Fri Mar 6 06:35:09 EST 2026
Electric Co-ops and Customer Solar
Residential solar PV systems for customers served by South Carolina Electric Co-ops are subject to sizing constraints based on historical consumption. This is likely driven by wholesale power agreements between the Electric Co-ops and their electricity suppliers. The co-ops are probably capping self-generation to historical consumption to prevent net-positive grid-sell (aka export, sell back, back feed).
The local co-op states in their interconnection guidelines. "System Sizing Limit: due to our wholesale power agreement, we are limiting the size solar PV connected to the annual kWh use divided by 1,800 for the AC kW size or the average monthly kWh use/150."
This document is an attempt to make sense of that guideline.
System Sizing Formula Analysis
The sizing limit is defined by the following ratio:
System Sizing Limit (kW) = Annual Consumption (kWh) / 1800 hours
or expressed monthly
System Sizing Limit (kW) = Average Monthly Consumption (kWh) / 150
Both these equations yield the same value.
The divisor 1,800 represents the expected annual specific yield (kWh/kW panels) based on approximately 5 peak sun hours per day in South Carolina. (1800 = 365 * 5)
Example Case
An example: A house with an annual consumption of 13,000 kWh and a daily consumption of 36 kWh. The maximum allowable "System Sizing Limit" or the "AC kW size" is 7.2 kW.
It is unclear whether "System Sizing Limit" and "AC kW size" are defined by the inverter nameplate capacity, the installed panels, the grid-sell power or is it really a kWh grid-sell energy limit.
Oddly the limit is given in kW but it seems the real goal is to limit the kWh (energy) delivered to the grid. The "AC kW size" can be converted to daily kWh by mutliplying by the 5 hours of sun. This would, of course, yield 36 kWh. Logically the limit would apply to the actual sell back energy. The logical limit would be selling back no more than 36 kWh of energy per day with assumably a maximum power of 7.2 kW.
The sizing limitation is apparently based on perfect panel location and sun angle with ideal sun exposure. In the case of homes governed by HOAs the ideal panel location is often not available. This example is constrained to the east and west roof areas towards the back of the house. The front faces south but is not permitted to have any panels. These HOA restrictions increase the number of panels required to acheive the same yield. This adds more confusion to the meaning of the "System Sizing Limit" or the "AC kW size". Does the Co-op allow for actual yield as opposed to ideal yield?
Compliance Options
There are a few configurations of the Sol-Ark 15K-2P inverter to limit grid-sell. To provide a margin, the example house would target grid-sell of less than 30 kWh per day.
- Method A: Time-of-Use (TOU) export. Restrict grid-sell to a specific window (e.g., 10:00–15:00) with an maximum power limit of 6 kW. Maximum grid-sell would be 30 kWh in ideal sun conditions.
- Method B: Constant power limiting. Apply a maximum grid-sell power limit of 3 kW with no TOU constraints. This assumes there will never be more than 10 hours of ideal sun.
- Method C: No grid-sell. Configure the inverter to never grid-sell.
Verification
A Co-op bi-directional meter would record any sell back that exceeded the 36 kWh per day.
Suggested Re-write for System Sizing Limit
In accordance with our wholesale power agreement, the maximum allowable capacity for a Solar PV system is limited based on the facility’s historical consumption. The maximum AC kW rating shall not exceed the value calculated by either of the following methods:
- Annual Basis: Total annual energy consumption (kWh) divided by 1,800.
- Monthly Basis: Average monthly energy consumption (kWh) divided by 150.
