Battery Sizing With Existing Solar: Why Most People Get It Wrong
Most people buy battery storage like it is a standalone upgrade. But if you already have solar panels, battery size is not a product choice, it is a numbers choice. The real question is how much electricity you still buy from the grid after the sun goes down, and whether your solar system can reliably refill the battery capacity you are paying for across the year.
This is where sizing goes wrong. Homes pick a Tesla Powerwall, Powerwall 3, or another home battery because it is popular, eligible for a battery rebate, or bundled in an easy installation package, then assume the capacity will automatically translate into lower electricity bills. In practice, the savings come from timing and fit: your energy usage pattern, your night usage, your solar export, and the seasonal difference between summer peak sun hours and winter output. If those numbers do not line up, you either buy too little capacity to cover the hours that matter, or you buy more solar battery capacity than your solar panels can regularly charge, which is why some batteries feel underused even though the system looks “big.”
This article breaks sizing down into a simple framework for solar batteries in Australia. First, we will clarify what you are sizing for, backup power during outages, bill savings, or both. Then we will walk through the key numbers, daily kWh, electricity use after dark, and solar export, and show how to translate them into a sensible battery size. Finally, we will pressure test it against the charging reality of your solar PV system in summer versus winter, so you end up with a solar battery system that matches your energy goals instead of a battery that only makes sense on a perfect sun day.
Solar Battery Sizing Goals: Backup vs Bill Savings
Bill Savings: Battery size only makes sense once you’re clear on what you’re optimising for, because “backup” and “savings” are not the same sizing problem. Bill savings sizing is about reducing the grid electricity you buy during the hours that cost the most, typically evenings and early mornings when your solar panels are not producing. In that mode, the best battery size is the amount of capacity you can actually cycle most days, because a battery that rarely fills or rarely discharges is just unused spend, not lower electricity bills.
Backup power: Sizing starts from a different question: what do you want to keep running during outages, and for how long? That pulls sizing toward a more resilience focused target, sometimes “backup days,” because you are designing for a worst case window, not only a normal weekday. It also forces clarity about priorities. Keeping fridges, lights, internet, and device charging going is a very different load profile to trying to run air conditioning, induction cooking, or electric vehicle charging during grid outages. The more you want to protect, and the longer you want it protected, the more battery capacity you need, regardless of whether you would normally use that capacity for daily savings.
Most households want both outcomes, but one should lead. If savings is the primary goal, you size around night usage and expensive hours, then add only as much extra capacity as your solar system can reliably refill for backup. If backup is the primary goal, you size for the loads you want protected, then treat bill savings as the secondary benefit that depends on your tariff, feed in tariff value, and how consistently the battery can be charged by your existing solar PV system.
What Size Battery Should I Get If I Already Have Solar Panels?
If you already have solar panels, the “right” battery size is the one that covers the grid electricity you actually want to avoid, and that your solar PV system can realistically recharge. That means you size from your usage profile first, then sanity check it against your solar export and your seasonal solar panel output. The aim is not to buy the biggest home battery you can afford; it is to buy enough battery storage capacity to materially change your imports and electricity bills, without paying for capacity your system cannot use.
Start with the number that drives almost everything: your night usage, meaning how much electricity you use from roughly late afternoon through to the next morning. This is the period when your solar system stops carrying the home and you start buying from the grid. If you have smart meter data, pull a few representative weeks and look at consumption by time of day. You can usually find this in your electricity retailer app or online account under Usage, Insights, or Energy use, as long as you have a smart meter. If you do not, you can still estimate from bills and habits, but smart meter data gives the cleanest energy profile. If your retailer portal does not show hourly or daily breakdowns, you can request interval data using your NMI via your retailer or local network distributor. Night usage is your baseline target because it defines how much battery capacity you can actually discharge for savings on a normal day.
Once you have night usage, decide what you want the battery to cover. Most homes are not trying to cover the entire night; they are trying to cover the most expensive or most painful part of the evening, or to reduce early morning imports. A simple sizing method is to set a coverage target, then translate that into usable kWh. For example, if your typical night usage is 10 kWh and your goal is to cover about 70 percent, you are aiming for around 7 kWh of usable battery capacity for savings. If your goal is closer to “cover most of the night most days,” you push the usable target higher. The key is using your own electricity use, not averages.
Now fold in your daily kWh. Daily energy consumption matters because it tells you whether your night usage is a small slice of your day or most of it. Two households can both use 20 kWh per day; one might use 15 kWh during daylight hours and only 5 kWh at night, the other might be the reverse. The first household usually needs less battery storage for bill savings; the second usually needs more, or it needs to address demand shifting first. Daily kWh is also a cross check. If the numbers you are using imply night usage that is almost your whole day, it usually means you are counting something twice, or you have a major overnight load that should be identified explicitly. Daily kWh is on every electricity bill, and many retailer apps show it as a daily average or as a day by day usage chart.
Then check solar export, because export is the most direct indicator of how much surplus solar energy is available to charge a battery. If you export a lot, a battery can capture more of that solar energy instead of sending it out for a feed in tariff. If you export very little, you may already be self consuming most of your solar panel output, which means the battery will often be charging from a smaller surplus or relying more on shoulder periods. Either way, export tells you whether a given battery size is likely to reach a high State of Charge on typical days. You can usually find export as a separate import vs export view in your retailer app if you have a smart meter; otherwise, your bill will show solar feed in tariff credits, which at least tells you your exported energy over the billing period.
A practical way to connect export to battery size is to think in “charge opportunity.” If your household regularly exports 8 to 12 kWh on clear days, that suggests there is enough excess solar to refill a battery in that range, assuming your daytime loads do not spike and your inverter and battery system can charge at a sensible rate. If export is more like 2 to 4 kWh most days, a very large battery capacity may not be used often unless you expand the solar system size, shift loads, or plan to add electrification loads that will change your energy usage. This is the part most battery size calculator tools gloss over; a bigger battery is only valuable if there is a reliable way to fill it. If your app does not show daily export clearly, use the bill credit as a rough starting point, then confirm with interval data once you have it.
The last input that matters, even before we get to summer versus winter, is your tariff structure. If your plan has high peak pricing, time of use, or real time pricing, a smaller battery that consistently covers the expensive hours can outperform a larger battery that rarely fills. If you are on a flat tariff with a strong feed in tariff, the savings maths shifts, because exporting solar energy might be relatively valuable compared to storing it. If you are with a retailer like Amber, the Amber app and market signals can make timing even more important, because charging and discharging decisions interact with price spikes and low price periods. The battery does not exist in a vacuum; it exists inside your electricity plan. Your tariff type is shown on your bill in the rates table, flat rate versus time of use peak and off peak, and it is usually repeated in your plan details inside the retailer app or online account.
Two important notes so you do not size to the wrong number. First, what matters is usable capacity, not just the headline “battery capacity” marketing figure. Every battery system holds some reserve, and performance depends on how it is configured. Second, if you care about outages, you may choose to keep a higher reserve State of Charge for backup power, which reduces how much capacity is available for daily savings. That is not a deal breaker, but it must be accounted for when you decide what battery size actually meets your energy goals.
So the sizing path is simple and defensible. Use smart meter data to quantify night usage, set a coverage target to convert it into a usable capacity number, use daily kWh as a cross check, then pressure test that number against your solar export to confirm your existing solar system can fill it often enough to matter. From there, choosing between solar batteries, whether lithium ion batteries like Lithium Iron Phosphate, or specific options like Tesla Powerwall, Tesla Powerwall 3, Sungrow SBR series, SunGrow batteries, Sigenergy SigenStor, SolarEdge compatible systems, or VoltX Energy, becomes a selection step, not a sizing guess. The next section checks the piece that trips up most solar owners: whether your solar system is big enough to charge that battery in summer and winter.
Is Your Solar System Big Enough to Charge a Battery? Summer vs Winter
When you already have solar, battery size should be checked against seasonal charging, summer versus winter. Summer flatters most solar batteries because peak sun hours are longer and solar panel output is higher, so the battery reaches a higher State of Charge more consistently; winter exposes the limit because shorter days, lower sun angle, cloudier weeks, and higher heating loads reduce surplus solar energy and solar export, so the battery fills later, fills less, or may not fill on typical days. The quickest sanity check is to compare a few summer days and winter days in your retailer usage data or monitoring portal and look at export; thin winter export usually means a large battery capacity will not refill reliably unless you shift demand into daytime or increase solar system size. For retrofits, an AC coupled battery can be a practical pathway and, where plan rules allow grid charging, it can top up in cheaper periods and discharge into peak pricing, which can soften winter performance even when solar charging is limited or your are inhibited by the size of your existing solar system.
When a Bigger Battery Actually Makes Sense
For backup, a bigger battery makes sense when you want more than “lights and internet” coverage, or you have a high baseline load that will drain a small battery quickly, and you want longer runtime through outages. The variables that need to stack up are your essential loads and their kWh over the outage window you care about, how many backup hours or backup days you are targeting, and whether the system can actually deliver it, meaning the battery and inverter setup is configured for backup, with enough power output headroom to run what you expect during grid outages.
For savings, a bigger battery only makes sense if you have consistently high night usage that a smaller battery would not cover, or you are adding after dark loads like electric vehicle charging, induction cooking, or electric hot water top ups. The variables that need to stack up are enough charge opportunity to refill the extra battery capacity, meaning regular surplus solar export on typical days, plus a tariff structure where avoiding peak pricing is worth more than exporting on a feed in tariff; in retrofits, an AC coupled battery system with plan approved grid charging can also make a larger battery workable by topping up in cheaper periods and discharging into peak pricing when winter solar output is limited.
