How to Enter a Time of Use Tariff in a Storage EMS

Why does the EMS need the tariff at all?

Any algorithm that decides when to charge or discharge, time of use arbitrage, demand charge reduction, self consumption with an export credit, reasons about prices. The same numbers value the energy the site imports, exports, and shifts, so the savings figures reported back to the customer are only as good as this page. The Financials page holds the tariff on a rate schedule tab and reports what it was worth on a savings dashboard tab.

Two switches in the general settings deserve care. Demand charges enabled switches on billing against the billing period peak; on a commercial tariff where the demand charge is a large share of the bill, leaving it off makes demand charge reduction look worthless in reporting. Nonbypassable charges enabled adds a per kWh charge on imported energy regardless of what the battery does. Utilities levy these to recover fixed costs, and they matter to dispatch because they narrow the spread between cheap and expensive hours; a battery cannot arbitrage a charge it cannot avoid.

How are seasons, day groups, and periods structured?

Most commercial tariffs price the same kilowatt hour differently in summer and winter. A season is a set of months that share one price structure, with a maximum demand charge that applies across the season. A season that wraps the turn of the year needs two month ranges on one season: winter at the demonstration site is January to May and October to December.

Within a season, a day group is a set of weekdays that share one daily price shape. Most tariffs need two, weekdays and weekends, but a tariff with a distinct Saturday schedule needs three. Within a day group, a rate period is a named price band such as on peak, mid peak, or off peak, with one or more hour windows and its own energy and demand rates. A period that occurs twice a day, off peak from midnight to the morning shoulder and again from evening to midnight, is one period with two windows and one pair of rates.

• Windows are entered on whole hours, start times from 00:00 to 23:00 and end times from 01:00 to 24:00. A tariff whose periods change on the half hour is approximated to the nearest hour, and the approximation is noted in the project documents so a later discrepancy against the bill is not mistaken for a fault.
• Everything remains a draft until Save Changes, and Discard abandons the whole draft. Changing the tariff changes how stored energy is valued in reporting from that point on, so a mid year rate change is dated in the project documents to explain the step in reported savings.

Worked example: a two season commercial tariff

This is the tariff configured at the Alpine Ridge Energy Center demonstration site, a summer and winter commercial rate with three weekday periods in summer and two in winter. The general settings are an energy charge of 0.1178 dollars per kWh, a demand charge of 22.5 dollars per kW, a sell back price of 0.0435 dollars per kWh, currency USD, demand charges on, and nonbypassable charges off.

The Summer season runs June to September with a maximum demand charge of 22.5 dollars per kW and two day groups. The Winter season runs January to May and October to December, entered as two ranges, with a maximum demand charge of 16.8 dollars per kW and two day groups.

Winter, and the checks before saving

The winter shape is different because the winter peak sits in the early evening rather than the afternoon, which is why the two seasons are not copies of each other. Confirm the on peak window matches the site’s real demand peak; the tariff describes when the utility charges most, and the dispatch algorithm will move energy to meet that window.

Before saving, confirm that the twelve months are covered exactly once, June to September by Summer and the rest by Winter, and that each day group’s color bar is continuous from midnight to midnight. A month covered by no season has no time of use prices, and a month covered by two is ambiguous.

What does the tariff mean for the battery?

The spread between off peak and on peak is what a storage algorithm arbitrages. In summer at the demonstration site that spread is 0.0712 to 0.2145 dollars per kWh, roughly 0.14 dollars per kWh, against a round trip efficiency of about 92 percent from the PCS charging and discharging efficiencies of 0.96 each. Every kWh charged off peak and discharged on peak earns the spread less the efficiency loss, and every kWh of the on peak demand ceiling that the battery holds down avoids the 28.4 dollars per kW demand rate for the month.

The savings dashboard tab reports what the tariff was worth over any month the site has stored data for. A newly commissioned site has none until it has run for a while, and the tab says so rather than reporting zero.