- Turn nameplate or metered watts and a timer schedule into kWh for a month.
- Identify which device in each method runs continuously, which cycles, and which swings with season.
- Price a kWh from a bill and see where timers, insulation and pump sizing cut use without cutting light, oxygen or flow.
Energy cost calculator
kWh = watts × hours/day × days ÷ 1,000. Cost = kWh × your tariff.
Energy (kWh): 43.2; Energy cost (your currency): 6.48
One device at constant power. Add other devices, duty cycles and fixed charges separately. Price is an illustrative input, not a market quote.
01 / One equation and where its inputs come from
kWh = watts × hours per day × days ÷ 1,000. The operating-costs chapter sets that line inside a full cost ledger; this guide takes electricity alone, device by device. Virginia Cooperative Extension BSE-301NP prints the same method: wattage × hours used per day ÷ 1,000 for daily kWh, times the days used, times your utility’s rate per kWh. The calculator on this page runs one device at constant power; add devices one schedule at a time.
BSE-301NP says the nameplate wattage is the power the appliance draws, and that where only amperes are printed you multiply by the supply voltage (120 V for most United States appliances, 240 V for large ones). Hours come from the timer or switch; days from the month or the crop cycle.
02 / What draws power in each method
In DWC the load is an air pump: Oregon State EM 9455 states that DWC requires effective aeration, through air pumps and air stones or ozone injection; running the pump round the clock is general practice, since nothing in the method switches it off. In NFT it is the water pump; Oklahoma State HLA-6442 notes constant flow with no timer required, and UKY CCD-SP-20 describes a continuously running pump. Ebb and flow is timer-cycled: EM 9458 lists a timer controlling the pump that floods the trays, and CCD-SP-20 places drip-fed Dutch buckets under flood and drain, whose soak time and frequency it calls variables a grower adjusts with crop size and growing conditions.
Around the reservoir sit a light on a timer, a fan, and in season a thermostatic heater or chiller. UConn’s Horizontal Air Flow Systems fact sheet gives the scale: 1/10 to 1/15 horsepower circulation fans draw about 100 watts, and a 100 W fan running 24 hours a day uses 2.4 kWh (printed as kilowatts).
| Device | Runs | Hours from |
|---|---|---|
| Air pump (DWC) | Continuously | 24 h (EM 9455 requires aeration; round-the-clock running is general practice) |
| Water pump (NFT) | Continuously | 24 h (HLA-6442, CCD-SP-20) |
| Water pump (ebb and flow, drip) | Timer cycles | Minutes per cycle × cycles per day |
| Grow light | Timer | Photoperiod (UMN: 12–14 h for hydroponic lettuce and herbs) |
| Fan | Switch or controller | Hours on |
| Heater or chiller | Thermostat | Unknown until metered; changes with season |
Reading: Hydro hints: Deep water culture ↗ · Hydroponics (HLA-6442) ↗ · Irrigation in Hydroponic Systems: An Illustrated Overview (CCD-SP-20) ↗ · Hydro hints: Ebb and flow ↗ · Horizontal Air Flow Systems (fact sheet, J. W. Bartok, Jr., updated 2013; no publication number shown) ↗ · Lighting for indoor plants and starting seeds ↗
03 / Rated watts, measured watts and duty cycle
The nameplate is an upper bound for a steady load and a poor guide to a cycling one. BSE-301NP estimates a refrigerator’s hours at maximum wattage as one third of the time plugged in, because it cycles on and off to hold temperature; a thermostatic reservoir heater or chiller behaves the same way, with a fraction unknown until measured.
General practice, not from the cited documents: a plug-in energy meter between device and outlet accumulates kWh. Leave each device on it for a representative week, including the coldest night for a heater, and divide by the days; that daily kWh replaces the nameplate. Timed loads are arithmetic: a pump on 15 minutes in every hour runs 25% of 24 h, 6 h.
04 / A light’s watts are its input, not its output
UMN’s indoor-lighting page says watts measure the energy needed to produce light, not its intensity. Kusuma, Pattison and Bugbee’s 2020 review gives the ratio as efficacy in µmol per joule: 1.72 for a 1,000 W double-ended HPS fixture against 2.78–3.0 for the highest-efficacy LED fixtures it tabulated. Buy by the daily light integral your crop needs (the grow-light chapter); hours are UMN’s 12–14 a day for hydroponic lettuce and herbs.
UMN’s small-scale hydroponics page works an example: a 9-watt LED (PPF 16 micromoles per second) for one or two 5-gallon buckets, 14 hours a day all year, is 5,110 hours and, in its words, approximately 45,990 watts a year (watt-hours; about 46 kWh); at 12 cents per kilowatt-hour, which it calls a common price in 2020, about 45 cents a month or $5.44 a year. A dated illustration, not a current price; by the equation the same light is 9 × 14 × 30 ÷ 1,000 = 3.78 kWh a month.
Reading: Lighting for indoor plants and starting seeds ↗ · From physics to fixtures to food: current and potential LED efficacy (Horticulture Research 7:56, 2020) ↗ · Small-scale hydroponics ↗
05 / Heating, cooling and the tariff
Climate control is the swing item. University of Alaska Fairbanks HGA-00336 states that nationally heating constitutes 65 to 85 percent of annual energy cost for a year-round commercial greenhouse. BSE-301NP lists a portable heater at 750–1,500 W; at a modest duty cycle that outruns every pump in the room, so meter a winter week and a summer week before calling a figure annual.
A tariff has a fixed charge that does not move with use, an energy charge per kWh, and sometimes tiers or time periods. The U.S. Energy Information Administration says some utilities offer time-of-day pricing to reduce peak demand, that prices are usually highest in summer, and that residential customers usually pay the highest retail prices. HGA-00336’s shortcut: divide a recent bill’s total by its kWh; its example is $130 for 650 kWh, 20 cents per kWh. Cutting use without touching the crop: a timer at the published photoperiod (UMN); fans off when idle, as UConn’s power relay turns HAF fans off when exhaust fans run or vents open; an insulated reservoir so a heater cycles less (general practice); a pump chosen for measured head, as the submersible pump chapter sets out; light bought by efficacy.
Reading: Controlling the Greenhouse Environment (HGA-00336, reviewed October 2019) ↗ · Energy Series: Estimating Appliance and Home Electronic Energy Use (publication 2901-9014, BSE-301NP, 2020) ↗ · Electricity explained: Prices and factors affecting prices (web page, accessed 17 September 2026) ↗ · Lighting for indoor plants and starting seeds ↗ · Horizontal Air Flow Systems (fact sheet, J. W. Bartok, Jr., updated 2013; no publication number shown) ↗
06 / A worked month: one DWC unit and one NFT unit
Both months use stated assumptions, not typical values; put your own metered figures in the calculator on this page. DWC bucket: air pump metered at 3 W for 24 h, 30 W LED for 14 h, 30 days. Air: 3 × 24 × 30 ÷ 1,000 = 2.16 kWh. Light: 12.6 kWh. Month: 14.76 kWh; at an illustrative 0.15 currency units per kWh, 2.21 units. The light is 85% of it.
Four-channel NFT: pump metered at 18 W for 24 h, 120 W LED for 14 h, 20 W fan on with the light. Pump: 18 × 24 × 30 ÷ 1,000 = 12.96 kWh. Light: 50.4 kWh. Fan: 8.4 kWh. Month: 71.76 kWh, 10.76 units. The continuous pump is 18% of the month; the light is 70%.
- Read each nameplate; where only amperes are printed, multiply by supply voltage to get watts.
- Put every thermostatic or cycling device on a plug-in meter for a week that includes its worst night; divide by 7 for daily kWh.
- For timed pumps write hours as minutes per cycle × cycles per day ÷ 60.
FIELD QUESTIONIn winter you add a 100 W reservoir heater to the NFT unit above. A plug-in meter across one week reads 6.72 kWh. What is its duty cycle, its 30-day kWh, and its rank among the unit’s loads?
Continuous running would be 100 × 24 × 7 ÷ 1,000 = 16.8 kWh; 6.72 ÷ 16.8 = 0.4, a 40% duty cycle. Month: 6.72 ÷ 7 × 30 = 28.8 kWh. Second behind the 50.4 kWh light, ahead of the 12.96 kWh pump; the month rises from 71.76 to 100.56 kWh, a 40% increase from one seasonal device. Insulate the reservoir, meter another week and compare.
Parts & buying criteria
Build your own parts & cost worksheet →
Showing aerated deep water culture. Quantities describe the teaching model. Specify real working volume, support, fittings and instruments for your installation.
01Nutrient reservoir1 · illustrated quantity+
Stores the measured nutrient solution beneath or beside the growing area.
Inspect: Compare the surface with the diffuser and the lower roots; freeboard is deliberate. There is no pump intake in this single-vessel example.
Maintain: Record level before refilling, keep light out and verify temperature, EC and pH.
Opaque, cleanable, intended-use container with volume marks and service access.
Check size, materials and operating conditions with your chosen supplier before ordering.02Lid & root support1 · illustrated quantity+
Holds the plant above the chamber or reservoir without pinching its crown.
Inspect: Separate the cover and inspect the basket or collar opening.
Maintain: Check fit as plants grow and keep unused openings covered.
Rigid compatible support with removable, correctly fitted baskets or collars.
Check size, materials and operating conditions with your chosen supplier before ordering.03Air pump & diffuser1 · illustrated quantity+
Delivers air below the water surface in this aerated DWC example.
Inspect: Trace the thin line from the dry pump to the diffuser. Bubbles are illustrative.
Maintain: Check actual delivery and dissolved oxygen; preserve backup air and prevent back-siphoning.
Rated output at the installed depth and diffuser resistance; accessible replaceable parts. Power it through ground-fault protection (GFCI/RCD) and keep the pump and its plug above the water line.
Check size, materials and operating conditions with your chosen supplier before ordering.Evidence beside the lesson.
Source checks: September 2026. The geometry, inspection exercises and worksheets are original teaching material. The named organizations have not endorsed or reviewed this site.
Published recipe rates and planning ranges retain their source context. Calculator equations are accounting tools; they do not predict uptake, yield, toxicity or safe stocking. Model dimensions, water speeds, roots and fish counts are illustrative. Verify species, crop, source water and product labels for a real system.
- Virginia Cooperative Extension, Virginia TechEnergy Series: Estimating Appliance and Home Electronic Energy Use (publication 2901-9014, BSE-301NP, 2020) ↗
The formula wattage × hours used per day ÷ 1,000 for daily kWh, multiplied by days used for the period and by the utility’s rate per kWh for cost; the nameplate wattage as the power drawn; estimating watts from amperes × voltage (120 V for most United States appliances, 240 V for large ones); a refrigerator cycling on and off so that hours at maximum wattage are estimated as one third of the time plugged in; and the nameplate wattage list including portable heater 750–1,500 W.
- Oregon State University ExtensionHydro hints: Deep water culture ↗
Aerated water culture layout and operation.
- Oklahoma State University ExtensionHydroponics (HLA-6442) ↗
System categories and recirculating nutrient delivery.
- University of Kentucky Center for Crop DiversificationIrrigation in Hydroponic Systems: An Illustrated Overview (CCD-SP-20) ↗
Illustrated overview of DWC, NFT and flood-and-drain, including drip-fed individual containers (bags or buckets) draining to a shared gutter; cycle frequency and soak time adjusted with crop size and conditions.
- Oregon State University ExtensionHydro hints: Ebb and flow ↗
Flood-and-drain operation; irrigation depends on media and plants.
- University of Connecticut Integrated Pest Management Program, College of Agriculture, Health and Natural ResourcesHorizontal Air Flow Systems (fact sheet, J. W. Bartok, Jr., updated 2013; no publication number shown) ↗
Circulation fans of 1/10 to 1/15 horsepower drawing about 100 watts; the worked example of a 100 W fan running 24 hours a day using 2.4 kilowatts (kilowatt-hours) a day and costing 36 cents a day at 15 cents per kWh; a power relay turning the HAF system off when exhaust fans run or vents open; and continuous HAF operation in fall, winter and spring.
- University of Minnesota ExtensionLighting for indoor plants and starting seeds ↗
Use of a timer; 12–14 hours per day for hydroponic lettuce and herbs and 16–18 for seedlings; 6–12 inches between light and hydroponic lettuce and herbs and 4–6 inches for seedlings; watts measuring the energy needed to produce light rather than intensity and lumens measuring brightness to the human eye.
- Kusuma, Pattison and Bugbee · Utah State University and Solid State Lighting Services · open access via PubMed CentralFrom physics to fixtures to food: current and potential LED efficacy (Horticulture Research 7:56, 2020) ↗
Efficacy defined as micromoles of photon output per second per watt of input power, simplifying to µmol per joule; the 1,000 W double-ended HPS figure of 1.72 µmol/J; tabulated 2020 fixture efficacies of 3.0 (blue plus red) and 2.78 (white plus red) µmol/J; calculated limits of 4.1 and 3.4 µmol/J; different LED colors having different efficacies; efficacy falling about 10 percent from 25 to 85 °C; and the statement that timing, angular delivery, spectrum and intensity also determine effectiveness.
- University of Minnesota ExtensionSmall-scale hydroponics ↗
Root support, transplanting, light, aeration and cleaning.
- University of Alaska Fairbanks Cooperative Extension ServiceControlling the Greenhouse Environment (HGA-00336, reviewed October 2019) ↗
The statement, citing Runkle and Both (2011), that nationally heating constitutes 65 to 85 percent of annual energy cost for a year-round commercial greenhouse; the method of dividing a recent electric bill’s total by the kWh used to find the average cost per kWh, with the example $130 for 650 kWh giving 20 cents per kWh; and the supplemental-lighting operating-cost example of 1.32 kW for 12 hours at $0.20 per kWh.
- U.S. Energy Information AdministrationElectricity explained: Prices and factors affecting prices (web page, accessed 17 September 2026) ↗
The statements that some utilities offer time-of-day pricing to encourage electricity conservation and reduce peak demand; that prices are usually highest in summer when total demand is high; and that retail electricity prices are usually highest for residential and commercial consumers because it costs more to distribute electricity to them. No price figure from the page is quoted in the guide.
The next useful connections.
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A boxed countertop garden and a drilled bucket meet the same needs; they differ in who chooses the light, the consumables, the meter and the spares.
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Interactive tool · 8 min + guided practice →