On your bill as average daily use.
North-facing and unshaded.
Average daily peak sun hours.
How much you use as it is generated.
Advanced options
What you get for exported power.
75–80% is realistic. 100% is not.
About 1.95 × 1.13 m.
0 to estimate from system size.
After rebates, if any apply.
0 to skip battery sizing.
This solar panel calculator sizes a system from real derate factors rather than nameplate ratings, then shows the thing that actually determines what you save — how much of the power you use as it’s generated, rather than how many panels you fit.
The short answer
System size = daily kWh ÷ (peak sun hours × derate). Using 20 kWh a day in Sydney needs about 5.8 kW — 14 panels of 440 W.
But the same system is worth $818 a year at 20% self-consumption and $1,898 at 70%. Identical hardware, 2.3× the value. When you use the power matters more than how much you generate.
On this page
How to use this solar calculator
Four ways in, depending on what you know:
- From my bill — enter the amount and period, and it works back to daily kWh
- Daily kWh — printed on most bills as average daily usage
- A system size — you’ve been quoted something and want to check it
- Roof space — work out what will physically fit
Then set your location for peak sun hours, your electricity rate, and — the input that matters most — your self-consumption percentage. If you don’t know it, 30–40% is typical for a household that’s out during the day.
Why a 6.6 kW system doesn’t produce 6.6 kW
Panels are rated under Standard Test Conditions: 1,000 W/m² of irradiance, cells at 25 °C, air mass 1.5. Those conditions essentially never occur on a roof, and the gap is not small.
| Loss | Typical | Why |
|---|---|---|
| Temperature | 10% | Cells run 45–65 °C on a roof, not 25 °C |
| Inverter efficiency | 3% | DC to AC conversion, 97% typical |
| Soiling | 3% | Dust, pollen, bird mess. Worse in dry spells |
| Wiring and mismatch | 3% | Cable runs, panel-to-panel variation |
| Minor shading | 2% | Vents, aerials, a chimney at some hour |
| First-year degradation | 2% | Light-induced degradation, then ~0.5%/yr |
| Compounded | 78% | These multiply, they don’t add |
So a 6.6 kW array behaves like 5.2 kW of real delivery. Industry practice puts the derate at 75–80%, and this build lands at 78% from the individual losses.
Panels lose about 0.35% of output per °C above 25. At a 55 °C cell temperature — ordinary on a summer roof — that’s 89.5% of rated output.
Which means hot sunny climates don’t gain as much as their sun hours suggest. Phoenix still beats Seattle comfortably, but the margin is narrower than the irradiance difference alone implies. Panels like bright conditions, not hot ones.
Peak sun hours
A peak sun hour is one hour at 1,000 W/m² — the intensity panels are rated against. It is not the number of hours the sun is up, because early and late sun is far weaker.
| Location | Peak sun hours | 6.6 kW annual output |
|---|---|---|
| Phoenix, AZ | 6.5 | 12,214 kWh |
| Darwin | 5.8 | 10,898 kWh |
| Los Angeles | 5.6 | 10,523 kWh |
| Perth | 5.4 | 10,147 kWh |
| Brisbane | 5.0 | 9,395 kWh |
| Adelaide | 4.8 | 9,019 kWh |
| Sydney | 4.4 | 8,268 kWh |
| New York | 4.2 | 7,892 kWh |
| Melbourne | 4.0 | 7,516 kWh |
| Hobart | 3.6 | 6,764 kWh |
| Seattle | 3.4 | 6,389 kWh |
| London | 2.8 | 5,261 kWh |
The spread is more than 2:1 between the extremes. Orientation and tilt matter too — in the southern hemisphere you want north-facing at roughly your latitude, and a west-facing array generates less overall but produces it in the afternoon when household demand is higher, which can be worth more.
Sizing the system
kW = Daily kWh ÷ (Peak sun hours × Derate)
Panels = kW × 1,000 ÷ Panel watts, rounded up
Derate of 0.78 · panels round up, so actual size exceeds the target
Worked example — $600 a quarter in Sydney at 33c/kWh
What actually drives your savings
This is the part quotes tend to gloss over, and it matters more than panel brand, inverter brand or a kilowatt either way on system size.
Feed-in tariffs have collapsed. You pay around 33c to buy a kilowatt hour and receive about 5c to sell one — a 7× difference. So a kWh you consume as it’s generated is worth seven times one you export.
| Self-consumption | Used | Exported | Annual value | Payback |
|---|---|---|---|---|
| 20% | 1,543 kWh | 6,173 | $818 | 6.8 yr |
| 30% | 2,315 kWh | 5,402 | $1,034 | 5.4 yr |
| 40% | 3,087 kWh | 4,630 | $1,250 | 4.4 yr |
| 50% | 3,858 kWh | 3,858 | $1,466 | 3.8 yr |
| 70% | 5,402 kWh | 2,315 | $1,898 | 2.9 yr |
| 90% | 6,945 kWh | 772 | $2,330 | 2.4 yr |
Same 6.16 kW system, same 7,717 kWh generated, Sydney, 33c retail and 5c feed-in.
The value goes from $818 to $1,898 — 2.3× — and payback from 6.8 years to 2.9. Nothing about the hardware changed. Only when the power got used.
Doubling the array instead would roughly double generation, but at 20% self-consumption most of the extra gets exported at 5c, so the additional value is small. Shifting load is nearly always the cheaper improvement.
Shifting your usage
Self-consumption is the one variable you control for free. Things that move load into the middle of the day:
- Dishwasher and washing machine on a delay timer to run at midday rather than evening
- Pool pump — often the single largest shiftable load in a house. Run it 10am–3pm
- Hot water on a timer or a heat pump unit set to heat at midday instead of overnight
- EV charging at home during the day where the pattern allows
- Pre-cooling — run air conditioning harder at midday and coast through the evening
A household that moves the dishwasher, washing and pool pump into daylight can realistically go from 30% to 50–60% self-consumption. On the example above that’s around $540 a year for changing some timer settings.
Roof space
| System | Panels at 440 W | Area | Sq ft |
|---|---|---|---|
| 3 kW | 7 | 15.4 m² | 166 |
| 5 kW | 12 | 26.4 m² | 284 |
| 6.6 kW | 15 | 33.0 m² | 355 |
| 10 kW | 23 | 50.6 m² | 545 |
| 13.2 kW | 30 | 66.0 m² | 710 |
That’s panel area only. Real installations need access paths, setbacks from roof edges and ridges, and clearance from vents and skylights — so allow roughly 20–25% more roof than the panel area suggests. Use the roof pitch calculator to convert your plan footprint into actual sloped roof area first.
Battery sizing
kWh = Evening and overnight load ÷ Depth of discharge
Size to the evening, not to daily generation
A battery’s job is to move your midday surplus into the evening. So the number that sizes it is what you use after dark — typically 8–12 kWh for a household — not how much the panels make.
At 90% depth of discharge, 8 kWh of evening load needs about 8.9 kWh of nameplate capacity. Sizing to daily generation instead buys storage that sits empty most of the year.
Every kWh cycled through it converts an exported kWh into a self-consumed one, so it earns the difference — around 28c per kWh at 33c retail and 5c feed-in, not the full 33c.
Multiply that by realistic annual cycles and compare against the installed price before committing. Batteries have got much better, but the arithmetic still needs doing rather than assuming.
Payback
Payback is system cost ÷ annual saving, and it’s sensitive to inputs people guess at. Three things move it most:
- Self-consumption — 6.8 years at 20%, 2.9 years at 70% on the same system
- Your actual electricity rate — not the headline rate, the one after daily supply charges and any discount
- Rebates — where they apply, these change the numerator substantially
Rebate schemes, feed-in tariffs and export limits vary by country, state and network, and they change. This calculator lets you enter the installed cost after any rebate rather than trying to model schemes that will be out of date shortly. Get real quotes and put those numbers in.
Solar sizing mistakes to avoid
- Using nameplate output. A 6.6 kW system delivers like 5.2 kW.
- Confusing peak sun hours with daylight hours. Sydney gets 4.4, not 12.
- Ignoring self-consumption. It’s worth more than 2× on savings.
- Sizing a battery to daily generation. Size it to evening load.
- Assuming hot climates are best. Panels lose 0.35% per °C above 25.
- Forgetting roof access and setbacks. Allow 20–25% over panel area.
- Believing a quoted payback without checking the self-use assumption.
- Oversizing for export. At 5c, exported kWh barely move the numbers.
Frequently asked questions
What size solar system do I need?
Daily kWh divided by peak sun hours times the derate factor. Using 20 kWh a day in Sydney at 4.4 sun hours and a 0.78 derate needs 5.8 kW — 14 panels of 440 W. Check the sizing against your roof space and any export limit on your network.
Why doesn’t my solar system produce its rated output?
Because panels are rated at 25 °C cell temperature under 1,000 W/m², which doesn’t happen on a roof. Temperature costs about 10%, inverter losses 3%, soiling 3%, wiring and mismatch 3%, minor shading 2% and first-year degradation 2%. Compounded, a real system delivers about 78% of nameplate.
What are peak sun hours?
Hours equivalent to 1,000 W/m², the intensity panels are rated against — not hours of daylight. Sydney averages 4.4 a day, Phoenix 6.5, London 2.8. Early and late sun is much weaker, which is why the figure is far lower than the hours the sun is up.
What is self-consumption and why does it matter?
The share of your generation you use as it’s produced rather than exporting. It matters because you pay around 33c to buy a kWh and receive about 5c to sell one. The same system is worth $818 a year at 20% self-consumption and $1,898 at 70% — 2.3× from timing alone.
How can I increase self-consumption?
Move flexible loads into the middle of the day. Dishwasher and washing machine on delay timers, pool pump running 10am to 3pm, hot water heating at midday rather than overnight, EV charging during daylight, and pre-cooling with air conditioning. Going from 30% to 55% is realistic and costs nothing but timer settings.
How much roof space do I need for solar?
About 2.2 m² per 440 W panel, so 6.6 kW needs 15 panels and 33 m² — around 355 sq ft. Add 20–25% on top for access paths, edge setbacks and clearance around vents. Only unshaded roof facing the right direction counts.
How big a battery do I need?
Size it to evening and overnight consumption, not daily generation. Divide that load by the depth of discharge: 8 kWh of evening use at 90% DoD needs about 8.9 kWh of nameplate capacity. A battery sized to generation sits empty most of the year.
Does heat reduce solar panel output?
Yes, by about 0.35% per °C above 25. Cells reach 45–65 °C on a roof, so a 55 °C cell produces 89.5% of its rated output. Panels want bright conditions, not hot ones — which is why hot climates gain less than their irradiance figures alone suggest.
Is it worth oversizing my solar system?
It depends entirely on self-consumption. If you use only 20–30% of what you generate, extra capacity mostly exports at around 5c and adds little value. Increasing self-consumption on the system you have is usually the cheaper improvement. Oversizing makes more sense with a battery or an EV to soak up the surplus.
What is a good payback period for solar?
Three to six years is common, but the range comes almost entirely from self-consumption. The same system pays back in 6.8 years at 20% and 2.9 years at 70%. When you see a quoted payback, ask what self-consumption rate it assumes — it’s the assumption doing most of the work.
How many panels is 6.6 kW?
Fifteen panels at 440 W, needing about 33 m² of roof. Panel wattage has risen steadily, so older 6.6 kW systems used 20 or more panels of 330 W. Check the actual wattage quoted rather than assuming a count.
Which direction should solar panels face?
Toward the equator for maximum total generation — north in the southern hemisphere, south in the northern — at roughly your latitude in tilt. West-facing generates less overall but produces it in the late afternoon when household demand peaks, which given the gap between retail and feed-in rates can be worth more.
Sources and method
- Derate built from individual loss factors compounded multiplicatively, giving 78% — within the 75–80% range used in industry practice
- Temperature coefficient of −0.35%/°C is typical for monocrystalline silicon; check the panel datasheet for the exact figure
- Peak sun hour figures are annual daily averages; seasonal variation is substantial, particularly at higher latitudes
- Self-consumption comparison holds system size and generation constant, varying only the split between used and exported energy
- Rebates, feed-in tariffs and export limits vary by jurisdiction and change frequently — enter your own post-rebate cost and current tariff rather than relying on defaults
Related calculators
Roof Area CalculatorHow much roof you actually have
Pool Volume CalculatorThe pool pump is a big shiftable load
Insulation CalculatorCut the load before generating
Square Footage CalculatorMeasure available roof
Cubic Yards CalculatorVolume for any material
How this solar calculator works out its numbers
Sizing divides daily consumption by peak sun hours times the derate factor, then rounds up to whole panels — so the delivered system is usually slightly larger than the calculated requirement, which the results reflect rather than hide.
Generation is actual system size × peak sun hours × derate × 365. The derate is a single compounded figure rather than a stack applied separately, because the losses multiply: 3% inverter loss on top of 10% temperature loss leaves 87.3%, not 87%.
Savings split generation into consumed and exported at your self-consumption rate, valuing each at the appropriate tariff. The comparison table then re-runs that split across six rates on the same system, which isolates the effect of timing from every other variable.
These are planning estimates, not a quote. Real output depends on orientation, tilt, shading through the day and across seasons, panel and inverter specifics, and local weather in a given year. Peak sun hour figures are annual averages and winter output can be half of summer. Export limits, rebates and tariffs vary by network and change often. Use this to sanity-check quotes and understand which variables matter — then get real numbers from installers who have looked at your roof.