Commercial Rooftop Solar with Demand Charges

Does commercial rooftop solar make sense with demand charges?

Commercial customers face a different bill than homeowners: they pay for energy ($/kWh) AND for peak demand ($/kW). Demand charges can be 30-60% of the bill. The question: does solar alone reduce demand charges, or do you need batteries too?

Quick answer

Solar alone modestly reduces demand charges (5-25% typical) but doesn't eliminate them. The biggest demand reductions come from solar + battery sized for peak shaving.

How commercial billing works

Energy charge ($/kWh)

• Total kWh consumed in the billing period.
• Typical commercial: $0.06-0.16/kWh (excluding fuel adjustment).
• Solar reduces this by offsetting daytime energy.

Demand charge ($/kW)

• Charged based on the highest 15-minute or 30-minute average power draw during the billing period.
• Typical: $8-25/kW per month (some markets up to $30+).
• Charged regardless of total energy used — just for the peak.

Time-of-Use (TOU) demand

• Some tariffs charge different demand rates for on-peak vs off-peak hours.
• Peak periods often 2-7 PM weekdays.

Fixed charges

• Customer charge (~$30-100/month).
• Distribution/transmission charges (per kW or per kWh).
• Various surcharges and riders.

Why solar alone doesn't fully solve demand

• Demand peak often occurs 4-7 PM when:
  • HVAC running at full load due to thermal lag from afternoon heat.
  • People leaving offices = elevators / appliances.
  • Solar production dropping rapidly (sun setting).
• Solar production peaks 10 AM - 2 PM, before peak demand window.
• Result: solar can offset 60-80% of energy but only 5-25% of demand peak.

How batteries solve demand charges

Battery + solar enables "peak shaving":

• Solar charges battery during midday (10 AM - 2 PM).
• Battery discharges during peak (4 PM - 7 PM), masking the demand spike.
• Demand charge reduction: 50-90%.

Sample math: 100 kW peak demand site

Demand charge savings of $7,200-10,800/yr add to energy savings. Payback on solar+battery investment improves dramatically.

Realistic commercial solar+battery economics

Sample: 200 kW solar + 400 kWh battery

• System cost: $1.80/W solar = $360,000 + $300/kWh battery = $120,000 + $480,000 total
• §48E (30%): -$144,000
• Domestic content bonus (10%): -$48,000
• Energy community bonus (10%, if applicable): -$48,000
• 5-year MACRS depreciation tax shield: -$130,000 NPV
• Net effective cost: $110,000
• Annual energy savings (250 MWh @ $0.10/kWh): $25,000
• Annual demand savings (50 kW reduction @ $15/kW): $9,000
• SREC income (where applicable): $5,000-15,000/yr
• Total annual savings: $39,000-49,000
• Payback: 2.2-2.8 years
• 25-year cash flow: $1M+ savings

What sizes commercial best for solar+battery

Sweet spot characteristics

• 50-500 kW peak demand with predictable peak window.
• Large flat roof (10,000+ sq ft).
• 10+ year remaining roof life.
• Tax-paying entity that can use ITC + depreciation.
• Predictable demand profile (office, manufacturing, agriculture).
• Located in state with high demand charges (CA, NJ, NY, MA, FL, TX).

Less ideal characteristics

• Erratic peak demand (call center after-hours, restaurants with mobile peak).
• Roof < 10 years old or aged + needing replacement.
• Multi-tenant building (lease structures complicate ownership).
• Tax-exempt entity that can't use ITC (consider PPA or direct pay).

Solar canopy / parking lot solar

Often better than rooftop for commercial:

• Larger area available (asphalt parking lots).
• Higher elevation = better cooling for panels.
• Bifacial gain over white-painted concrete.
• Doubles as customer/employee shade structure.
• Cost: ~$0.40-0.80/W premium over rooftop.

Common commercial demand charge mistakes

• Not knowing your demand profile. Pull 12 months of 15-minute interval data from utility before designing.
• Sizing battery too small. 100 kWh battery sounds big, but only 25 kW for 4 hours of dispatch — less than the demand spike.
• Not using smart battery dispatch. AI-driven battery management (Stem AI, Tesla Megapack) optimizes timing — key to maximum savings.
• Underestimating ratchet impact. Many utilities have demand "ratchet" rules where a single bad month locks in higher demand for 11 months. One outage can cost the year.
• Ignoring TOU windows. Demand charges can shift by hour. Battery dispatch must respect TOU rules.
• Not utilizing capacity tariffs. Some markets (PJM, NYISO) pay capacity rates for verified peak shaving.

Demand response programs

Some utilities pay you to reduce demand on their command:

• PJM RTO: capacity revenue + emergency response.
• NYISO: Special Case Resources.
• CAISO: demand response programs.
• ERCOT: Emergency Response Service.

Stacking demand response revenue on top of solar+battery can add 10-25% to annual savings.

How to evaluate commercial rooftop solar

Step 1: Pull 12 months of interval data

• Most utilities provide 15-minute or 30-minute interval data on request.
• Identify peak hours, peak magnitude, demand ratchet, TOU structure.

Step 2: Get qualified commercial bid

• Use installers experienced with commercial — not residential generalists.
• Ask for hourly production simulation (HelioScope, Aurora) overlaid with your demand data.
• Estimate energy and demand charge reductions separately.

Step 3: Confirm your CPA can use the credits

• S-corp / LLC pass-through: members can use credits at owner level.
• C-corp: corporate-level credit.
• Tax-exempt: use Direct Pay election or PPA structure.

Step 4: Verify roof structure + remaining life

• Roof load capacity: 4-7 lb/sq ft for ballasted; 3-5 lb/sq ft for penetration mount.
• Re-roof if < 10 years remaining; you don't want detach-and-reset on commercial.

Step 5: Get utility interconnection pre-approval

• Capacity at the substation/feeder.
• Backfeed limits.
• Review timeline (90-180 days typical).

Frequently asked questions