How Many Solar Watts Do I Need for Solar Panels in My Home?

Solar watts are the rated power of your solar panels, a measure of how much electricity each panel can produce in full sunshine. Add up the rating of every panel on your roof and you have the size of your solar system.

If you have solar panels, or you’re planning to buy them, that number decides how much of your power bill the system can cover. Too few watts and you keep buying power from the grid, while too many panels export surplus power, which earns less than the price you pay for it.

Take a Brisbane home that uses 18 kilowatt-hours (kWh) of electricity a day. Here’s what that means in terms of solar watts:

• 18 kWh a day divided by about 5 peak sun hours gives 3.6 kilowatts (3,600 watts) of solar panels
• Divide by 0.8 to allow for real-world losses and you get 4.5kW
• Add a 15% buffer and you land on about 5.2kW, which is 13 panels rated at 400W each

That’s how you calculate solar watts for solar panels. Read more of our guide to learn more about why solar watt calculations are important if you have solar panels.

How to Calculate Your Solar Power Needs in Five Steps

To sum up: divide your daily electricity use by your area’s peak sun hours, then allow for losses and add a buffer.

1. Find your average daily use in kWh on your power bill.
2. Divide it by the peak sun hours for Brisbane, which average about 5 across a full year.
3. Divide the result by 0.8 to cover real-world losses.
4. Add a 15% to 20% buffer.
5. Divide by the wattage of one panel to get a panel count.

What Watts and Kilowatt-Hours Mean on Your Power Bill

A watt (W) measures how fast something uses or makes electricity at one moment. 1,000 watts make one kilowatt (kW). A kilowatt-hour (kWh) measures energy over time, so a 1,000W heater running for one hour uses 1 kWh.

Solar panels are sold in watts, and your bill counts kWh. Peak sun hours link the two. One peak sun hour is an hour of full-strength sunlight, which is 1,000 watts landing on every square metre.

A 400W label describes the panel in lab conditions, with full-strength light and the panel itself at 25°C. Real roofs rarely match those conditions, so the calculation adds an allowance for losses.

Find Your Daily Use From Your Power Bill

Most bills show an “average daily usage” line in kWh. If yours only shows a total, divide the kWh for the billing period by the number of days.

Twelve months of bills give the best picture. Here’s where Queensland homes sit, based on smart meter data from 2025:

• The median household drew about 15 kWh a day from the grid
• One in five homes used less than 9 kWh a day
• One in five homes used more than 25 kWh a day
• A pool added roughly 6 kWh a day on average

Those are grid figures, so a home that already has a solar system may use more in total. Air conditioning in summer and electric heating in winter both push daily use up, so swap in your own number for our 18 kWh example.

Peak Sun Hours in Brisbane and Why They Change With the Seasons

Weather records measure sunlight in megajoules per square metre. Divide that figure by 3.6 and you have peak sun hours. Brisbane averages about 5.1 peak sun hours a day across a year.

Winter has fewer hours of sunshine and the sun sits lower, so output naturally drops. Overcast days and rain cut the light too, and those dips are already inside the monthly averages above.

That’s why the method uses the full-year average. Your solar system produces extra in summer and less in winter, and the yearly total is what saves you money on your bill.

A Worked Example for a Brisbane Home Using 18 kWh a Day

A solar inverter is the box that converts the direct current (DC) your panels make into the alternating current (AC) your home runs on. It loses a little energy in the process, and so do the wiring, the dust on the glass and the heat of a summer roof.

Common modelling allowances put the panel-side losses at around 14%, before the inverter. That makes 80% to 85% a sound figure for the whole system, and 0.85 instead of 0.8 gives about 4.9kW in our example.

The buffer covers panels losing a little output over the life of the system, plus your future energy needs when a new appliance or an electric car arrives.

As a check, a 5.2kW solar system produces about 21 kWh on an average Brisbane day (5.2kW × 5.1 hours × 0.8). That covers the 18 kWh with room to spare. Expect roughly 14 kWh in June and 28 kWh in December.

Factors That Change How Much Solar Power You Need

• Shading: A tree or a neighbouring building shading part of a panel in the morning or afternoon can pull down the output of the panels wired with it. Ask your installer to assess shade at the site visit.
• Roof Direction and Tilt: A north-facing roof gets the most sun across the year. East and west roofs produce slightly less in total but spread output across the morning or afternoon.
• Location: Peak sun hours differ across Australia, so a Brisbane figure won’t suit a home in Melbourne or Perth.
• Heat: Panels lose a small share of their output for each degree above 25°C, typically 0.3% to 0.4% per degree on current datasheets. A hot summer roof produces less per sun hour than the lab rating suggests.
• Dust, Debris, and Solar Panel Cleaning: Leaves, bird droppings and dust block sunlight. Rain washes off much of it, and heavy rain also brings cloud, which cuts output on its own. If build-up stays put, leave solar panel cleaning to a licensed professional rather than climbing up there yourself.
• Panel Quality and Ageing: Warranties differ by maker. REC’s published warranty for its Alpha Pure-R panels, for example, guarantees at least 92% of rated power in year 25.

Solar Watt Calculations for Single Panels (1kW, 5kW and 10kW)

This table uses Brisbane’s yearly average and the 0.8 allowance for losses. Treat the energy and appliance figures as estimates, since your home and roof will differ.

Will a 400W solar panel run a fridge?

Yes, in daylight. A fridge-freezer works fine on one panel in decent sunshine, because it draws roughly 80W to 200W when its compressor is on. The fridge keeps running at night, so that share comes from the grid or a battery.

What will 1,000 watts of solar run?

About 4 kWh a day on average, which covers a fridge, a TV, lights, fans and a laptop or two through daylight hours. A kettle alone draws roughly 1,800W to 2,400W, so heavy appliances need a bigger array. These are estimates, so check your own appliance labels.

Is 10kW of solar enough to run a house?

In energy terms, yes for most homes. A 10kW system (the same thing as 10,000 watts) makes around 41 kWh on an average day, and even the busiest 20% of Queensland homes use about 25 kWh or more from the grid.

Solar supplements grid power rather than replacing it, because panels only produce power in daylight. The inverter and network connection also cap how much a home can run or export at once.

How Panel Watts Add Up to Solar System Size

Your solar system size is the panel count multiplied by the rating of one panel, so thirteen panels at 400W make 5.2kW. Current panels are commonly rated around 400W to 450W.

Systems are sold in common sizes such as 5kW, 6.6kW and 10kW, so your sum usually rounds to the nearest one. How many panels fit on your roof is a separate question that our panel size guide covers.

How Many Solar Watts Does a Battery Need for Backup Power?

A battery bank is one battery or several wired together to store solar power for later. Battery capacity is how much energy it holds, in kWh, while its power rating in kW is how fast it can deliver that energy. A 5 kWh battery with a 2.5kW rating runs a 2.5kW load for two hours.

Your panels have to refill whatever the bank gives out, so it adds to your solar watts. This section covers a shed, caravan or small backup circuit. Whole-house batteries and full off-grid design are separate jobs that our battery sizing and off-grid guides cover.

What your solar system does during power outages

A standard grid-connected solar system switches itself off during power outages, with or without a battery. The shutdown protects line workers from live power on downed wires, so a sunny day won’t keep your lights on.

Backup needs a battery and an inverter designed for it, wired to run chosen circuits such as the fridge and security system. How long it lasts depends on battery capacity, your solar system size and what you run.

Sizing battery capacity for a shed or caravan bank

Take a load of 1.2 kWh (1,200 watt-hours) a day. Lead-acid batteries, including AGM batteries (absorbent glass mat, a sealed type of lead-acid design), are generally drained only about half before they’re recharged. Lithium iron phosphate (LiFePO4) batteries can give up around 80% of their capacity.

Round-trip efficiency is the share of energy you get back out of a battery. Makers quote about 80% for lead-acid and 92% for lithium. The table sizes the refill for June, because the bank has to fill in the sunlight-poor month.

Cold matters little in Brisbane. In cooler regions it trims usable capacity, and some lithium batteries can’t be charged below 0°C.

What keeps a battery charge topped up?

A charge controller sits between the panels and the battery. It manages the flow of current, which is crucial for reaching a full charge without overcharging, and it needs to be set for your battery type.

Charging speed follows your panel watts. In June, 500W of panels adds only about 1.3 kWh to the bank on an average day, so a bigger bank refills over several days. That’s why the sizing above uses June rather than the yearly average.

Voltage matters too. Most caravan and shed banks run at 12 volts (12V), and larger ones often use 24V. The benefits of 24V are lower current for the same watts and smaller cable losses, because current equals watts divided by volts. A 600W array carries about 50 amps into a 12V bank and about 25 amps into a 24V bank.

How Volteam Sizes Solar Around Your Bill and Your Roof

Volteam installs solar under QLD Electrical License #72286 and Clean Energy Council (CEC) accreditation. Our 10,000-plus installations span the whole of South East Queensland, so sizing a roof is work we do daily.

We start from your bill and your roof, not from a ready-made package. We look at how much you use, when you use it, which way your roof faces and what shades it. Then we recommend a size and panel count, from 5kW up to 13kW, and talk it through with you.

Frequently Asked Questions

Should I size solar for my whole bill or only my daytime use?

Panels only produce power while the sun is up. The cheapest solar power is the power you use as it’s made, because exports are usually paid at a lower rate than the price you pay to buy. If you’re mostly home in the evenings, a smaller system or a battery might suit you better.

Will Energex let me install any solar system size I calculate?

Energex sets the limits for South East Queensland. A home with the common supply setup, where one live wire comes in from the street, can usually carry up to 10 kVA of inverter capacity (kilovolt-amps, close to kW for home solar).

Exports to the grid are limited to 5kW on a standard connection, and an accredited installer confirms what applies at your address.

How does a new pool or an electric car change the maths?

Add the extra kWh to your daily use before you start. As a quick rule, divide the extra daily kWh by about 4 to get the extra kW of panels. A pool adding 6 kWh a day means around 1.5kW more, or four more 400W panels.

Does more battery capacity mean longer backup during power outages?

Yes, up to a point. Capacity sets how long the battery can supply power, while its kW rating sets which appliances can run at once. A large battery with a low rating can still struggle to start an air conditioner.