Grow Light Wattage to Cost Converter

What each input means

State selects the default commercial electricity rate from EIA Table 5.6.A [eia-epm-5-6-a]. The rate is a state-wide average, not your contract price.

Number of fixtures is the count of luminaires on the same schedule. If veg and flower rooms run different hours, run the tool twice and add the results.

Watts per fixture is input power at the meter. For double-ended HPS, include ballast losses: operators commonly budget 650 W for a retrofit LED replacing a fixture that actually draws 1,000 to 1,100 W. The DesignLights Consortium horticultural list sets minimum efficacy thresholds for qualified fixtures [dlc-hort-v3]; lower wattage at similar canopy light levels is the main efficiency gain.

Hours on per day follows the photoperiod. Flower is often 12 hours; veg 18. Greenhouse supplemental lighting may run fewer hours seasonally.

Your rate (optional) replaces the state average with your all-in cents per kWh when you know it from a bill or supply contract.

The math step by step

Step	Formula	Example (100 fixtures × 650 W, 12 h/day, 14.53 cents/kWh)
Lighting load	kW = (fixtures × watts) ÷ 1,000	(100 × 650) ÷ 1,000 = 65 kW
Daily energy	kWh/day = kW × hours	65 × 12 = 780 kWh/day
Monthly energy	kWh/month = kWh/day × 30.4	780 × 30.4 = 23,712 kWh/month
Monthly lighting cost	dollars = kWh/month × (cents/kWh ÷ 100)	23,712 × 0.1453 = $3,445/month

Annual lighting cost is roughly the monthly figure times 12. Seasonal changes in photoperiod or room count are not modeled; adjust fixture count or hours when rooms flip from veg to flower.

How this fits a full facility bill

Lighting was 66 percent of total electricity in the Northwest Power and Conservation Council survey of licensed producers [nwpcc-cannabis]. Cooling, dehumidification, and fans made up most of the rest. So a $3,445/month lighting line in the example above might sit inside a total facility energy bill of roughly $5,200/month at the same rate, before any demand charge.

Demand charges are based on peak kilowatts, not kilowatt-hours. One hundred 650 W fixtures create a 65 kW lighting block when they all start together. HVAC starting in the same interval pushes the billed peak higher. The demand charge estimator handles that side.

For a technology comparison, see LED vs. HPS energy cost. For project payback including rebates, use the LED retrofit ROI calculator.

Assumptions and limits

• All fixtures share one wattage and one schedule. Mixed rooms need separate runs.
• Wattage is constant for every hour the lights are on. No dimming, sunrise/sunset ramps, or partial fixture outages.
• The rate is flat across all hours. Time-of-use tariffs are not applied.
• Monthly days are fixed at 30.4, not calendar months.
• No demand charges, customer fees, taxes, or power-factor penalties appear in the output.

This tool answers one question well: "What do these lamps cost in kWh dollars?" It does not replace a commercial energy audit or interval-data review when you are negotiating a supply contract or rate-class change.

Fixture count and layout examples

Single flower room, 100 fixtures at 650 W, 12 hours. The default math in the table above applies: 65 kW, 780 kWh per day, about 23,712 kWh per month.

Two rooms, same schedule, 80 fixtures each at 600 W, 12 hours. Run once with 160 fixtures or twice and add. Total 96 kW lighting load, 1,152 kWh per day.

Veg at 18 hours, flower at 12, different counts. Example: 60 veg fixtures at 700 W for 18 hours plus 100 flower at 650 W for 12 hours. Veg: 42 kW, 756 kWh/day. Flower: 65 kW, 780 kWh/day. Combined daily kWh 1,536; monthly about 46,694 kWh at 30.4-day factor.

Rolling harvest with partial rooms dark. If only 70 percent of fixtures run on any given day, reduce fixture count proportionally for a rough average, or model each phase separately.

HPS to LED comparison with this tool

Run two passes with the same fixture count:

Technology	Watts per fixture	100 fixtures, 12 h, 14.53 cents
DE HPS	1,050	~$5,570/month lighting
Top-bar LED	650	~$3,445/month lighting

Difference is about $2,125/month in lighting kWh dollars only. HVAC savings are excluded here; the LED retrofit ROI calculator adds a cooling factor. Demand savings from lower simultaneous kW are also excluded; use the demand charge estimator after you estimate reduced peak.

DesignLights Consortium qualified products must meet minimum efficacy to list [dlc-hort-v3]. Lower wattage at target PPFD is the efficiency gain, not a license to under-light.

Time-of-use and real rates

EIA state averages hide time-of-use spreads. If your tariff charges 20 cents on-peak and 8 cents off-peak, flat-rate math overstates cost when lights run off-peak and understates when they align with on-peak windows.

Cannabis lighting often runs nights to shave on-peak overlap in TOU territories. Enter your weighted average cents per kWh from the bill total divided by kWh rather than the headline on-peak rate.

Contract supply rates may be fixed while delivery stays TOU. Split the bill and enter supply-plus-delivery blended rate for all-in comparison.

What lighting cost ignores

Every lighting kWh becomes heat indoors. Rule-of-thumb adds 25 to 40 percent HVAC kWh per lighting kWh saved in sealed rooms [nwpcc-cannabis]. Dehumidification load may shift when sensible heat drops; see dehumidification load indoor cannabis.

Auxiliary loads tied to photoperiod include exhaust fans, circulation fans, and irrigation pumps. Add them manually to monthly kWh if they are significant.

Demand charges apply to peak kW when all fixtures energize together. Staggered room starts lower peak below the kW sum from this tool.

Using lighting cost in procurement and rebates

Suppliers bid on total meter kWh, not lighting alone. Use this tool to explain to finance how much of the meter is lamps before you approve an LED capex or a supply contract.

Utility rebates often require DLC-listed fixtures and pre-approval [dlc-hort-v3]. FAQ on LED retrofit and rebate questions covers timing.

For full-facility estimates including HVAC placeholder, switch to the cannabis facility energy cost calculator. For industry intensity context, read kWh per square foot benchmarks.

Dimming, spectrum shifts, and control systems

Modern LED controllers dim fixtures or run sunrise/sunset ramps. This tool assumes constant wattage for every hour lights are marked on. Average wattage over the photoperiod is a better input than nameplate when dimming is aggressive.

Spectrum changes without wattage change do not affect this calculator. They may affect yield and HVAC load indirectly.

If controllers cut wattage 20 percent during last two hours of photoperiod, reduce effective watts by weighted average: example 650 W nominal with 20 percent reduction for 2 of 12 hours equals effective 642 W approximated as 640 W for screening.

Interpreting results for cultivators and CFOs together

Cultivation leads care about PPFD and DLI; finance leads care about monthly dollars. Translate tool output into both: show monthly lighting dollars and kWh per square foot by dividing monthly kWh by canopy area.

Example: 23,712 kWh/month on 8,000 sq ft canopy is 2.96 kWh/sq ft/month, about 36 kWh/sq ft/year if extrapolated linearly (seasonality makes linear extrapolation rough).

Use the annualized figure against facility type energy benchmark lookup bands for a quantity check independent of rate.

Common operator errors

Using diode wattage instead of input wattage understates cost. Using HPS nameplate without ballast overstates LED savings in comparisons. Forgetting removed HPS ballast heat when sizing HVAC after retrofit overstates cooling savings in other tools.

Count only fixtures on the same meter you pay. Tenant grow rooms on landlord meters need landlord bill data.

When in doubt, clamp a few fixtures and read the panel or use a meter for one hour at steady state, then scale.

Greenhouse and supplemental photoperiod notes

Greenhouse operators often run supplemental LEDs only in winter months. Annualize by weighting months: if supplemental runs five months at 12 hours and seven months at zero supplemental, average hours across the year rather than using peak winter hours alone.

High-pressure sodium supplemental in greenhouses uses the same math as indoor; enter measured wattage including ballast. Light-emitting diode supplemental typically runs lower wattage per fixture at similar micromole targets [dlc-hort-v3].

Security and egress lighting

Hallway and security lighting outside flower rooms adds kWh but usually does not add to peak kW if loads are small relative to cultivation. Include security kWh in manual adjustments when it shares the cultivation meter.

Emergency lighting on backup circuits may not run every day; exclude from daily hours unless it materially affects monthly totals.

Linking lighting cost to procurement RFPs

When suppliers request load breakdowns, attach lighting kWh from this tool plus a separate line for HVAC from engineering estimates. Transparent breakdowns reduce bandwidth disputes later.

If lighting cost alone exceeds budget, compare LED vs. HPS before renegotiating supply. Sometimes rate work and lamp work together beat either alone.

Training new facility managers: have them rerun this tool when fixture counts change after harvest flip so finance sees updated lighting exposure before the next bill closes.