Getting the most out of your solar PV system

Posted by   SFiunch   |   Categories :   Energy Source

Now that you have had your solar photovoltaic (PV) system installed you might be wondering how to maximize your financial return.

Calculating what you export

Reducing the amount of electricity you export

How to get an export meter fitted

One way of making the most of your system is to use more of the electricity your PV system generates at home rather than exporting it back to the grid. Financially this makes sense because electricity bought from your electricity supplier (current average 14.4p/kWh) costs more than what you get back through the Feed-in Tariff (currently 5p/kWh). Using current average electricity prices that’s a price difference of 9.9p for every kWh (unit).

You’ll need to be careful how you do this, though, as increasing your electricity usage to more than your solar PV panels are producing at any given moment can increase your electricity bills and not reduce them. Aim to reduce your electricity use first, and only after that to time your use to gain the maximum benefit. Householders who do not have an export meter fitted (which is normal for anyone with a less than 30kWp solar PV system) are deemed to be exporting 50% of the electricity generated back to the grid. Their FIT payment will be based on this amount no matter what their actual export is.

Click here for more information about solar from the Energy Smart website –

Example of an export meter: Iskra import-export smart meter

Calculating what you export

If you want to do a rough calculation of how much electricity you export over a year you can carry out a very rough estimate by:

**calculating the amount of electricity you used in the year before your PV system was installed in kWh (import year 0). You can get this from previous electricity bills
**calculate the electricity you’ve imported in the year since your solar PV system was installed (import year 1). Again using electricity bills
**make a note from your generation meter of the amount of electricity your solar PV system generated in year 1 (total generation). This should be over the exact same period as bullet above.

For example, if you imported 4,000kWh in year 0, imported 3,400kWh in year 1 and your total generation in year 1 was 1800kWh then you will be exporting 67% rounded to the nearest decimal point. This has been calculated using the following formula:

Total % exported = (1 – change in import over two periods (kWh)/total generation (kWh)) x 100
67% = (1 – (4,000 – 3,400)kWh/1,800kWh) x 100)

In this example, the fact that you’re exporting 67% of the electricity generated also implies that 33% is used on site which is the saving you make from import.

As mentioned at the start this is only a very rough estimate. The main weakness with this approach is that it assumes your consumption over the two years is identical. While this calculation offers some insights, it will never replace the accuracy of an export meter.

If you have had your PV system installed for less than a year, you can compare figures over a shorter period. This will be less accurate but not very far out – electricity usage does not fluctuate much over the year unless you use electricity to heat your home.

If you want to know whether you are likely to be exporting electricity at any given moment of time you can look at the graphic display on your inverter. This will show the current amount of electricity being generated by your PV panels which you can then compare to the total consumption of all the appliances in your home that are on. You can make a rough calculation of this by adding together the rated power of each appliance.

Or you can buy a monitor for £125-150 (based on the Geo Solo PV Energy Monitor and the Sunny Boy Beam). This is particularly useful if your inverter is in an out-of-the-way location such as an attic or garage. Using Bluetooth technology you can also monitor the output of your PV system remotely – although the cost of this might be more than the value of the electricity you save.

This process can also be managed automatically by installing a home energy management system that compares your electricity production and consumption in real time. If there is surplus electricity the management system decides whether the surplus should be stored (as heat or hot water) or exported to the grid. An example of this is the Energy and Micro-generator Manager or EMMA for short.

Reducing the amount of electricity you export

Steps that you can take to reduce the amount of electricity that you export in order of ease are:

Use electrical appliances during the day while your PV system is working, rather than at night – for example washing machines and dishwashers (easy)
Use the excess electricity generated to generate heat (medium)
Use the excess electricity generated to generate hot water (medium)
Store the electricity generated in batteries (hard)
Use electrical appliances during the day (easy)

You can adjust your behavior so that you use appliances with a high energy demand such as washing machines, dishwashers, irons and hoovers during the day when your PV system is generating more electricity. At the same time you can charge up devices such as laptops and mobile phones which run on batteries. What you are aiming for is to match the output of your PV system with your electrical consumption in real time. If you are not at home during the day to take advantage of the surplus electricity being generated, you can buy electrical appliances with a timer delay which will enable your appliances to start working during the day while you are out.

Use the excess electricity generated to generate heat (medium)

You might also consider using any excess electricity to top up the likes of electric storage heaters. If you do this you have to be particularly careful not to use more electricity than would otherwise be exported if you currently buy your electricity on Economy 7 or 10. This is because any additional electricity consumed during the day will cost you more than electricity consumed during the cheaper rates available at night using Economy 7 or 10 – making you worse off.
Use the excess electricity generated to generate hot water (medium)

One way of using excess daytime electricity in a more controlled way is to use it to generate hot water via an immersion heater. In order to be able to do this you will need:

**a standard hot water cylinder with an immersion heater fitted a PV (or wind) system ideally rated at 2kWp or more the PV (or wind) system and immersion heater connected to the same consumer unit
**an immersion relay to reduce the electrical consumption of your immersion heater from the usual 3kW to 1kW (alternatively to replace the existing 3kW immersion heater with a 1kW heater). If the electrical consumption of your immersion heater is not reduced in this way, there is a significant risk that you have to buy in electricity to make up the shortfall – costing you more rather than saving you money. But if your immersion heater is currently used to provide all your hot water (even if only at some times of the year), down-rating its output in this way is unlikely to be useful as you may find you have insufficient hot water for your daily needs a current sensing relay unit to detect how much electricity is being generated by your inverter and to switch over to the immersion heater at a set point – usually 1kW. This is done on the assumption that no more than 1kW is being consumed within the home and that any excess would normally be exported to the grid.

The advice in the UK is that hot water stored in a cylinder should be kept above, or periodically rise to, above 60°C to avoid the danger of Legionella bacteria. If your down-rated immersion heater cannot do this, you will need to ensure that your boiler comes on from time to time to provide this top-up heat.

This work needs to be done by a suitably qualified heating engineer: it is not a DIY job. The cost of having this kind of system installed is likely to be £300 to £500 including VAT. The amount that can be saved in this way will depend on the type of fuel that you would normally use to heat your hot water. The highest savings will be made if you currently use oil or LPG, the lowest if you use gas (estimated average cost 4.49p/kWh).

If you normally use gas to heat your hot water you will need to consider carefully whether it is more cost-effective for you to use the surplus electricity generated in this way – even taking account of the fact that your immersion heater will be more efficient than your boiler, the payback time is likely to be twice as long as for other fuels.
Store the electricity generated in batteries (hard)

Excess electricity generated by your PV system can be stored in batteries even if your system is grid connected. However, good quality batteries are expensive and will need replacing every 5 to 10 years depending on the amount they are used and the depth to which they are discharged.

Installing batteries is unlikely to save you money, although you might still consider installing batteries as a backup if you live in an area which experiences frequent power cuts. There are systems (sold as system backups or power routers) on the market that can automatically switch over to battery power in the event of a power cut.
How to get an export meter fitted

If you are still exporting more than 50% of the electricity you generate, you will be getting paid for less than that. If you wanted to have an export meter fitted the way to do this is to contact your energy supplier and request one.

Having an export meter only makes sense financially if you have a relatively large system and you can either get an export meter fitted for free or at low cost.

To receive payment under the Feed-in Tariff, the installation and maintenance of the export meter must be done by your energy supplier rather than your PV system installer.

As the work done by an export meter will be superseded by smart meters which are due to be installed in every household (at your energy supplier’s cost) between 2014 and 2019 you might prefer to wait rather than paying now. Smart meters with export meter functionality may be rolled out by some energy suppliers as early as 2013.

April 15, 2017

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