Reducing gas reliance with a solar diverter

In September 2021 we installed a solar diverter called iBoost as a measure to reduce our gas consumption.

One year on I looked at the figures to review how effective this gadget has been.

Spoiler: in 14 months it displaced 1.89kWh per day on average, around 18% of our total gas usage. We might be able to improve this with a new hot water tank.

What’s a solar diverter?

A solar diverter detects when our rooftop solar panels are generating more than we are using.

Rather than sending that excess to the grid (called “exporting”), it “diverts” it to an immersion heater in our hot water tank.

By heating the water in the tank, we don’t need to use gas to heat that water. The idea is that the excess solar displaces some gas usage by pre-heating the water.

Gas reduction so far

In the 422 days since installation (22/09/21 to 18/11/22), the unit has displaced 797 kWh of gas energy with solar electrical energy.

Extrapolating gives a daily, yearly and lifetime estimate:

^*Estimated based on 422 day period. It’s probably a bit better since that period contained two October and November months which are overcast.

From smart meter readings, I estimate that the tank takes about 5 kWh per day to heat. We’ve therefore reduced the gas usage for water heating by a bit over a third.

Return on investment

This project wasn’t about money, it was about reducing gas reliance. But it’s useful to understand the cost of different measures. Let’s take a look at the numbers and see how it works out.

There are two costs to consider: the installation and the running cost.

Installation cost

It cost £450 to install the unit. That includes a morning’s work of an electrician.

Running cost (or income)

When we export excess energy to the grid we get paid around £0.05 for every kWh (called a “unit”).

At the time of installing, it cost £0.04 to buy gas from the grid.

So back then it would effectively cost us £0.01 per unit to heat the hot water using the excess solar rather than buying gas. (But don’t panic, that only works out £9 / year.)

However, since installing, gas prices shot through the roof. At £0.07 per unit, we now make £0.02 per unit for saving gas. Great!

Lifetime cost

Assuming the device works for 10 years and displaces 6,893 kWh over its lifetime.

Scenario 1: Gas prices stay high, averaging £0.07 over the 10 year lifetime. The saving of £0.02 becomes £0.038 per day, £13.79 per year, £138 over the lifetime. That subsidises the £450 installation cost to a lifetime cost of £312.

Scenario 2: Gas prices drop again. It seems unlikely they’ll drop below their previous value, so let’s say an average of £0.05 over the 10 year lifetime. That means the gas unit rate versus export unit rate is neutral. The lifetime cost is therefore £450.

Cost per tonne of carbon saved

Again, assuming a lifetime gas reduction of 6,893 kWh.

According to this website, the carbon intensity of burning domestic gas is 0.23314 kg CO₂e per kWh.

That’s a carbon saving of 1.60 tonnes over the 10 years.

That works out as £195 to £285 per tonne of carbon for the two gas price scenarios.

For comparison, it currently costs Climeworks about £900 to permanently remove a tonne of carbon from the atmosphere. So at today’s prices, this measure is about 3-5x cheaper than burning gas then sucking back up the carbon it releases into the atmosphere. (Lesson: don’t think carbon capture gives us an excuse to carry on burning stuff.)

Note that this doesn’t take into account the carbon cost of manufacturing the iBoost device, solar panels, inverter and so on. I’ll try to improve this.

Possible improvements

Replace the hot water tank?

Even on very sunny days, the iBoost never diverts more than 2.5 kWh. The gas kicks in to heat the water even after these days. I believe the water tank takes about 5 kWh to heat, so the iBoost is only heating about 50% of the water.

My best theory is that the immersion heater is not long enough to heat the bottom of the tank. This is a bit strange as it’s a 32” immersion which matches the tank’s documented maximum. But perhaps the tank designers intended that an immersion is only a backup so doesn’t need to heat the whole tank?

More investigation required, but if that’s the case, we could upgrade to a tank with two immersion heater positions. The iBoost supports this configuration, heating one first then the next. The tank would also have to have a primary coil, so it would be quite cramped. Need to look into this.

Conclusion

Saving 0.16 tonnes of carbon per year is great and it’s nice to be moving towards full electrification.

But how much of our entire gas usage has that knocked out?

In the same period (22/09/21 to 18/11/22) that we diverted 797 kWh, we still consumed 3,535 kWh of gas.

Assuming the iBoost measured correctly, we would have consumed 3,535 + 797 = 4,332 kWh without it.

(As a sanity check, this roughly tallies with a comparable period, 22/09/2018 to 18/09/2019 in which we consumed 4,821 kWh of gas.)

18% gas reduction so far

So the iBoost reduced our gas consumption by 797 / 4,322 = 18%

I’m pleasantly surprised how large that reduction was.

Insulate first…

For the remaining gas usage, I’m going to take fabric first approach. That means improving the insulation of the building before installing renewables. Despite subscribing to this approach, I got distracted by cool shiny technology (solar, battery, iBoost) when I should’ve started with the building.

In summary, if you already have solar PV, a hot tank with an immersion, and you’re exporting lots of electricity, a solar diverter is a good option (but sort out your draughts, loft, windows and doors first!)