Working out what lighting costs a vertical farm over a winter on an intelligent electricity tariff

To know what lighting costs a vertical farm over a winter on an intelligent electricity tariff, price every lit period at the rate actually published for it, then add them up. A typical day built from a few sample days can miss the season’s price pattern, and over a winter those misses add up. This post sets out the method step by step, so you can run it on your own room with your own tariff, and says what the result leaves out.

Fix the light before the clock

Start from the crop’s daily light integral (DLI) and the average PPFD across the canopy, not one reading under a single fixture. The hours of light follow from the formula in our earlier post on DLI and tariffs, and the room’s lighting power multiplied by those hours gives the energy needed each day. With a fixed target, steady output and constant power, that daily energy stays the same through the season. What changes is the price of each hour.

Price every period, not a typical day

Some intelligent tariffs publish a separate price for each short period of the day, and those prices can move a great deal from one day to the next. Others use fixed price bands. Either way, for a season take every published rate in the window, price each lit period at its own rate, and add them up.

Two details are easy to miss. If the window includes the night the clocks change, that day is an hour longer or shorter, and a schedule that runs across the changed hour lights for an hour more or less. Each rate has to land on the clock time it really applied to. And where published rates are missing, say how much of the lit time you could price rather than filling the gap with an average, because a filled gap is a guess presented as a price.

Hold one start time for the season

With the hours fixed, try every start time the tariff’s price periods allow and price each one over the whole window. The useful answer is the cheapest single start time held for the full season. Choosing a new start each day looks cheaper on paper, but it moves the crop’s dark period every night, and each crop’s tolerance for that needs checking first. Set the cheapest start beside the one the room uses today, and you have the honest size of the choice.

When comparing start times on the same tariff, keep the standing charge separate: moving the lights changes what you pay per unit, not the fixed daily charge. When comparing tariffs, include it, because it differs between them.

What the result leaves out

• Heat. Lights warm the room, so moving them from night to day shifts when that heat arrives, and the heating and cooling bills can move either way. A lighting figure on its own cannot show the net effect.
• Your contract. A published tariff rate is not a negotiated commercial supply agreement.
• The crop and the team. The cheapest start still has to suit the plants and the people who work in the room.
• Metered reality. A calculation from rated power is not a meter reading, so measure during a trial before counting any difference as a saving.

Running it in BeeGrow AI

BeeGrow AI’s saved tariff comparisons apply this method to your own lighting options. They price each lit period against the rates published for the last 30 or 90 complete days, or for tomorrow once its rates are out, and include the standing charge in each total. For an option saved with an optimal start, they search every possible start and recommend the cheapest one only where the rate data is complete or very nearly so. Each result shows how much of the lit time could be priced. BeeGrow recommends a start time. It does not move your lights. More detail is on the energy and tariff optimisation page.