Picture running your own mini power station from your roof. Immune to electricity price rises. Saving the world at the same time. Great! Now we’re going to show you how to get started. By answering the number one question on the internet about backup power and going off-grid: How many solar batteries are needed to power a house? You’ll also get the best tips all in one place to save money (and space) on your solar battery system. No need to search for old power bills, just spend 5 minutes reading this article. We promise it’ll be worth it.

How many solar batteries are needed to power a house? A Summary

Here is how many solar batteries are needed to power a typical Australian house, state by state. Use this as a guide for your own circumstances.

SCENARIOS	QLD	VIC	TAS	NSW	SA	ACT
Grid tied solar battery (Nightime only)	Solar: 4 kW Solar panels: 12 Battery: 16.4 kWh Powerwalls: 1.2	Solar: 4.8 kW Solar panels: 14 Battery: 17 kWh Powerwalls: 1.3	Solar: 9.5 kW Solar panels: 27 Battery: 31.5 kWh Powerwalls: 2.3	Solar: 5 kW Solar panels: 15 Battery: 18.7 kWh Powerwalls: 1.4	Solar: 3.9 kW Solar panels: 12 Battery: 15.8 kWh Powerwalls: 1.2	Solar: 5.6 kW Solar panels: 16 Battery: 23.4 kWh Powerwalls: 1.8
Backup solar battery (1 day of backup)	Solar: 6 kW Solar panels: 18 Battery: 23.3 kWh Powerwalls: 1.7	Solar: 6.8 kW Solar panels: 20 Battery: 24.3 kWh Powerwalls: 1.8	Solar: 12.8 kW Solar panels: 37 Battery: 44.9 kWh Powerwalls: 3.3	Solar: 6.8 kW Solar panels: 20 Battery: 26.6 kWh Powerwalls: 2	Solar: 5.4 kW Solar panels: 16 Battery: 22.6 kWh Powerwalls: 1.7	Solar: 7.8 kW Solar panels: 23 Battery: 33.5 kWh Powerwalls: 2.5
Off grid solar battery (2 days no sunshine)	Solar: 11 kWh Solar panels: 31 Battery: 46 kWh Powerwalls: 3.5	Solar: 13.5 kW Solar panels: 39 Battery: 48.5 kWh Powerwalls: 3.6	Solar: 25 kW Solar panels: 71 Battery: 90 kWh Powerwalls: 6.7	Solar: 13.7 kW Solar panels: 40 Battery: 53.3 kWh Powerwalls: 3.9	Solar: 10.8 kW Solar panels: 31 Battery: 45.2 kWh Powerwalls: 3.3	Solar: 15.6 kW Solar panels: 45 Battery: 67 kWh Powerwalls: 5

Here’s what each of the different scenarios means:

1. Grid tied solar battery = to meet your nighttime energy use with solar and battery. The lowest power demand of our 3 scenarios.
2. Backup solar battery = to backup your entire energy use for 1 day. Longer than overnight, so more battery power is needed.
3. Off grid solar battery = to power your whole house for 2 days of no sunshine. By far the highest power demand.

5 surprising tips to save you money: A Summary

By focusing your aims and your battery and solar sizing, you’ll save money. Because you’re not buying power capacity you don’t need.

Here are our top 5 tips:

1. Where to start? Size your battery first. Then your solar panels. Why? Because your solar needs to be big enough to do 2 things: 1) meet your daily energy use & 2) charge your battery within a day. So it’s best to know your battery size first!
2. Size your emergency backup batteries and off grid batteries is to have just enough backup power to meet your needs. 1 day for emergency back up batteries. 2 days for off grid batteries.
   • Then size your solar to charge your battery within a day, if you can afford it.
3. Size your grid tied batteries to meet your nighttime energy use and eliminate most of your power costs.
   • Then size your solar to at least meet your entire daily energy needs with the solar system. That way, you can add a battery and eliminate most of your power bill.

Now that you’ve seen the answers, keep reading to find out what’s behind them. We’ll even show you how to work them out for yourself. It’s easy.

What is a solar battery?

A solar battery is a device that stores electricity, in this case made by solar panels. It holds the electricity for later use when the sun isn’t shining, like at night or on cloudy days.

Solar batteries are also called ‘solar generators’ or ‘portable power systems’.

Solar panels generate DC (direct current) power to charge the battery. An inverter system converts the DC power into AC power. AC power is what our appliances use.

When you need the stored energy, the solar battery releases it so you can use it in your home instead of getting electricity from the power grid.

Solar batteries are even helpful when you’re connected to the power grid. Because they let you use more of the solar power you generate, reducing your reliance on the grid and on ballooning power costs. This is why people that have rooftop solar like to add batteries these days.

Solar batteries usually use lithium-ion technology, which is good at storing a lot of energy in a small space. LifeP04 batteries are a type of lithium battery growing in popularity because they can cycle (charge) thousands of time, giving you more bang for your buck.

What do solar batteries do?

1. Solar batteries can power an entire home all of the time (off-grid solar battery system) or just some of the time (grid tied solar battery system).
2. A grid tied solar battery can also provide backup power in an outage. This will add between $1500 and $3000 to the install price of the solar battery. That extra cost is for wiring.
3. Backup solar battery systems may not be connected to your home or to the grid at all. They can be 100% portable. These are often referred to as ‘portable power stations‘. You can take them with you on road trips, camping, use them in RVs or for any outdoor activity. What a great feature!
4. We have a portable solar battery system – the Ecoflow River 2 Pro – that is the backup power for our Tiny House. The added portability means more bang for our buck. Because we can use the system for more than just backup power.

If you’ve gotten this far, you’re probably asking, Are solar batteries any good? We tackle the costs and benefits of solar batteries in this article.

Why do you want solar batteries?

If you’re thinking about solar, have you thought about why?

This is always your first step before investing in any equipment.

How may solar batteries you need, depends on why you want them in the first place. How you want to use them. Have a think about it now. Do you want:

1. A grid tied solar battery system?
2. Your own off grid solar battery power station?
3. A backup solar battery system (portable or fixed)?

To help you decide, we answer the question ‘how many solar batteries are needed to power a house’ for these 3 scenarios. Because there are vastly different power needs, costs and benefits to each.

But first, lets explain how to calculate how many solar batteries are needed to power your house. So you can too, when the time comes.

How many solar batteries are needed to power your house – a step by step guide

Tip #1 – Where to start? Size your battery first. Then your solar panels. Why? Because your solar needs to be big enough to do 2 things: 1) meet your daily energy use & 2) charge your battery within a day. So it’s best to know your battery size first!

Battery sizing in 4 steps

Follow these 4 steps to work out how many battery are needed to power your house, whether it’s for grid tied, off grid or backup power:

Step 1: Work out your energy consumption

Get your power bill and find your kWh energy usage figure. Try to find a bill that covers your peak season energy use. In cold places that’s in winter. In hot locations, its summer.

Bills are quarterly or monthly in Australia. Divide your energy usage on your bill by 30 (monthly bill) or 90 (quarterly bill) to get your average daily consumption in peak season. Make sure you add peak and off peak consumption. This will give your your Total Energy Use.

We use peak season energy use to make sure you have enough battery power when you need it most.

Step 2: Decide how long you need the battery power for

Do you want the battery to provide power when the grid is down? Or do you need to it to run your whole house in poor weather and no sunshine (off grid solar battery)? Or should it just meet your nighttime load to save on power bills?

Write down how many hours or days of battery power you need. The longer the time, the bigger and more expensive the battery. So keep reading for the best tips on how to reduce solar battery costs.

Step 3: Work out your energy storage needs (in kWh)

Tip # 2 – Size your backup batteries and off grid batteries to have enough backup power for when the grid goes down for a few hours. Or a few days of bad weather & no sunshine. Time matters!

To size a battery for backup and off-grid systems, multiply your daily energy use (kWh) by the desired backup duration (days).

Typically, it’s 1 day for grid tied backup power.

2 days for off grid backup power.

Given solar panels still produce between 25% and 45% of their average output on cloudy days, you’ll actually get more power than you plan for.

Here’s how to work out your energy storage needs for backup power:

Daily energy use x 2 (days) = battery size in kWh for off grid systems.

Tip # 3 – Size your grid tied batteries to meet your nighttime energy use and eliminate most of your power costs.

For grid tied batteries, its different. You need to work out your nighttime load (kWh) instead. Typically, you use around 70% of your power at night. Here’s how it works out:

Daily energy use x 0.7 = battery size (kWh) for grid tied systems.

Step 4: Account for ’round trip efficiency’ & ‘depth of discharge’

There are 2 things to know about solar batteries.

1. They lose energy when charging and discharging (round trip efficiency – RTE).
2. They are typically not discharged beyond a certain depth (depth of discharge – DoD) to prolong their lifespan.

Lithium batteries have a round trip efficiency of 90% and a DoD of 80% so lets use these figures.

RTO and DoD are important because they reduce the amount of delivered energy from a battery. So you have to factor them in. To do so, just divide the total energy storage from Step 3 by 0.9 and then 0.8.

Battery size in kWh / 0.9 / 0.8 = the adjusted battery size (kWh) you need.

If you’re in the US, battery size is often measured in Amperage. So you’ll need to take 2 additional steps:

Step 5 (US only): Choose the battery voltage

Determine the voltage of the battery system you intend to use. Common residential system use a 120 volt battery bank (in the US). Off grid systems could be 48V, 24V or even 12V. The choice depends on the specific system design and components.

Step 6 (US only): Calculate the battery capacity

Divide the adjusted energy storage capacity (from step 4) by the battery voltage (from step 5). This will give you the required battery capacity in ampere-hours (Ah).

Solar system sizing in 3 steps

Here are the steps to work out how much solar is needed to power your house. Don’t worry, we run through these with a live example down below to demonstrate:

Step 1: Work out your peak season total energy use

This is the same as Step 1 above. Use your own power bill (find your ‘energy consumption’ in kWh, or use the table below.

Step 2: Work out your peak season daily energy use

Tip # 4 – Size your grid-tied solar to at least meet your entire daily energy needs. That way, you can add a battery and eliminate most of your power bill.

We need to know our daily energy use. Follow the same process as in Step 2 above. Or use the table below.

Step 3: Calculate your solar system size using the solar output factor for your area

Solar output factor tells us how much energy we can expect a solar system to generate, based on its location. It’s a good guide to use here. You’ll find the solar output factor for all Australian capital cities in the table below.

Just divide your daily energy use (kWh) by the solar output factor to get the size of your solar system.

Solar system size (kW) = Daily energy use (kWh) / Solar output factor (kWh)

Tip # 5: For backup and off grid batteries, size your solar to charge your battery within a day, if you can afford it.

For backup and off grid solar batteries, its best to size your solar to your battery. To do this, simply divide your adjusted battery size (kWh) (battery sizing, Step 4) by the solar output (kWh) for your area. Here’s how:

Adjusted battery size (kWh) / solar output (kWh) = solar system size

Answer: How many solar batteries are needed to power a house?

The answer to this question is summarised in the table below.

If you don’t know your household energy use (or can’t be bothered to find it), use this table as a guide.

To make it easy for you, we show how many solar batteries are needed to power an average home, in different states of Australia. You’ll see big differences, depending on where you live. This is because energy use varies across Australia.

Southern states use way more energy in winter. But that’s also when and where solar generates less power. So in cold climates, you need more solar batteries. But stick with us to the end where we share tips to keep your solar battery costs low.

Solar batteries needed to power a house - typical household power use

States	Typical power use peak season (kWh)	Daily power use peak season (kWh)	Solar output (Capital Cities) kWh	Solar battery use	Solar size needed (kW)	Number of solar panels (rounded up)	Battery size needed (kWh)	Number of batteries (Tesla Powerwalls)
Queensland	1513 (Summer)	16.81	4.2	Grid Backup Off grid	4 6 11	12 18 31	16.4 23.3 46.7	1.2 1.7 3.5
Victoria	1572 (Winter)	17.46	3.6	Grid Backup Off grid	4.8 6.8 13.5	14 20 39	17 24.3 48.5	1.3 1.8 3.6
Tasmania	2913 (Winter)	32.26	3.5	Grid Backup Off grid	9.5 12.8 25	27 37 71	31.5 44.9 90	2.3 3.3 6.7
New South Wales	1726 (Winter)	19.18	3.9	Grid Backup Off grid	5 6.8 13.7	15 20 40	18.7 26.6 53.3	1.4 2.0 3.9
South Australia	1485 (Winter)	16.28	4.2	Grid Backup Off grid	3.9 5.4 10.8	12 16 31	15.8 22.6 45.2	1.2 1.7 3.3
Australian Capital Territory	2168 (Winter)	24.10	4.3	Grid Backup Off grid	5.6 7.8 15.6	16 23 45	23.4 33.5 67	1.8 2.5 5.0

Average peak season energy consumption numbers are from this 2020 report for the Australian Energy Regulator.

Next, lets run through the Queensland example in the table, to answer the question: “How many solar batteries are needed to power a house”.

1. Grid tied solar battery system

How many solar batteries are needed to power a house connected to the power grid?

Here’s how we worked it out so you can too:

Batteries

Around 30% of our energy use is in the daytime when the solar panels are generating. We use 70% of our power at night when they’re not. So here’s how to calculate our nighttime load:

16.81kWh (daily energy use) x 0.7 = 11.8kWh of nighttime load.

This means your battery should be big enough to deliver 11.77 kWh of power at night. But don’t forget, we still have to account for roundtrip efficiency (90% for lithium batteries) and depth of discharge (80% to 100%). Following Step 4, here are the calculations:

So then how many solar panels are needed to power your house and charge that battery?

Solar system

Step 1 above starts with our energy use. The average Australian household uses between 16 and 20kW hours of electricity a day in peak use season. But you can see in our table, it varies. Especially in cold climates like Tasmania and the ACT. Then it’s typically 25 to 33kWh a day!

Following the steps to size a solar system, you can see from the table that average home in Queensland needs at least a 4 kW system.

But most Australian’s upsize their solar. They get more than they need. Because the kWs are cheaper the more you can fit on your roof. According to the Clean Energy Council, the average grid tied solar system in Australia in 2022 was 8.8kW!

To get the number of solar panels in a 4 kW system, you divide the solar system size (kW) by the panel size (kW). We’ve used 350 watt panels. But rooftop solar panels can run from 300 watts to 450 watts. If you want to save space, more watts per panel will help you do so.

For example: 4kW / 0.35kW = 12 solar panels.

Huzzah! We’ve now worked out how many solar panels are needed to power our house (or an average house where we live using the table above).

2. Backup solar battery system

How many solar batteries are needed to power a house during a power outage?

Batteries

Firstly, we work out how much battery storage we need for our energy use:

Next we work out battery size:

Solar system

And then we calculate the solar system size based on the battery size:

3. Off grid solar battery system

How many solar batteries are needed to power a house off grid?

Batteries

Firstly, we work out how much battery storage we need for our energy use:

Next we work out battery size:

Solar system

Peak season energy use = 1513 kWh

Solar power output factor = 4.2 kWh

Here are the steps:

2. Determine the size of solar system needed based on battery size:

So, an 11 kW solar system and 46.7 kWh of battery storage will provide enough power to cover the average house in Queensland during peak energy use season, with 2 days backup power.

What to do when buying a solar battery

Tip # 4 – When buying a battery, pay attention to its maximum output. This is the total energy your battery can send out at once. And if your load > max output, your battery will trip off!

Here’s a real life example. The Tesla Powerwall has a max output is 5kW. If all the devices you want to ru at once are > 5kW, then your Powerwall won’t run it all. This isn’t a problem. It just means you have to manage your energy use when using your battery.

Here are our top things to pay attention to when it comes time to buy a solar battery:

1. The battery’s maximum output. This limits everything you can power from the battery at the one time. The Tesla Powerwall for example is a 13.5 kWh battery with a 5 kW maximum output. If you wanted to run a 7 kVa air conditioner, you couldn’t do that with one Powerwall. Maximum output adds together when you link a number of batteries together.
2. Battery chemistry. Lithium batteries come in different chemistries, such as lithium iron phosphate (LiFePO4) or lithium-ion (Li-ion). Each chemistry has its own characteristics, including energy density, cycle life, and cost. LiFeP04 is one of the best performing lithium chemistries.
3. The battery smarts. Built in battery management systems can help regulate the performance of your battery (voltage, current) and will protect your battery from overheating. A battery with a good App helps you monitor performance and get the best, most efficient use of the battery power.

How to save money on a solar battery system

Have a critical backup circuit

Power only your critical loads for backup. Get an electrician to put all of your critical loads on one circuit. Things like your fridge, freezer, a few lights, powerpoint, internet. Then have your solar battery connected to this circuit. This way, you can have critical backup power but need a smaller solar battery.

Diversify large loads

Our table of average energy use shows that Tasmanian’s use 32 kWh a day on a average in winter.

We live in Tasmania. Inland Tasmania, so it’s cold. In July 2022 (peak winter), we used an average of 28 kWh or power a day. Most from hot water and space heating. Even though most of our space heating is from a wood heater. We do have electric hot water.

That kind of energy use needs a lot of solar batteries. A 9.5 kW solar system and 2.3 Tesla Powerwalls just for a grid tied solar battery in fact. That’s double most other states.

Go off grid, and solar battery becomes cost prohibitive for most Tasmanians.

But you can diversify your energy use to reduce how many solar batteries you need in cold climates.

Loads with heating elements like heaters and hot water systems use a lot of electricity. Instead, by using a wood heater for space heating, and continuous gas hot water you can reduce your power use by more than 30%.

The continuous gas hot water system in our Tiny House for example uses around 50 watts of electricity and is easy to run off a battery. Compare that with the 3000 watt electric hot water system in our house!

Reducing your energy use by 30% makes solar battery backup in cold climates more affordable.

Instead of 12.8kW of solar power, you’d only need 6kW. You’d be able to backup your total energy use for 1 day with 2 Tesla Powerwalls of battery storage instead of 3.3.

Improve energy efficiency at home

We bought an old cottage when we moved to Tasmania. It leaked heat like a sieve.

So we installed thicker installation in the cladding as well as in the floor. We replaced the single glazed windows with double glazed ones. Our winter power bill dropped noticeably. And so would the cost of a solar battery system.

In a Nutshell

We’ve given you the answers you need to the number one question about backup power and energy independence on the internet: How many solar batteries are needed to power a house? No matter where you live, if you’re interested in a grid-tied, backup, or off-grid solar battery system, you now have the numbers you’ve looking for. The next step is up to you. To take the plunge. Make the investment. Save the planet, and some money. What are you waiting for!

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