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A consumption rate ($/kWh) can be used to calculate a bill in different ways, depending on whether there is excess solar production and whether selling to the grid is allowed. Many utilities have designed complex meter structures to keep track of the purchases from grid and the sales to the grid. Feed-in tariffs, Net metering, Buy-all Sell-all are all different types of consumption rates and have different definitions around the world. To keep it simple, we use the following five rate transaction types to define how a consumption rate is used to calculate the bill. A tariff can have many consumption rates and each consumption rate can have only one rate transaction type. Here’s a list of the possible rate transaction types:

 

Transaction Type

How the bill is calculated

Net Meter

cost = Net kWh * $/kWh

Net Excess

IF Net kWh<0, the cost applies= Net kWh * $/kWh

Net Purchase

IF Net kWh>0, the cost applies= Net kWh * $/kWh

Import

For the sum of instantaneous energy (kWh) coming from grid,

apply cost= Purchases kWh * $kWh

Export

For the sum of instantaneous energy (kWh) sent to grid,

apply cost = (-) Sales kWh * $/kWh

 

These transaction types are further explained below:

Understanding Net Meter, Net Excess, and Net Purchase:

The diagram below shows the transaction between solar panel, house (customer) and utility using a single meter.

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The house has only one meter, which can “move forwards” when buying power from the grid and “move backwards” when selling excess to the grid.  

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For the first hour in the month of January, the load was 10 kW, and the solar panel produced 20 kW. In that hour, the load after self-consumption is -10 kW. Since it is a negative load, we could export energy to the grid. Therefore, in the first hour in January, the system sold 10 kW to the Grid. After the first hour in January, meter A now has “moved backwards” and has recorded -10.

In the next hour, the load after self-consumption is 10 kW, meaning the system needs to purchase 10 kW from the grid. Now the meter will move forward by ten and will read zero (-10 + 10 =0). This goes on until end of the month when the meter is read by the utility.

In this type of meter, only the monthly “net” reading affect the utility bill – not the instantaneous meter readings. If the total purchases at the end of the month have been greater than total sales, then the meter will have a positive value. If the total sales at the end of the month is greater than total purchases, then the meter will have a negative value.

To represent meters that record the “net” consumption over a period of time, we use the following three rate transaction types:

(1)Net Meter

A consumption rate with “NetMeter” will be calculated as = Net kWh * $/kWh.

In the above example, the Net kWh were calculated over the period of a month. Some utilities will have different charge periods. The charge period options for “NetMeter” rates are: Hourly, Daily, Monthly, and Annually.

(2)Net Purchase

A consumption rate with “NetPurchase” will be calculated ONLY IF Net kWh>0. The cost = Net kWh * $/kWh.

In the above example, the Net kWh were calculated over the period of a month. Some utilities will have different charge periods. The charge period options for “NetPurchase” rates are: Hourly and Monthly.

(3)Net Excess

A consumption rate with “NetExcess” will be calculated ONLY IF Net kWh<0. The cost = Net kWh * $/kWh

In the above example, the Net kWh were calculated over the period of a month. Some utilities will have different charge periods. The charge options for “NetExcess” rates are: Hourly and Monthly.

 

Understanding Import and Export

The diagram below shows the energy flows between a solar array, a house (customer), and the utility.

cr1

 

The system has two meters, each of which can only “move forwards”.  Meter A will record any electricity sold to the grid. Meter B will record any electricity purchased from the grid. Another way to look at this is that Meter A and B will record instantaneous exports and imports, respectively. There is no “netting” at the end of the month or charge period.

To represent Meter A, we have:

(4)Export

A consumption rate with “Export” will be calculated as = (-) Sum of instantaneous Sales kWh * $/kWh

To represent Meter B, we have:

(5)Import

A consumption rate with “Import” will be calculated as = Sum of instantaneous Purchases kWh * $/kWh

The estimated cost for these two rate transaction types is calculated on a monthly basis. Comparing Transaction Types:

Below is an example of the different calculations by rate type with a monthly charge:

 

 

 

$/kWh

0.5

0.2

0.5

0.5

0.2

Month

Energy Purchased

Energy Sold

Net Energy

Net Purchase

Net Excess

Net Meter

Import

Export

 

kWh

kWh

kWh

$

$

$

$

$

Jan

72,071

61,007

11,064

5,532

0

5,532

36,036

-12,201

Feb

59,490

57,856

1,634

817

0

817

29,745

-11,571

Mar

60,907

67,186

-6,278

0

-1,256

-3,139

30,454

-13,437

Apr

52,656

61,070

-8,414

0

-1,683

-4,207

26,328

-12,214

May

56,282

60,666

-4,384

0

-877

-2,192

28,141

-12,133

Jun

60,384

51,916

8,468

4,234

0

4,234

30,192

-10,383

Jul

62,993

52,762

10,231

5,116

0

5,116

31,497

-10,552

Aug

68,213

56,097

12,116

6,058

0

6,058

34,107

-11,219

Sep

62,066

63,651

-1,584

0

-317

-792

31,033

-12,730

Oct

61,313

64,410

-3,097

0

-619

-1,549

30,657

-12,882

Nov

71,771

52,437

19,334

9,667

0

9,667

35,886

-10,487

Dec

72,843

55,752

17,091

8,546

0

8,546

36,422

-11,150

 

 

 

 

 

 

 

 

 

 

The table shows that the estimated utility bill depends significantly on which transaction type is defined.

You can add multiple consumption or energy charges to your tariff. Click on the Add Rate button on the right corner under the consumption tab.

 

TB_Consumption_add_Rate

 

This will open a pop-up window as shown below.

 

tb3

 

 

 

Follow the steps below to add a consumption rate.

Step 1: Enter the name of the consumption rate.

tb4

Step 2: Select the rate type and the corresponding charge period.

 

tb5

 

As discussed in the beginning of this article the following charge periods are available for each rate type

Rate Type

Charge Periods

NetMeter

Hourly, Daily, Monthly and Annually

NetPurchase

Hourly and Monthly

NetExcess

Hourly and Monthly

Import

Monthly

Export

Monthly

 

Step 3: Specify the cost

 

a. Flat energy rate - If it is a constant energy rate ($/kWh) then enter the cost in box for Cost ($/kWh)

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b. Tiered energy rate- If it is a tiered energy rate, then check the box that asks “Has Tiers”. This displays a table for you to enter the tiers and their costs. Enter the Limits for each Tier and the costs in $/kWh. The table in the picture below, has the following tiers - from 0 to 50 kWh, the rate is 0.5 $ per kWh. Between 50 -100 kWh the rate is 1 $ per kWh. Any consumption above 100 kWh would be charges 2 $ per kWh.

So if a customer consumed 300 kWh in the billing period, then the cost = ( 50 kWh * 0.5 $/kWh) + (100 kWh *1 $/kWh) + (150 kWh * 2 $/kWh) = 425 $

 

Tip: Be sure to add a last tier with a really high upper limit for all consumption that exceeds the highest tier, will fall into this category

newtb3

 

Step 4: Specify the Period

 

a: If this rate to only apply during the summer season, then change the starting and end dates accordingly.

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b: If this rate has a Time of Use, then check the box that says “Has Time of Use”. You may add one or more days and times times to this rate.

 

tb8

 

You can specify the days of the week, and the time ranges in which the rate applies on each of those days. You can add multiple times of use by clicking the "Add a Time of Use" button.

cr-8

 

Step 5: Specify if this rate applies during Holidays

tb9

 

 

If this is checked, it would mean that this rate would be applied to any grid purchases during the holidays. To add all the public holidays in your region, go to the Add Holidays Section.

 

If you want to import real time prices, then refer to this article.

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