Sizing My Sister’s Battery To Store Free Daytime Electricity

The good news is, my sister is getting a battery. The bad news is, she says I have to decide how large it should be, and god knows what she’ll do to me if I get it wrong. To find out how I decided what battery capacity to recommend, while taking into account her average electricity consumption, modest 6.6kW solar system, and the fact she’ll be topping up her battery with free daytime electricity — read on. But if you only want to know how I rate my chances of getting out of this unharmed, then also read on.

What Size Battery Do You Actually Need?

These days, if you have solar — or have a roof you can put it on — the odds are getting a home battery will be a good financial decision. It will come down to individual circumstances, but it’s normally now worthwhile thanks to:

• Falling battery prices
• The federal battery rebate
• High electricity prices
• Affordable and reliable solar

But what can really make a battery pay is topping it up with free grid electricity during the day. Free charging can supercharge your savings, and I’ll make sure my sister takes full advantage of it. Nearly all major retailers now offer Solar Sharer plans with free periods in NSW, South East Queensland, and SA. While Victoria’s own version of Solar Sharer only goes live in October, it’s still possible to find plans with a free electricity period, and in WA and rural QLD it’s possible to get plans with cheap, but not free, daytime electricity.

Over the past year, my sister’s household consumed just under 6,500 kWh of grid electricity. Based only on this, my guess was she’d want around 22 kWh of battery capacity, plus maybe some extra to provide reliable backup power. But after estimating the benefits of charging her battery with free electricity, I ended up recommending a little more.

My Sister’s Current Setup

At the moment, my sister currently has:

• A home in rural NSW
• A 6.6kW solar system
• A mean left hook
• A mean right hook
• An entire garden shed full of mean hooks

If I don’t want to be on the hook for her missing out on potential savings by recommending a battery that’s too small, or causing her to pay more than she has to for a battery that’s too large, then I need information on her electricity consumption patterns.

The most accurate way to do this is to get an NEM12.csv file of smart meter data. The problem with this is, it’s a huge file stuffed with numbers that has to be interpreted. Our Battery Calculator can help, but at the moment, it can’t take into account topping up a battery with free grid electricity. Soon, we’ll have a new and improved version of our battery calculator able to do that, but I don’t know exactly when it will be ready. Despite this, you can still get some useful information out of our current battery calculator, and I’ll explain how to do that below.

If you already have solar, and aren’t intending to install more when you get a battery, then — provided you’re certain you’ll top it up with free or cheap grid electricity during the day — you can roughly estimate your required battery capacity by looking at your electricity bills.

Fortunately, I have all my sister’s electricity bills. And if I pay them on time, nothing bad happens.

Battery Sizing Rules Of Thumb

My sister has solar and isn’t planning to expand it anytime soon, so I could look at her bills and use a very basic rule of thumb that says, for those who already have solar, that battery capacity should be equal average daily grid electricity consumption — plus some extra, depending on how important backup power is considered. But there are a couple of problems with this…

1. It’s for batteries that are only charged with solar and not topped up from the grid.
2. It’s just a rule of thumb, and I’m not sure how well it works anyway.

If I had to guess — and I kind of have to because my mental problems compel me to do so — I would say a better rule of thumb for homes that charge their battery every day with free electricity would be for battery capacity to equal average daily grid consumption + 25%, plus extra for backup as desired.

Dissecting My Sister’s Bills

So my next step was hacking into my sister’s electricity account by cunningly asking for her login details. This let me see her consumption over the past 12 months and work out that her average daily grid consumption was 17.8 kWh. So, using my rule of thumb, she should get a battery of around 22 kWh, plus whatever she wants for backup.

But to find out if that’s right, I’m going to have to dig a lot deeper, and I’ll start by working out her seasonal daily averages:

• Spring: 15.4 kWh
• Summer: 12 kWh
• Autumn: 13.5 kWh
• Winter: 30.1 kWh

Looking at the results, I’m like — Wow! What are you doing in winter, woman? Leaving your wardrobe door open and attempting to heat the whole of Narnia?¹

Digging a little deeper to find the months with the highest and lowest daily consumption gives:

• June: 32.8 kWh
• February 8.1 kWh

While this extreme seasonal variation complicates working out how large a battery she should get, it does mean she’s a perfect candidate for charging her battery with free daytime electricity, as there’s no way her 6.6 kW solar system could reliably charge a battery to keep up with her winter consumption.

NEM12 Smart Meter Data

If you’re planning to get solar and a battery at the same time, or you have solar but will expand it when you get a battery, then you can’t just use your electricity bills, as they won’t show the reduction in your consumption from getting solar or expanding it. But if you have a smart meter, you can request a NEM12 file of electricity consumption data. We have a page explaining how to get it here. My sister is with Origin (the owner of SolarQuotes), and with them, you can get it by logging into your account and clicking on usage …

…and then clicking on the red “Request data” towards the bottom of that page:

To see what your electricity consumption is like over a full year, you’ll need to have had a smart meter for at least a full year. A NEM12 file covering 12 months or more can be used with our Battery Calculator to estimate how much you would have saved if you’d had a battery over the past year — and soon it should be able to handle topping up for free from the grid.

It only took a few minutes for my sister’s NEM12 file to be emailed to me. Origin also sent along an explainer document containing this graph of my sister’s average grid electricity use by time of day:

Looking at that, you can see my sister somehow manages to use most of her electricity at 4 o’clock in the morning. But I want to make it absolutely clear that this is not because she’s a vampire, werewolf, undead, or any other mythological creature of the night. Her hideous strength is entirely natural.

While it’s nice to have this graph, if that were all I looked at, I would entirely miss my sister’s outrageous winter consumption. Fortunately, I can get graphs giving a breakdown of my sister’s seasonal consumption by telling our Battery Calculator that she doesn’t have a battery.

Getting ‘No Battery’ Battery Calculator Results

If you want graphs of your seasonal grid electricity consumption and solar generation, you can load an NEM12 file with at least 12 months of smart meter data into our Battery Calculator. But you’ll have to tell it that you don’t have a battery. This is easy, but how to do it is not immediately obvious.

First, enter your postcode at the top of the Battery Calculator page and then enter an electricity plan — any plan will do, it doesn’t matter. Then, where it says “Battery”, click on “My battery isn’t on this list” and tell it the “Usable Battery Capacity” is 0.01 kWh, which is almost the same as no battery at all. Then set your battery reserve to 100% so even that tiny battery capacity won’t have any effect. After that, load up your NEM12 file and hit “Calculate Your Battery Savings & Payback”. Since you told it your battery is tiny, and not being used anyway, you’ll get some nice graphs of your seasonal grid electricity consumption and solar generation.

Here’s the graph of my sister’s average solar generation and grid electricity consumption in Summer:

That big orange sail through the day shows her solar system is producing more electric energy in total than she consumes, but it’s nearly all being sent into the grid for a low feed-in tariff. This is because there’s usually no one home during the day except her fearsome hounds, she doesn’t have a battery yet, and she didn’t listen to a word I said about shifting loads to the middle of the day when you get solar. But, to be fair, she’s never listened to me before in her life, so it’s not as though I was expecting her to start.

If she installed a suitably sized battery, she could almost eliminate her grid electricity consumption — in summer.

Here’s a graph of her hefty winter consumption:

This shows her surplus solar generation is swamped by her winter electricity use, so she’d have no hope of meeting her overnight consumption by storing her surplus solar, no matter what size battery she got, making charging with free daytime electricity ideal.

Battery Savings Without Free Charging

It’s easy enough to work out how much my sister would save with a battery on her current electricity plan — if she didn’t charge from the grid during the day. All I have to do is use our battery calculator. With a 0% battery reserve, it gives the following annual savings:

• 5 kWh battery: $672  ($134 per kWh)
• 8kWh battery: $939  ($117 per kWh)²
• 10 kWh battery: $1,070  ($107 per kWh)
• 16 kWh battery: $1,282  ($80 per kWh)
• 24 kWh battery: $1,385  ($58 per kWh)

This shows that while total savings increase as battery capacity increases, the savings per kWh drop off pretty fast after 10 kWh. With free daytime charging, my sister’s savings will be higher, but she will still see a similar drop off in savings per kWh. However, it will be at a higher battery capacity.

Because small batteries have the highest return per kWh, it can make it look as though getting a small battery is a smart move. But the trick to getting the best financial return is not maximising your savings per kWh. Instead, your goal should be to install the largest reliable battery you can, that still provides a return you consider acceptable. If you get a battery smaller than this, you will miss out on savings you consider worthwhile.

Choosing A Solar Sharer Plan

Before I can work out how much my sister will save, I have to decide which electricity plan she should switch to. This will be whichever will save her the most money once she has a battery. What I definitely shouldn’t do is fall into the trap of choosing a plan with high rates and use that to claim the battery is saving her more money than she would with a more sensible plan. You may laugh, but I’ve seen people do this. Some of them sell batteries.

Because my sister’s consumption is highest in the early hours of the morning, the best plan for her will have a low rate after midnight, along with a free daytime period. The best one I’ve found so far is from the electricity retailer Covau, and they call the plan “SolarMax NSW Essential Residential TOU”. Its charges are:

• Peak 5pm-9pm: 45.3c
• Off-peak 6am-11am, 2pm-5pm, 9pm-12am: 27.48c
• Shoulder 12am-6am: 16.5c
• Free period (Shoulder 1) 11am-2pm: 0c for first 24kWh, then 27.48c
• Daily supply charge: $2.30

Because of the low 16.5c rate early in the morning, my sister would be considerably better off with this plan before even getting a battery. This is despite the hefty $2.30 daily supply charge, which is 50c a day more than what she pays now, or $182.50 more a year.  Her supply charge was already high because rural NSW’s Essential network charges like a wounded bull that was wounded in the process of eating a bear full of cocaine.

The Solar Sharer 24 kWh Cap

Not every plan with free daytime electricity is a Solar Sharer plan, but every Solar Sharer plan has a cap of 24 kWh of free electricity per day. Also, if you happen to have a battery that obeys the laws of physics — which is all of them these days — there will be charging and discharging losses. Home batteries typically have a round-trip efficiency of around 90%, which means if you charge a battery with the full 24 kWh of free electricity, you’d only be able to get about 21.6 kWh out of it.

But because my sister’s solar will be able to charge her battery a little before the free period starts on most days, and it can potentially top up the battery after the free period ends, she can still financially benefit from having a battery larger than 21.6 kWh.

One important thing to note is that if your battery can only charge at 5 kW, you’ll never be able to charge it with more than 15 kWh of free electricity in a three hour period. Some Solar Sharer plans offer four hours of free electricity, but if you want to make full use of a three-hour Solar Sharer free period, your battery will need an inverter that can charge it at 7-8 kW or more.

VPP Payments

Joining a Virtual Power Plant (VPP) can provide extra payments, and I say my sister should join one, because I consider the extra money worthwhile — provided you find a VPP that suits you. You can see a table comparing them here. While the payments aren’t high, I’d expect them to be at least $150 a year. Also, NSW offers a payment for joining a VPP, and I think she should definitely take advantage of it.

Calculating Battery Savings

Because our Battery Calculator can’t currently account for topping up your battery for free, working out how much my sister would save based on her past consumption isn’t easy. I tried to use an LLM, or AI as kids call them these days, but the damn thing ended up hallucinating more than I do — and believe me, that’s a lot.

If I wanted a 100% accurate savings estimate based on her past consumption, then my only choice was to examine her electricity consumption on every single one of the past 365 days, calculate the battery savings for each one, and then add them together.

So thank god I didn’t want that! That would be really tedious! I’d be happy with a 99% accurrate estimate. Even 95% would be fine with me. Looking at every day would take forever. Maybe even two forevers stapled together. But if the estimate doesn’t have to be perfect, I can just use a sample of days to estimate her savings.

A brother who loves their sister and wants to be highly confident their estimate is correct would sample a minimum of fifty days. But a scumbag who didn’t respect their sister and just wanted a quick, rough estimate so they could kick back and watch the anime, “Delicious in Dungeon” would sample half that. I’m confident you have no doubt about what I chose to do.

After ensuring my sample of days was representative, I calculated how much a battery could save my sister on each day and used that to calculate what her annual savings would be for battery capacities from 1 to 50 kWh, without setting a battery reserve. Then I made this graph:

The graph shows savings per kWh falling with increasing battery capacity. This happens gradually at first, becomes more noticeable after 10 kWh, and then after 20 kWh there’s only a small increase in total savings from each additional kWh of capacity. This occurs for two reasons:

1. As battery capacity increases, the number of days my sister will use all its stored energy decreases.
2. You only get 24 kWh of free electricity per day on a Solar Sharer plan.

This drop in savings per kWh happens for every home battery, but how it happens depends on circumstances. A home with higher electricity consumption, especially during the peak period, will see the falloff occur at a higher battery capacity. A large solar system also helps improve the return from having a battery. If my sister expanded her solar, it would modestly improve the return. But when she gets an EV — which she eventually will, despite her saying she doesn’t want one — then she should definitely increase her solar capacity, unless the cap for free electricity has increased significantly.

While the graph above shows how much you can expect to save per year by battery capacity, it doesn’t tell you how big a battery you should get. In a follow-up article next week, I’ll go into how to work that out, and reveal how much battery capacity I recommend my sister get.

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