The Real Math Behind LED Retrofit Payback
Every variable that actually matters for payback calculation. How we model it on a real project, the common mistakes in spreadsheet templates, and why sub-3-year payback is realistic for commercial CRE.
Most LED retrofit spreadsheets you'll find online get the math wrong in at least one of three ways: they use bulb wattage instead of system wattage (understating energy savings), they ignore maintenance cost reduction (understating total savings), or they use a flat utility rate that doesn't exist for any real commercial customer (distorting payback). This guide walks through how we actually calculate payback on real projects, the variables that matter most, and why we can commit to sub-3-year payback targets with confidence on commercial CRE retrofits.
- Use system wattage, not bulb wattage — HID ballasts add 10-15%
- Maintenance savings are often larger than energy savings over 15 years
- Pull actual utility rate schedules from OpenEI URDB, not state averages
- Rebate capture can reduce project cost 10-50% in our Southeast service area
- Sub-3-year payback is achievable on almost every commercial exterior retrofit when priced correctly
The simple formula (and why it's usually wrong)
The formula most spreadsheets use is: Payback = Project Cost / Annual Savings. Simple enough. The problem is the inputs. Get any one of them wrong and the payback number is fiction. Here's the version we actually use on project quotes, with every input defined:
- Project Cost = Total installed cost (fixtures + labor + materials + permits + commissioning) MINUS rebates captured
- Annual Savings = Energy savings + Maintenance savings + Controls savings (if applicable)
- Energy Savings = (Existing kW - New kW) × Burn Hours × Utility Rate
- Maintenance Savings = (Existing $/fixture/year - New $/fixture/year) × Fixture Count
- Utility Rate = Time-of-use weighted average from actual billing data, NOT state average
Variable 1: Existing wattage (use system, not bulb)
The #1 mistake in retrofit math is using bulb wattage instead of system wattage. A 400W metal halide bulb is actually a 458W system including the ballast — that's 14.5% more energy than the label suggests. Over a 15-year hold period, using bulb wattage instead of system wattage understates total energy savings by tens of thousands of dollars on a typical commercial project. Always multiply bulb wattage by 1.10-1.15 to get system wattage.
Variable 2: Burn hours
Burn hours per year varies wildly by property type. A shopping center parking lot runs about 4,380 hours (12 hours per day, dusk to dawn). A 24/7 manufacturing plant runs 8,760 hours. A sports facility that only lights for games might run 500-1,000 hours. Getting this wrong changes payback by 2-4x. We pull actual utility billing data or photocell timer records whenever possible rather than guessing.
Variable 3: Utility rate (not what you think)
Most spreadsheets use a flat rate like $0.12/kWh. That's wrong for almost any commercial customer. Real commercial rates have demand charges ($/kW), time-of-use multipliers, and often facility-specific riders. For a rigorous calculation we pull the specific rate schedule from OpenEI URDB and model demand charge savings separately from energy charge savings. Controls that dim during peak demand periods have very different ROI from controls that just save kWh.
Variable 4: Maintenance cost reduction
This is the variable most spreadsheets skip entirely, and it's often the biggest one. A typical HID fixture costs ~$17.25/year in maintenance (labor + materials + truck rolls). LED drops that to ~$0.54/year. On a 100-fixture parking lot, that's $1,670/year in maintenance savings alone. Over a 15-year fixture life, that's $25,000 just from maintenance — often more than the energy savings.
- Bucket truck rental: $300-$700/day
- True cost of a truck roll: $250-$1,000 including labor
- Emergency after-hours rolls: 1.5-2x standard labor
- LED driver warranty: 5-10 years vs HID ballast 1-3 years
Variable 5: Rebate capture
Utility rebates can reduce project cost by 10-50% depending on the program. The big ones in our Southeast service area:
- TVA EnergyRight (AL, TN, MS, KY western): $0.13/kWh saved + 15% bonus
- Georgia Power Custom Savings: $0.10/kWh saved, $75K/building/year cap
- Duke Energy Smart $aver (NC, SC): $27-$264/fixture prescriptive, 75% project cost cap
- Entergy (MS, LA): Up to 80% project cost for small commercial
- Cleco Power Wise (central LA): Up to 100% for small commercial under 100kW
The 89-project benchmark
Across 89 real commercial retrofits in our Southeast portfolio, we see consistent per-fixture economics that let us commit to sub-3-year payback targets. The portfolio-wide averages:
- Annual savings per fixture: ~$139 (range $120-$160)
- Installed cost per fixture: ~$245 (range $220-$280)
- Annual kWh saved per fixture: ~1,069
- Average energy reduction vs legacy HID: 70%+
- Typical project gross margin: 25-40%
Payback-driven pricing
Most commercial LED vendors price cost-plus — they quote their cost plus a margin, and payback is whatever it ends up being. We do the opposite: we target a sub-3-year payback and price the project to hit it. The logic: Target Price = Annual Savings × 3.0. If the target price delivers at least 35% margin on our cost, we quote the target price. If the target price would leave margin below 35%, we floor the quote at 35% margin and show the actual (slightly longer) payback. At typical margins around 45%, sub-3-year payback is achievable on almost every commercial exterior retrofit we quote.
What kills payback assumptions
Three things that regularly break payback models in the field:
- Site access restrictions forcing weekend or night work — raises labor cost
- 480V sites requiring higher fixture costs — adds ~40% to per-fixture spend
- Pole-heavy exteriors (many high poles, few low fixtures) — drives cost up to $432/fix vs $178 mixed
- Missing rebates — a missed TVA application alone can move payback from 2.5 to 3.5 years