Solar diverter - maybe not the best financial investment - better for the planet?

ricwallar

Six months ago we installed 3kW solar PV, 6.5kW battery & an iBoost solar diverter on our gas boiler system, which kicks in to augment domestic hot water after the battery has reached 100% and delays exporting to the grid (presumably until the water is fully up to temperature).

We’re on an Octopus tariff at (rounded) rates per kWh of 28p electric, 7p gas and 15p export from the solar. The iBoost cost £385 to install.

The gas boiler is under 2 years old (replacement) and probably 90% efficient (heat pump was not viable option). So 1kWh of clean “free” solar saves about 1.1kWh of fossil fuel gas when the iBoost is working. But we’re losing about 7p/kWh when it’s working (7p saved, 15p lost x 90%).

We’ll obviously turn off the iBoost (& the boiler) when we’re away from home and not needing hot water at all, but it seems we didn’t make the best decision installing the iBoost, apart from the saving on fossil fuel energy. All the numbers are pretty small (apart from the initial cost!).

Does that seem logical?

Reed_Richards

I think the iBoost and equivalent devices date back to the days of the Feed-In Tariff, where you were paid the same amount no matter how much electricity you actually did or didn't export.  So you were strongly incentivised to use all the solar electricity you could.  Even then, they were so expensive that the pay-back time must have been long.  

ricwallar

Yes, I think that’s right. With our second PV system in 2013 (we’ve had 3, on 3 different properties since 2011) we included an Immersun diverter (at only about £150 if I remember rightly) with no battery obviously, and with an oil-fired boiler. By then FIT was down to a really low sum and oil had rocketed, so it was a no-brainer to install the diverter. Probably fond memories of its performance is one factor leading to the recent decision. 

Magnitio

The boiler may be 90%+ efficient, but it is heating water that then gets pumped from the boiler to the heat exchanger in the cyclinder. It will depend how far it is from your boiler to the cylinder and how well insulated that pipework is, but there will be heat losses. The pump and the boiler may also be consuming 20-40 watts of electricity. During winter, that excess heat goes into the house (assuming the boiler and pipework are within the insulated part of the house). When the heating season is over, you could compare energy usage from a period of 7 days using gas with another 7 days on immersion. I'm sure it will still work out financially better to use gas, but for the majority of the time it will be better environmentally to use the immersion.

Merlin139

We had an I-Boost installer with our system in May 2014. As of Monday this week it had diverted 12005.3 kWh since install. It was an extra £200 when we had our SolarEdge System installed. Paid for itself a few times over.

We have just had a smart meter installed and yesterday I set about the process of moving onto Fixed Octopus Outgoing paying 15p per kWh exported moving away from the deemed export element of FIT. We get paid for an average of 2100 kWh per year for Export. 

We will be turning our I-Boost off once Outgoing starts.

The I-Boost has one advantage. On a really sunny day when it diverts around 3kWh the water in the tank gets very hot! My DW has to have Hot Hot showers! Asbestos Skin!  

Screwdriva

I view diverters as suboptimal from a fiscal and environment standpoint. The financial aspect has been covered quite well but from an environmental standpoint, using electricity for 1:1 heating can't hold a candle to swapping the older cylinder out for a Mixergy iHP. It is the most efficient way to heat hot water using solar energy or time of use tariffs. No gas or oil needed. 

andyg9053

I read last year that making electric from gas was not as efficient as making heat from your boiler. The article went on that it was greener to put your excess solar back in to the grid and use gas to make hot water. With octopus rates of 15 p for electric export and 4p for gas import it is also more cost effective 

ricwallar

Thanks for the comments, interesting. The boiler efficiency (assuming 90%) is the area that I'm most fuzzy about because I imagine, as has been said, there must be some losses between boiler & cylinder (3 year old 200 litre s/s Tempest with a 2 year old Baxi system boiler about a metre away, indoors, uninsulated pipes).
I guess the issue financially is that the difference between the saving on gas at 7p cost and the export rate at 15p lost, even allowing for less efficient heating by the boiler, is a loss-making exercise.
The saving on fossil fuel use is of course nice but it's so marginal that it doesn't really justify the £385 installation cost & the loss of export tariff when the iBoost is operating. Unless I'm missing something.
Never mind, should have thought of it before the install, I don't think we really appreciated that we'd be getting 15p from Octopus, cheers Greg👍

Netexporter

There is an argument that home batteries are largely redundant, as you can use the grid as a virtual battery. However, batteries do give you some security against power cuts, which may be relevant, particularly in more rural areas.

michaels

With the battery would it not make more sense to be on a time of use tariff.  WE only use 8p overnight electricity with our V2H battery.  Our SEG is 12p so in theory we should also probably turn off the iBoost when we are certain the battery will see us through the peak rate window - unfortunately our iBoost is an old one with no web interface so adjusting the operation involves a trip to the loft (or I guess disconnecting the sensor/sender unit on the mains supply).

Magnitio

andyg9053 said:

I read last year that making electric from gas was not as efficient as making heat from your boiler. The article went on that it was greener to put your excess solar back in to the grid and use gas to make hot water. With octopus rates of 15 p for electric export and 4p for gas import it is also more cost effective 

If all of our electricity was generated from gas or coal, then this would be accurate. On days when there is plenty of renewable energy on the grid, then it is better to use your solar instead of gas. From an environmental point of view it's pretty trivial though.