (16,200 kWh / 1,500 kWh/kWp), significantly over-sizing the system.
PV System Offset Sizing
Calculating the required PV system DC array size to meet a target percentage of a client's annual electricity consumption, considering local production factors.
- Annual consumption is the baseline.
- Target offset percentage determines required annual PV production.
- Divide required production by site-specific kWh/kWp factor to get DC size.
Memory trick: Offset is target usage divided by system yield per kWp.