Revealed: How Home Batteries Cut Over Ten Times The Emissions They Create

Here are two facts about home batteries:

1. Using a home battery reduces greenhouse gas emissions.
2. Making and installing a home battery increases greenhouse gas emissions.

The good news is the first effect is much larger than the second.  Pinning down how much home batteries reduce emissions isn’t easy, but I’m going to take a stab at it.  Hopefully, by the time you get to the end of this post, you won’t want to take a stab at me.

How Much Do Home Batteries Cut Emissions?

Based on my analysis of Australia’s largest grid, the National Electricity Market (NEM), a typical home battery will pay off its emissions debt in under 18 months.  Over its lifespan, a solar battery with 15kWh of usable capacity will cut emissions by more than 10 times the amount caused by its creation and installation, reducing emissions by more than 16 tonnes over a 15 year lifespan.

That’s just for the typical use case of a home battery charged with solar energy that could have been exported to the grid, where it may have reduced fossil fuel generation had the battery not been in use.

If however, a home battery is only charged with solar energy that otherwise would have gone to waste, the climate benefit is far larger, cutting emissions by somewhere around 28 to 1.

These figures don’t include indirect effects on emissions, which my recent analysis shows can offer even greater climate benefits.

Upper & Lower Bounds For Emission Reductions

When a home battery is charged with rooftop solar, it can have one of two effects on emissions:

• Emissions increase because the solar energy would otherwise have been sent into the grid, where it would have reduced fossil fuel generation.
• There’s no effect on emissions because there is already so much clean energy available that some of it is going to waste, or the solar energy wouldn’t have been sent into the grid in the first place.

I’ll start by considering a home battery that never increases fossil fuel generation by being charged.  This can happen if a home is zero export limited and can’t send rooftop solar power into the grid, or the solar system produces much more energy than can be exported, allowing the battery to only be charged with clean energy that otherwise would have been wasted.

A Nasty Discharge For Fossil Fuel Generation

When a home battery is discharged at night, it will reduce either coal or gas generation.  Renewables normally won’t be affected.  This is because coal and gas have fuel costs while wind power’s fuel — moving air — is free.  It is possible hydro generation will be cut back, but this leaves water in the dam that will later be used in place of fossil fuels, so it’s always coal and gas that get cut — there’s just a delay sometimes.

Over the past year, 50.3% of Australia’s generation came from coal, while only 7.1% came from gas.  This makes it appear that discharging a battery should mostly cut coal, but gas generation is concentrated in the early evening when home batteries do most of their discharging.  However, gas generation is in decline and nearly at half of its peak 11 years ago.  Batteries are hastening its fall, so I expect a home battery will reduce considerably more coal generation than gas over its lifespan.  But to keep things simple, I’ll assume a 50/50 split between gas and coal.

Over the past year, the average emissions from fossil fuels in the National Electricity Market have been:

• Coal:  971g per kWh
• Gas:  534g per kWh

These figures are mostly carbon dioxide (CO2), but also take into account methane leaks and nitrous oxide by adding in an equivalent amount of CO2.  This makes them CO2 equivalent emission figures.

With a 50/50 split, the average emissions reduction from discharging 1kWh of battery energy is 753g.  If a 15kWh battery were completely discharged overnight, it would lower emissions by 11.3kg.  If this were done every night, the emission reductions over 1 year would come to 4.1 tonnes, which is more than the roughly 3 tonnes the average petrol passenger car emits annually.  But discharging a home battery’s entire capacity every night isn’t realistic.

Home Battery Capacity

Unless used in a strange way — possibly by strange people — a home battery will discharge an average of under 100% of its usable capacity every night.  If you’re even getting close to 100%, it means you should have bought a bigger battery.  Ideally, a battery should be able to provide all your electricity consumption from late afternoon to a couple of hours after sunrise on nearly all nights.  Batteries also degrade over time.  This may be under 1% per year for good quality ones, but it still increases the amount of battery capacity that should be installed.

It was common for home batteries to have an average capacity factor of 70%, but this will be heading down as the size of batteries has increased since the federal battery rebate was introduced.  While I’m considering a battery with 15kWh of usable storage when new, which isn’t a lot these days¹, I’ll still assume it has a modest capacity factor and it discharges an average of two-thirds of its usable capacity when new overnight.  This gives it a capacity factor of 66.6% and means it discharges an average of 10kWh per night.

With a 50/50 split between coal and gas, it will reduce emissions daily by an average of 7.53kg per day.  This comes to 2.75 tonnes of avoided emissions per year.  With a 15 year lifespan, the battery will reduce emissions by over 41 tonnes.

Typical Home Battery Emission Reductions

So far, I’ve considered a battery charged with energy from rooftop solar that couldn’t have been sent into the grid.  But if it could have, then sending that energy into a battery instead can increase fossil fuel generation and thus emissions.  If there’s already so much clean energy being supplied that some is going to waste, then it’s not a problem.  Charging your battery will just result in less clean energy being wasted.  But at other times, fossil fuel generation will increase to fill the gap.

While this is a drawback, it may not seem like a real problem, as the energy stored in the battery will later be used to reduce fossil fuel generation.  But there are two ways it can increase emissions:

1. Losses from charging and discharging.
2. Increased fossil fuel generation during the day often comes from high-emission coal power, while discharging a battery in the evening often reduces gas generation, which has lower emissions.

Using assumptions I consider conservative for losses and fossil fuel generation reductions, I worked out the typical home battery would annually reduce emissions by over a tonne.  But the trouble with this was, my assumptions were just assumptions.

To get an estimate of typical emission reductions that isn’t just based on my opinion, but should still be roughly correct, I’ll examine the difference in average emissions between day and night.

Average Emissions: Day Vs. Night

Solar now supplies over 21% of Australia’s electricity, and this has dramatically reduced daytime emissions.  For several hours on either side of noon, when home batteries get most of their charging done, the average emissions per kWh of electricity generation are around 320g.  During the time batteries discharge, from late afternoon until after sunrise the next morning, the average emissions are around 660g per kWh.  Subtracting the daytime average from the nighttime average² will give a reasonable, back-of-the-envelope estimate.

Based on this, if a 15kWh battery discharges an average of 10kWh overnight, it will reduce emissions by just over 3kg per day, or 1.11 tonnes per year.  Over a 15 year lifespan, this would come to 16.6 tonnes.

While I expect home battery emission reductions to increase as daytime solar generation rises and gas generation declines, I’m okay with using this as a lower bound estimate for emission reductions from home batteries.  It’s a simplistic way to estimate emission reductions, but I’m a simplistic guy.

Location Affects Emissions

The emission reduction estimates are for the National Electricity Market as a whole and will vary depending on location.  But because the states are interconnected and trade electricity, even in the two states without coal power stations — SA and Tasmania — a home battery can and will reduce coal generation.  So differences in emission reductions between locations shouldn’t be extreme.

When I said the states are interconnected, I didn’t mean Western Australia.  They’re off on their own with a separate grid where the difference between average emissions during night and day tends to be larger than in the NEM states.  This increases the potential for emission reductions, but WA also generates more electricity from gas than any other state, which limits them.  For now, I’d expect home battery emission reductions in WA to be considerable, but not as high as in the NEM.

Home Battery Embodied Emissions

Now I’ve estimated that the typical 15kWh battery will reduce emissions by more than 16 tonnes over a 15 year lifespan, I need to work out how much emissions result from the manufacture and installation of batteries and inverters.

It’s even possible for battery recycling at the end of its life to slightly reduce emissions, although that would only be a tiny amount, so I’m not going to worry about it.

Battery Manufacture Emissions

Luckily for me, estimating emissions from making battery cells is easy, as I can simply use someone else’s work.  For example, a paper with a title that has little interest in brevity…

Estimating the environmental impacts of global lithium-ion battery supply chain: A temporal, geographical, and technological perspective.

The above report says emissions from making lithium-iron battery cells are around 55kg of CO2 equivalent per kilowatt-hour (kWh).  The paper is nearly two years old, so the figure should be a little lower now, because it declines as technology improves and energy gradually becomes cleaner.  But instead of lowering it, I’ll bump it up a little.  This is because the usable amount of a home battery is typically 90-99% of its total or nominal capacity.  The amount depends on the exact type of battery cell and how much the manufacturer cares about avoiding battery degradation.  I’ll increase emissions by just over 5% to 58kg per kWh to allow for this.

So for 15kWh of usable battery cell capacity, we’re looking at 870kg of CO2 emissions.  While not good, that’s not too bad.  However, this is only for the battery cells.  The required supporting electronics and other components all add to it.

Inverter Emissions

Every home battery needs an inverter.  Either a hybrid inverter that’s shared between the solar panels and the battery, or its own dedicated battery inverter.  This also results in emissions.

Thanks to a Fronius report, we know the amount of emissions resulting from the manufacture of their 10kW Fronius GEN24 Plus hybrid inverter.  And thanks to Fronius having a mild interest in title brevity, that report is called:

Fronius GEN24 Plus — A Benefit For The Environment Life Cycle Assessment (LCA)

It says that manufacture of inverter components, assembly, and transport result in around 345kg of emissions.

However, there are a couple of reasons why a lower figure would be more accurate:

• A hybrid inverter is also used by the solar panels, so inverter emissions should be shared with the solar system.
• Most home batteries with 15kWh of usable storage will have a smaller 5kW inverter, rather than the 10kW one in the study.

On the other hand, I wouldn’t be at all surprised if Fronius takes more care in sourcing low-emission components than lower-cost manufacturers, so that may bump up average inverter emissions.  This shouldn’t add much, but I’ll err on the side of caution and allow for 200kg of emissions from battery inverter manufacture, transport, and installation.

The Rest Of The Battery System

By themselves, the battery cells are likely to weigh 90kg or more.  But the total weight of a 15kWh battery system is 160-240kg.  So there’s around an extra 100kg to account for.  A lot of this will be the battery system’s structure.  This protects components and holds them in place and will mostly be steel and plastic that’s likely to average around 3kg of emissions per kg.  But there will also be several kg of electronics with a much higher average emission amount.  So, in addition to cell manufacturing emissions, I’ll add 350kg of emissions to account for the balance of battery system components.  This includes any additional components required for installation, such as fuses, cabling, and battery stickers.

Transport Emissions

To allow for transport emissions from getting the typical home battery from China to an Australian home, I’ll add 70kg.  This is only 70% the amount per kg for a Fronius inverter, but shipping distance from Shanghai is less than half that from Austria.  (While it is possible to ship things out of landlocked Austria, I’m still not sure where they keep the Austrian navy.)

Installation Emissions

While Fronius doesn’t consider emissions from installation to be worth mentioning, they are likely to be more involved for batteries, so I’ll allow 20kg of emissions for extra materials that may be required, such as bollards if a battery is being installed in a garage, or a shade if it’s in direct sunlight.

Total Emissions Estimate

If I list all the emissions from the various parts of a battery system and then total them, I get this:

• Battery cells:  870kg
• The inverter’s share:  200kg
• Other battery components:  350kg
• Battery transport:  70kg
• Installation:  20kg
• Total:  1,510kg

This is around 100kg of emissions per usable kWh, or around 1.5 tonnes for a 15kWh battery. While that’s a lot of emissions for something the size of a small bar fridge, it’s way less than the reduction in emissions resulting from use.

Home Battery Emission Savings

The estimated annual emission reductions for a 15kWh home battery that discharges an average of 10kWh overnight, and is charged with solar energy that can’t be exported to the grid, is 2.75 tonnes.  This means it would offset the 1.51 tonnes of emissions from its manufacture and installation in under 7 months.  With a 15 year lifespan, its emission reductions would be 28 times more than it caused.

For a more typical installation, where the battery is mostly charged with solar energy that could have been sent into the grid, a battery of similar size and capacity factor would reduce emissions by 1.11 tonnes per year.  This would let it pay off its emissions debt in a little over 16 months.  With a 15 year lifespan, it would reduce emissions by 11 times more than those resulting from its creation and installation.

 

While my figures are only estimates, it’s clear to me there’s no need to worry about home batteries increasing emissions.  For a typical battery, the emission benefits are likely to exceed the emission cost by around 10 times or more.  So if you care about reducing emissions, install a home battery.

This is the second in a two-part series on battery emissions. Read the first, on how batteries reduce emissions, here.