Estimating microgrid sizing for minimal export

By John-Ross Cromer

The commercial solar landscape is rapidly evolving. With declining solar export compensation and federal tax credits, the industry must shift from simple grid-tied arrays to sophisticated energy management solutions that optimize value while minimizing costly grid interaction.

Commercial vs. Residential: A Different Game

Unlike residential customers who face primarily energy charges ($/kWh), commercial electric bills include demand charges—fees based on peak power consumption ($/kW). This dual billing structure fundamentally changes the solar value proposition. Net metering buyback policies historically available to residential only apply to energy rates, representing just one part of commercial bills, resulting in design errors if that “cover the rooftop” philosophy is blindly applied to commercial sites.

The cheapest way to avoid selling back to the grid at a loss is to reduce the size of the solar array. Most behind-the-meter commercial batteries are 2-hour or 4-hour systems. Because most commercial projects are not doing larger 6+ hour batteries required for keeping 100 percent energy offset behind the meter and instead focus on peak energy management, under-sizing commercial projects is a valid starting strategy when economics are the project priority.

The Paper Napkin Method

Only two questions need to be asked to mentally estimate ideal commercial microgrid size for economic priority:

Question 1: What is the facility’s demand charge? This kilowatt figure represents peak power consumption. By keeping solar arrays smaller than this limit, the battery no longer needs to manage “excess” solar production and can be sized to focus in on Time of Use (TOU) shifting or peak shaving.

Question 2: Is the business energy use consistent seven days per week? For consistent seven-day operation, perhaps target solar arrays at 80 percent of the demand charge. For buildings with weekend closures or other seasonal variations, a much smaller target, such as 40 percent, is a more conservative starting point.

Demand charges and facility energy use patterns are typically known to facilities managers. Knowledge of BESS application can better inform the mental math. Smaller, 208V service buildings have more variability in their load profiles favoring 2-hour batteries, while 480V buildings have smoother loads favoring 4-hour batteries. Knowing the answers to the above with a little field knowledge enables paper napkin estimates of project sizes that are realistic enough to consistently hit performance goals.

Taking a Load Off on the Weekend

Consider a 200kW demand charge office for a building closed on weekends. An optimized 80kW solar array with 160kWh battery effectively peak shaves and time-shifts without significant grid export, delivering higher effective generation rates and quicker payback. If the building were on 480V service, the recommended battery size might be 320kwh instead, and the solar array might be slightly larger.
For the solar array size, it can be assumed the first 80kW goes directly into the weekend load profile. After that, additional solar can be added back into the system based on the battery size, using a rough estimate of 1kW of additional PV per 8kWh battery capacity.

Sol-Ark’s Product Differentiation

Commercial economics treat solar batteries as optimization tools, not just backup accessories. Sol-Ark hybrid inverters scale for whole building backup between main service panels and utility transformers, while also supporting load-side interconnection for individual distribution panels.

For the 200kW office example with an 80kW weekend peak load, if on a 208V service, the best match is a Sol-Ark 30K 208V inverter, resulting in a 102kW PV/90kWac/180kWh system. If on 480V with a 4-hour battery, the recommendation might be to use the Sol-Ark 60K inverter, resulting in a 140kW PV array on 120kW inverters with 480kWh batteries. These systems would not be large enough for “whole building backup”, but would be large enough to power a handful of essential distribution panels such as refrigeration, lighting, and security.

Sol-Ark can do 100 percent offset, taking buildings fully off-grid. But the company can also deliver smaller systems, precisely sized for economic optimization, which is a much larger market. Industry sales reps can use this mental math estimating technique to improve conversations with potential commercial system owners. At that point, final project sizing can be determined using computer software, engagement with Sol-Ark’s internal sales team, or even guided by third party controller services such as EnergyToolbase, Wattch, New Sun Road, or Ampra Grid.

John-Ross Cromer is Head of Training for energy technology company Sol-Ark (www.sol-ark.com).

Q4 2025