Grams per Watt Calculator & Grow ROI Estimator
See your grams-per-watt efficiency and what a harvest really costs.
Result
ROI here covers light electricity only and excludes other inputs. Use it as an efficiency benchmark, not a full financial projection.
History
What it is
This calculator measures grow-light efficiency and running cost. It returns grams per watt (a standard efficiency benchmark), the total kWh used over a cycle, the electricity cost, the energy cost per gram, and — if you enter a sale value — a basic ROI percentage for one grow cycle.
Who should use it
Growers comparing lights or setups, and anyone wanting to know what a harvest actually costs in electricity and whether the numbers make sense.
How to use it
- Enter your harvest yield in grams.
- Enter the light's power draw in watts (use real wattage, not the model number).
- Enter run hours per day and the length of the cycle in days.
- Enter your electricity price per kWh.
- Optionally enter a sale value per gram to get an ROI percentage.
- Read grams per watt, energy used, energy cost, cost per gram and ROI.
Example calculation
Worked example
A 400 W light run 12 hours a day for 70 days uses 336 kWh (0.4 kW × 12 × 70). At $0.16/kWh that is $53.76 of electricity. A 400 g harvest is 1.0 g/W and an energy cost of about $0.13 per gram. At a $5/g sale value, ROI on energy is roughly 3,600%.
Formulas used
g/W = yield / watts • kWh = watts/1000 · h/day · days • ROI% = (yield·sale − energyCost) / energyCost · 100
How to read your result
Grams per watt is the headline efficiency figure — 1.0 g/W is a solid result for many setups. Remember this ROI counts electricity only; it ignores nutrients, equipment, rent and labour, so treat it as a light-efficiency benchmark rather than a full business model.
Recommended ranges
| g/W | Rating |
|---|---|
| < 0.5 | Below average |
| 0.5 – 1.0 | Average |
| 1.0 – 1.5 | Good |
| 1.5 – 2.0 | Excellent |
| > 2.0 | Expert / commercial |
Common mistakes to avoid
- Using the marketing wattage instead of actual power draw at the wall.
- Counting only the light and forgetting fans, pumps and dehumidifiers in your real energy bill.
- Comparing g/W across very different light intensities without context.
- Treating the energy-only ROI as total profit.
Frequently asked questions
How many grams per watt is good?
Around 1.0 g/W is a good benchmark for many home setups; 1.5 g/W and above is excellent and usually reflects strong genetics, high light and a dialled-in environment.
How much does a grow light cost to run?
Multiply kilowatts (watts ÷ 1000) by hours per day and by days, then by your price per kWh. A 400 W light for 12 h over 70 days at $0.16 costs about $54.
What does grams per watt mean?
It is dry yield divided by the light's wattage — a simple measure of how efficiently your light converts electricity into harvest.
Does this include all my costs?
No. It covers light electricity (and ROI on that). Nutrients, equipment, ventilation power, rent and labour are not included.
Should I use real or rated wattage?
Always use real power draw measured at the wall. Many LED fixtures draw far less than the number in their name.