Inputs
Example data table
Sample scenarios to sanity-check inputs. Values are illustrative.
| Scenario | Fixtures | W/fixture | Dim level | Hours/day | Rate | Estimated Year 1 savings |
|---|---|---|---|---|---|---|
| Retail aisle | 60 | 35 | 70% | 12 | $0.18 | $730–$980 |
| Office open area | 120 | 28 | 65% | 10 | $0.14 | $820–$1,050 |
| Warehouse | 80 | 150 | 60% | 14 | $0.12 | $2,200–$3,000 |
| Parking | 40 | 90 | 50% | 11 | $0.20 | $1,000–$1,450 |
| Hospital corridor | 90 | 32 | 75% | 24 | $0.16 | $900–$1,250 |
Formula used
- Annual hours: Hours/day × Days/year
- Effective power (kW): Fixtures × (W/fixture) × (Level/100)e ÷ 1000
- Annual energy (kWh): Effective power × Annual hours
- Energy savings ($): (Baseline kWh − Dimmed kWh) × Rate
- Demand savings ($): (Baseline kW − Dimmed kW) × Coincident factor × Demand charge × 12
- Total Year 1 savings: Energy savings + Demand savings + Maintenance savings
- Simple payback: Control cost ÷ Total Year 1 savings
- NPV: −Cost + Σ [Savingst ÷ (1+discount)t], with savings escalated yearly
The exponent e models nonlinearity between dim level and power draw. Choose “linear” for a conservative, quick estimate.
How to use this calculator
- Enter the number of fixtures and their typical wattage.
- Set the baseline level (often 100%) and your expected average dim level.
- Add operating hours and days to match actual schedules.
- Input your electricity rate; include demand charges if billed.
- Add any annual maintenance savings and the one-time control cost.
- Choose analysis years, escalation, and discount rate for financial metrics.
- Press “Calculate savings” to see results above the form.
- Use CSV/PDF buttons after a calculation to export results.
Notes
- For safety-critical areas, confirm minimum illumination before dimming.
- For mixed-use spaces, use a weighted average dim level.
- Demand savings depend on how lighting contributes during peak periods.
- Measure real wattage if drivers behave differently when dimmed.
Operational profile and baseline assumptions
Start with a clear operating profile: fixture count, wattage, and annual hours. Many facilities run 3,000 to 6,000 hours yearly. At $0.12 to $0.20 per kWh, small percentage reductions deliver meaningful savings. Baseline output is often 100%, but daylit or low occupancy spaces may already average 80% to 90% across seasons.
Modeling dimming behavior and nonlinearity
Power does not always fall in perfect proportion to light level. Drivers and control gear can create nonlinear behavior, so the calculator offers curve choices. A moderate curve reflects common dimming electronics, while an aggressive curve can represent high efficiency at low levels. When possible, validate with short interval meter data and adjust the curve for your fixtures.
Financial outputs that matter to decision makers
Year 1 savings combine energy savings, optional demand savings, and maintenance savings. The model escalates savings annually and discounts cashflows to estimate net present value. Simple payback communicates speed, while NPV and IRR support capital planning. For portfolios, compare projects by NPV per dollar invested and implementation effort, not only payback.
Demand charges, coincidence, and peak strategy
If your tariff includes demand charges, reducing coincident kW can be as valuable as cutting kWh. Coincidence captures the portion of lighting reduction that occurs during system peak. Typical coincidence factors range from 0.3 to 0.8 depending on schedule and climate. Pair dimming with scheduling and occupancy control to improve peak alignment.
Reporting, governance, and next-step actions
Use the charts to review baseline versus dimmed energy, then track projected savings over time. Export CSV for audit trails and budgeting, and export PDF for approvals. Document assumed dim level, hours, and control cost, then re-run sensitivity cases. Next steps include a pilot, verification measurements, and updating assumptions before full rollout. For governance, record baseline measurements, control setpoints, and commissioning dates. Assign an owner for ongoing tuning, and review savings quarterly against utility bills. Where rebates apply, capture documentation early to protect incentive timelines and maintain tenant comfort targets.
FAQs
Does dimming always reduce wattage by the same percent?
No. Some drivers reduce power nearly linearly, while others are nonlinear at lower levels. Use the dimming curve option to better reflect real behavior, and validate with measured wattage or short-interval metering.
How should I pick a dim level for the calculation?
Use the average output level across the hours you entered. If levels vary, estimate a weighted average based on time at each level, or run multiple scenarios and compare savings ranges.
When do demand charges matter in this calculator?
If your bill includes a $/kW-month charge, enter it and set a coincidence factor. Demand savings depend on whether lighting reduction occurs during your billing peak period, not just total yearly energy.
What should I include in control cost?
Include hardware, installation labor, commissioning, networking, and any required sensors. If you expect periodic software fees, add them as a negative “maintenance savings” value by reducing the annual savings accordingly.
Why can NPV be positive while payback looks long?
Payback focuses on early cashflow, while NPV values savings across the full analysis life and discount rate. Long-lived savings with escalation can produce strong NPV even when Year 1 savings are modest.
Can I use this for LED retrofits with controls?
Yes. Set the wattage to the post-retrofit fixture draw, and treat dimming level as the controlled operating level. For combined retrofit and controls, include total installed cost and compare against the appropriate baseline.