Energy Matters Forums

[EnergyMatters]

Question:

What does a bi-directional meter do?

Answer:

A bi-directional meter is like a standard electricity meter, except that it’s designed to measures electricity flow in two directions. When you install grid connected solar power system, you need a meter that can tell you not just how much energy you’ve consumed but also how much you’ve fed back into the grid.

Most states have feed in tariffs, premium rates consumers get paid for the clean energy their solar power systems contribute back into the grid. Data from the bi-directional meter is used by your electricity company to tell how much power they need to pay you for. 

Most electricity meters currently installed on houses can not measure electricity from a grid connect solar system being returned to the mains grid. Some meters, such as electro-mechanical (or spinning disc) meters may accurately measure power but the data is not always accepted by electricity companies.

Most bi-directional meters will be "interval" or "smart" meters. These meters record consumption and feed-in data in half hour blocks. Interval meters are either remotely or manually read, depending on the model. Manually read interval meters download data into hand held probes, carried by meter readers at the end of every billing period. With remotely read interval meters, data is sent back to the distributor using a device like a mobile phone. Some remotely read meters can even de-energise and re-energise remotely, removing the need for a distributor to come and pull out the fuse. 

Depending on where you live, installation of a bi-directional meter may be free. We can tell you if this will be the case in your situation. For example, in Victoria from July 2009 onwards every home will be receiving a new smart meter in a staged rollout. 

[markd]

Hi and thanks for this.

A question though - are you sure the net power put back on the grid is measured in half hour blocks? My impression with mine is that the credit is applied instantaneously as soon as production exceeds household demand. This is then accumulated in fractions of kw/hr credits. Is this not correct?

Mark

[wandifred]

Hi EM,

I have a question in regards to net FIT vs gross FIT. I can see on my bi-directional meter the net electricity I've exported but I don't know how a gross tarrif would be calculated? Does one's inverter need to be consulted?

Cheers,
Fred

[SolarShane]

In order for a gross reading to be shown at the meter the physical wiring from the inverter would have to go directly to the meter. That means all the electricity that leaves the inverter goes straight to the meter, and therefore the meter measures gross. But in Victoria we just wire to the switchboard. And therefore the meter just sees Net.

But the inverter should tell you how much electricity its generated from day one. Mine do. A good 1kw system and site should almost give you 1460 kilowatts per year in Victoria.

Cheers,
Solar Shane

[SolarShane]

Billing will definitely be based on what's happening at the various intervals of 1/2 hours. But I don't know if we know yet how exactly people will get the premium feed-in tarriff....

Example

In a half hour interval:

200 watts used
400 watts exported

Would it be (if electricity is bought at 17.5 cents / kwh)

.2 x .175 = 3.5 cents
.4 x .60 = 24 cents

therefore you get 21.5 cents credit for that half hour interval or....

would it be:

400-200 = 200
.2 x .60 = 12 cents

therefore you get 12 cents credit for that half hour interval

Cheers,
Solar Shane

[markd]

You would get 12 cents.

Someone needs to explain to me why my meter records 0.1 increments in kwh and within a half hour block it changes more than once when I am net exporting power to the grid. If it only does it in half hour blocks then unless you are net exporting for a full half hour, every half hour you won't get a cent in credit.

Keep em coming....

Mark

[micke]

Bi-directional meters record instantenous power on seperate registers one for import and one for export. Most electronic bi-directional meters have built in interval data storage which record data such as power, voltage, current and frequency.

The interval data is usally sampled every 30 minutes but depending on brand and model can be altered to suit the intended use.

At the end of the billing cycle the retailer will get the total power from the import and export registers and subtract the total power from the previous billing cycle, the retailer will then know how much power the customer has imported and exported and will send out the bill or credit.

[markd]

Thanks Micke,

Elsewhere on this forum I have posted this little gem:

On the issue of 30 minute blocks - I have read the specs on the AMPY EM1200 series meters (as used in SA) and they do indeed store data in 30 minute blocks. Whether that means that is the means by which is recorded is another thing I guess.

The SA Govt fact sheet covers this issue and says ...' You will need a meter that separately records the electricity you return to the grid so that, even if it is only for a second, the meter will keep track of any electricity you return. When the meter reader comes around at the end of the month or end of the quarter, the total amount of electricity you have returned and the total amount you have consumed will be read and passed onto the state’s electricity distributor, ETSA Utilities.'

more info at : http://www.climatechange.sa.gov.au/uplo ... _sheet.pdf

So, are you saying it samples once every 30 mins (ie whatever the kwh you are putting onto the grid is at that moment) or that it stores a value every 30 mins which would be the cumulative actual net amount that you have generated in the previous 30 min period? They are different in my mind.

Mark

[micke]

Hi Markd,

The AMPY EM1200 meter you refer to is a single phase 2 element bi-directional meter, meaning it can register 2 import tariffs (general and off-peak) and one export tariff (feed-in). This meter has electronic registers which record instantenous power and interval data storage. The electronic registers are the same as the old electromechanical registers, that is when power flows the register records it, much like a cars odometer. In addition to the standard registers the EM1200 has interval data storage. The data from these registers is what the retailers use to bill the customer.

The interval data storage has nothing to do with the normal operation of the meter it is just an optional extra so to speak, the AMPY EM1200 meter you refer to can record 12 different data points at 30 minute intervals, this data can used by the electrical distributor to determine how there system is operating, or record pulses from a gas or water meter.

I work for Ergon Energy (electrical distributor and retailer) in Queensland and we are trialing a version of this meter called the EM1242 on the Townsville Solar Cities Project (Magnetic Island Solar Surburb). We are recording data every 30 minutes to determine a load profile of Magnetic Island over a year, the EM1242 also record pulses from household water meters, to determine water consumption per household and sends this data to a wireless in house display to inform the customer.

Mick

[markd]

Thanks Mike,

very interesting and what a bonus for what will I hope happen in the future. An in-house display that gives you live data (or close to it) on gas, water and electricity would be fantastic. I have just bought the Fronius remote display second hand so I can know what the system has delivered each day and live - really good with great range too - which was the problem I had with 'the owl' hooked up to the feed in side. More accurate too.

So what you are saying is that it is the cumulative net amount you produce at any moment that is recorded (like an odometer), which then is read by the meter reader in half hour blocks when they download this from the meter via its optical link. So it is still the cumulative total for the billing cycle that is considered to be your total net production. The half hour data points merely gives an idea of when you are producing it - a profile of your generation pattern in net terms.

Is this correct?

Sorry - I am just trying to clarify the opening statement in this thread - 'These meters record consumption and feed-in data in half hour blocks'. I had the impression from this that the meter takes a sample each half hour of what may be occurring at that time - but it doesn't, it fixes a data point each half hour which records the cumulative amount you have generated and updates that total each each half hour.

Correct?

Mark