Sizing the systems
Solar panel calculator
A 400 watt panel does not make 400 watt-hours in an hour of sun — it is rated in a laboratory at 25°C with the light square on, and no roof has ever been in that condition. Between the rating and your meter go 14 per cent of system losses, 4 per cent of inverter, and three to nine per cent of cell temperature. Then there is the roof: west-facing makes 78 per cent of south-facing, which is in no proposal because the installer cannot change it. This works out what your roof will actually make.
Written and edited by Pablo Ruiz Quintero
Last reviewed 2026-09-21 · Nothing you type leaves your browser
Your house and your roof
The two numbers that decide this are on your electricity bill and on your roof. The second one — which way it faces — is not in any proposal, and it moves payback by years.
Add up twelve bills, or take one month and multiply by twelve if it was an average one. The US average is about 10,800.
Washington DC, Nashville, Kansas City, Portland
How much of your usage to cover. A hundred is the usual ask; more rarely pays where exports are credited below retail.
Stand with your back to the house looking out over the slope. This is the single input a proposal will not show you.
6:12. The tilt curve is much flatter than people expect — this is rarely the problem.
The current volume product. About 22.6 ft² of roof each, and the size most racking is designed around.
Of the slope that faces the right way. Leave blank if you have plenty.
Every kilowatt-hour you export cancels one you import, at the same price. It is the best case and it is disappearing state by state.
Used only to pick a regional labour rate. It stays in your browser.
Net cost
$19,096
22 panels · 8.8 kW · $3 a watt
11,288 kWh in year one — 103% of what you use
About 10 years to pay back, saving $1,919 in the first year.
Nameplate is not production
The gap a proposal does not show you.
- Nameplate × sun hours14,454 kWh
- Roof orientation costs−0%
- Losses, inverter and heat−22%
- What you actually get11,288 kWh
1283 kWh a year per kW installed, here, on this roof. Ask any proposal for that number — a good installer says something near it without pausing.
The same system, on a different roof
| Facing | Of south | kWh / yr | Payback |
|---|---|---|---|
| Due south | 100% | 11,288 | 10 yr |
| Southeast | 95% | 10,723 | 10.5 yr |
| Southwest | 96% | 10,836 | 10.4 yr |
| Due east | 78% | 8,804 | 12.8 yr |
| Due west | 78% | 8,804 | 12.8 yr |
| Due north | 52% | 5,870 | 19.1 yr |
Identical hardware, identical price, different roof. This is the variable nobody can sell you and nobody puts in a proposal, and it is worth more than the brand of panel.
Where the output goes
Every step is known in advance, and none of it is optional.
Shopping list
- 400 W — standard residential panels
8.8 kW of nameplate · about 622 ft² of roof with spacing and setbacks22
What it costs
National average. Panels, inverter, racking, electrical, permits and installation, with the 30 per cent federal credit already taken off. A main panel upgrade, a new roof and battery storage are separate and ticked above. Utility interconnection fees and any HOA approval are not included.
$0 a kWh over twenty-five years against $0 from the utility today — that comparison, not the payback year, is the honest one, because it does not assume what the utility will charge in 2040. If the roof is near the end, do that first, and insulation reduces the bill you are trying to cover for a tenth of the money.
Worth knowing
- 22 panels, 8.8 kW of nameplate, making about 11,288 kWh in year one. Multiplying nameplate by peak sun hours — which is what a hopeful quote does — gives 14,454. The gap is 22 per cent, and none of it is optional.
- Where it goes: 14 per cent in system losses, 4 in the inverter, 5 in cell temperature, and 0 because this roof faces due south at 26.6°. Ask any quote what derate factor it used; if the answer is a blank look, the number is nameplate.
- The pitch is doing almost nothing wrong: 6:12 gives 100 per cent of optimal here. That is the reassuring part of this page — the tilt curve is very flat, so a roof at the wrong angle costs a few per cent, while a roof facing the wrong way costs tens.
- $3.10 a watt installed, so $27,280 before anything else. About 41 per cent of that is soft cost — permits, design, sales and overhead — which is why the same hardware installed in Australia costs roughly half. It is worth knowing what you are actually buying.
- Panels last thirty years and a shingle roof twenty. Putting an array on a roof with eight years left means paying to remove and refit it — always price the roof first. Nothing on this page accounts for that unless you tick it, and it is the single most expensive thing people discover afterwards.
- $30 per cent federal credit taken as $8,184. The federal residential clean energy credit. It is a credit against tax owed rather than a rebate, so it is worth the full amount only if your tax bill is at least that large — and it can be carried forward if it is not. Verify the current rate before signing anything: this is the number most likely to have moved.
On this page — 5 sections
Nameplate is not production
The short version
- A system makes about 80 per cent of what nameplate times sun hours suggests, before orientation is counted.
- A west-facing roof makes 78 per cent of a south-facing one. It is in no proposal, and it moves payback by years.
- Roof pitch barely matters. Flat gives 89 per cent of optimal, very steep 91. The direction is what costs you.
- How exports are paid moves payback more than the price of the panels.
A 400 watt panel does not make 400 watt-hours in an hour of sun. It is rated in a laboratory at 25°C, at exactly 1,000 W/m², with the light hitting it square on — and no roof anywhere has ever been in that condition.
Everything between that rating and your meter is known in advance, and it is the part a hopeful proposal leaves out.
kWh a year = kW × peak sun hours × 365 × orientation × derate
- peak sun hours
- the day's solar energy per square metre ÷ 1,000 W/m² — four to six and a half across the US
- orientation
- how far your roof is from due south at the optimal tilt, as a fraction
- derate
- system losses (14%), inverter (4%) and cell temperature (3–9%) — together about 0.80
Worked through — 8.8 kW on a south roof at 4.5 peak sun hours: 8.8 × 4.5 × 365 = 14,454 kWh if you stop there. With a 0.78 derate it is 11,288 — and the 3,166 kWh difference is not pessimism, it is arithmetic everybody in the industry knows.
| Where it goes | Loss | Why |
|---|---|---|
| Soiling | 2% | Dust, pollen and bird mess between rains. Worse in a dry climate, precisely where the sun is best. |
| Shading | 3% | The default allowance for partial shade. A single tree branch across one panel can cost far more than this if the inverter is a string inverter rather than microinverters. |
| Module mismatch | 2% | No two panels are identical, and a string performs like its weakest member. |
| DC and AC wiring | 2% | Resistance in the run from roof to inverter to panel. Longer runs cost more of this than people expect. |
| Connections | 0.5% | Every connector is a small resistance. |
| Light-induced degradation | 1.5% | Silicon loses a little output in its first hours of sun, permanently. It is normal and it is already in the warranty. |
| Availability | 3% | Days the system is off — an inverter fault, a utility outage, maintenance. Three per cent is about eleven days a year. |
National Renewable Energy Laboratory, PVWatts Calculator, version 8 · checked 2026-09-21
The one input nobody puts in a proposal
An installer can offer you a better panel, a better inverter, a better price. What they cannot offer you is a different roof — so the direction yours faces, which is the single largest variable after location, appears in no proposal anywhere.
It is not a small effect. A west-facing roof produces 78 per cent of what the same roof would facing south, which means 28 per cent more panels — and 28 per cent more money — for the same electricity.
| Roof faces | Of due south | Panels for the same output |
|---|---|---|
| Due south | 100% | 22 |
| Southeast | 95% | 24 |
| Southwest | 96% | 23 |
| Due east | 78% | 29 |
| Due west | 78% | 29 |
| Northwest | 62% | 36 |
| Due north | 52% | 43 |
One nuance worth knowing where exports are badly paid: a west roof produces less in total, but it produces later in the day — closer to when a house actually uses power and when the grid is short. Under avoided-cost rules that timing has real value, and it narrows the gap that raw annual kWh shows.
National Renewable Energy Laboratory, National Solar Radiation Database, solar resource by location · checked 2026-09-21
How your exports are paid decides half the answer
A solar system does not line up with a house. It makes most of its power in the middle of the day and a house uses most of its power in the evening, so a large share of what you generate goes to the grid and comes back later — and the terms of that exchange are worth more than any hardware decision.
| Arrangement | Exports worth | What it means |
|---|---|---|
| Full net metering | 100% | Every kilowatt-hour you export cancels one you import, at the same price. It is the best case and it is disappearing state by state. |
| Partial | 60% | The common successor: exports are worth something, but less than what you pay. It makes self-consumption matter, and it is where a battery starts to earn. |
| Avoided cost only | 25% | Exports are bought at wholesale. Under this, only the power you use as you make it is worth retail, and the whole arithmetic changes. |
| Full net metering | Avoided cost only | |
|---|---|---|
| Payback on the same system | About 10 years | About 18 years |
| Does system size still help? | Yes, up to 100% offset | Only up to what you use live |
| Is a battery worth it? | Weak case — the grid is free storage | It starts to earn |
| Does a west roof hurt as much? | Yes — annual kWh is what counts | Less, because afternoon output is used |
| What to optimise | Total annual production | Matching production to your own use |
US Energy Information Administration, Electric Power Monthly, average residential price by state · checked 2026-09-21
What you are actually paying for
19%
Panels
The part everybody shops for
24%
Inverter, racking, electrical
The rest of the hardware
16%
Installation labour
Two or three days on a roof
41%
Permits, design, sales
Why the same kit costs half in Australia
The hardware is a minority of a US residential solar system. Soft costs — permitting, design, interconnection paperwork, sales commission and overhead — are around 41 per cent, and they are the whole reason identical panels on an identical roof cost roughly half as much in Australia.
That is not an argument against buying. It is an argument for shopping on the installed price per watt rather than on panel brand, because the brand is a fifth of what you are paying and the sales process is twice that.
Get every proposal to state these
- The derate factor, or the specific yield in kWh per kW per year. Without it the production figure is unfalsifiable.
- Installed price per watt of DC nameplate, which is the only number that compares across proposals.
- The roof's azimuth and tilt as they measured it, not as assumed.
- What shading they modelled, and with what tool. A single tree matters more with a string inverter than with microinverters.
- Whether the main panel needs upgrading, and whether that is in the price.
- The roof's remaining life, and what removing and refitting the array would cost if it needs replacing.
- Which net metering regime the savings assume, and whether existing systems are grandfathered in your state.
- Whether degradation is in the twenty-five-year projection, and at what rate.
- What the federal credit is assumed to be, and that it is a credit against tax owed rather than a rebate.
MaterialsEstimator, Residential solar installed price survey · checked 2026-09-21
What goes wrong
01Believing a production figure with no derate behind it
Nameplate times sun hours overstates output by about a fifth before orientation is even counted. System losses take 14 per cent, the inverter 4, and heat three to nine more.
Instead — Ask for the derate factor or the specific yield. Around 0.80 and 1,200 to 1,800 kWh per kW are the sane ranges, and a proposal outside them needs explaining.
02Ignoring which way the roof faces
It is the largest variable after location and it is in no proposal, because the installer cannot change it. A west roof makes 78 per cent of a south one.
Instead — Put the azimuth into the arithmetic before you get quotes. It tells you how many panels you will need, which tells you the real price.
03Worrying about roof pitch
The tilt curve is very flat — flat roof 89 per cent of optimal, very steep 91. People agonise over this and ignore the direction, which costs ten times as much.
Instead — Check the direction first. If it is south-ish, stop worrying about the angle.
04Assuming full net metering
It is being rewritten state by state and the direction is one way. A payback built on retail export credit in an avoided-cost state is wrong by most of the saving.
Instead — Find out which regime applies to you, and whether existing systems are grandfathered. Then run the numbers on that, not on a national average.
05Putting an array on a roof near the end of its life
Panels last thirty years and shingles twenty. Re-roofing under an array means paying to remove and refit it, and it is thousands for nothing.
Instead — If the roof has under ten years left, do it first. It is the most expensive avoidable mistake on this page.
06Buying a battery for the payback
Under full net metering the grid is already free storage with perfect round-trip efficiency. A battery stores nothing you were not already exporting.
Instead — Buy it for outage resilience, which is real and which no payback calculation captures — or under avoided-cost rules, where it genuinely starts to earn.
07Shopping on panel brand
Panels are about 19 per cent of what you pay. Soft costs are 41, and that is where proposals differ most.
Instead — Compare installed price per watt, and treat the panel brand as a warranty question rather than a performance one.
08Judging it purely on payback year
Payback requires guessing what the utility will charge in fifteen years, which nobody knows. It makes an uncertain projection look like a fact.
Instead — Compare the levelised cost of your own electricity against what you pay now. If yours is comfortably lower, the decision holds without forecasting anything.
- 01Add up a year of kWhNot dollars — kilowatt-hours. Twelve bills, or one average month times twelve. This is the only input that matters more than the roof.
- 02Find which way the slope facesStand with your back to the house looking out over it. South is 100 per cent; every degree away costs output, and no installer can change it.
- 03Take the pitchIt matters far less than people think. A flat roof gives about 89 per cent of optimal and a steep one 91.
- 04Work out the specific yieldPeak sun hours × 365 × orientation × derate. That is kWh per kW installed, and it is the number to ask every proposal for.
- 05Divide your usage by itThat is the system size in kW. Divide by the panel wattage for the panel count, and check it fits the roof with a quarter added for spacing.
- 06Check how exports are paidFull net metering, partial, or avoided cost. It moves payback more than the price of the panels does.
- 07Price the roof firstPanels last thirty years and shingles twenty. An array on a roof with eight years left is a cost you will pay twice.
Insulation reduces the bill you are trying to cover, costs a tenth as much, and pays back faster than any array. It is the unglamorous first move and it makes the solar system you eventually buy smaller and cheaper.
Before you buy generation, cut the load- Nameplate
- The DC rating stamped on the panel, measured in a laboratory at 25°C and full sun. It is a specification, not a forecast, and no roof ever matches the conditions it was measured in.
- Peak sun hours
- The day's solar energy on a square metre, divided by 1,000 W/m². It is not hours of daylight — it is the number that makes kW × hours give kWh.
- Specific yield
- Annual kWh per kW installed, on your roof. It bundles location, orientation and every loss into one number, and it is the figure worth asking any installer for.
- Derate factor
- The ratio of real output to nameplate output. Around 0.80 in practice. A proposal without one is a proposal that has not done the arithmetic.
- Azimuth
- Which way the roof slope faces, in degrees, where 180 is due south. It is the single input with the largest effect that nobody can sell you.
- Tilt
- The angle of the slope. Optimal is roughly your latitude, and the curve is surprisingly flat — a few per cent, where azimuth costs tens.
- Inverter
- The box that turns panel DC into house AC. About 96 per cent efficient at full load, and a permanent four per cent off the top.
- Microinverter
- One small inverter per panel instead of one big one for the array. Costlier, and it stops a single shaded panel dragging a whole string down.
- Net metering
- The arrangement for what you export. Full net metering credits exports at retail; most successors credit them at much less, which changes the whole calculation.
- Degradation
- The slow annual loss of output, about 0.5 per cent a year and normally warrantied. After twenty-five years a panel makes roughly 88 per cent of what it did new.
- LCOE
- Levelised cost of energy: what your own electricity costs per kWh over the system's life. It is a fairer comparison than payback because it does not require guessing future utility prices.
- Soft cost
- Permits, design, sales commission and overhead — about 41 per cent of a US residential system, and the reason identical hardware costs half as much in Australia.
Common questions
- How many solar panels do I need?
- For a house using 11,000 kWh a year on a south-facing roof in the middle of the country, about 22 panels of 400 W — 8.8 kW. But the same house with a west-facing roof needs 28 to make the same electricity, and a north-facing one 42. The panel count is a function of the roof, not just the bill.
- How much electricity will my solar panels actually make?
- Around 80 per cent of what nameplate times peak sun hours suggests, before orientation is even considered. System losses take about 14 per cent, the inverter another 4, and cell temperature three to nine depending on climate. A proposal that quotes the naive figure is overstating by a fifth, and every part of that gap is known in advance.
- Does the direction my roof faces really matter?
- More than any other single thing, and it is in no proposal because the installer cannot change it. Due south is the reference. West or east makes 78 per cent of that, southwest 96, and due north 52. On a west roof you need about 27 per cent more panels for the same output — same price per watt, more watts.
- What about the roof pitch?
- Much less than people expect, and that is the reassuring part. Optimal tilt is roughly your latitude, but the curve is very flat: a nearly flat roof gives about 89 per cent of optimal and a 12:12 roof about 91. Unless the roof is extreme, the pitch is not your problem — the direction is.
- What is a derate factor and why should I ask for it?
- It is the ratio between what a system is rated at and what it produces. About 0.80 once orientation, soiling, shading, mismatch, wiring, inverter efficiency and cell temperature are counted. Ask any installer what derate they used. Someone who knows their job answers with a number immediately; someone who does not has quoted you nameplate.
- How long does solar take to pay for itself?
- On a south-facing roof in the middle of the country with full net metering, about ten years. In California with high electricity prices it can be five; in Washington with cheap hydro power it is closer to eighteen. The electricity price you are displacing and the export arrangement move this far more than the price of the hardware.
- Is net metering going away, and does it matter?
- It matters enormously and it is changing state by state. Under full net metering every exported kWh cancels an imported one at retail. Under avoided-cost rules exports are bought at wholesale — a few cents — so only what you use as you generate it is worth full price. The same system can pay back in ten years or eighteen depending purely on which regime you are in.
- Is a battery worth it?
- It depends on what you want from it, and the honest answer is not what it is sold as. It stores nothing you were not already exporting, so on full net metering it adds nothing to the payback — it buys you power in an outage, which is a different thing worth buying. Under full net metering the grid is already a perfect battery that costs nothing, so the payback case is weak. Under avoided-cost rules it starts to earn, and in a place with outages it buys something no arithmetic captures.
- Why is US solar so much more expensive than Australia or Germany?
- Because the hardware is a minority of the price. Panels are about 19 per cent of a US residential system and soft costs — permits, design, sales commission and overhead — are around 41. The same panels on the same roof cost roughly half in Australia, and none of that difference is technology.
- Should I replace my roof before installing solar?
- If it has less than about ten years left, yes, without hesitation. Panels last thirty years. Removing and refitting an array to re-roof underneath costs thousands and buys nothing, and it is the most expensive thing people discover after the fact. Price the roof first, always.
- Do solar panels lose output over time?
- Yes, about 0.5 per cent a year, and it is normally in the warranty. After twenty-five years a panel makes around 88 per cent of its original output. It is slow enough not to change a buying decision but real enough that a twenty-five-year projection should include it — and many proposals do not.
- Is payback the right way to judge solar?
- Not really, because it requires guessing what the utility will charge in 2040. Levelised cost of energy is fairer: what your own electricity costs per kWh across the system's life, compared with what you pay now. If your own is comfortably below the current retail price, the decision holds up without forecasting anything.
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About the author
Pablo Ruiz Quintero — Editor
Editor of MaterialsEstimator. Decides which documents count as a source, what every default value is, and where the line sits between what the reader parses and what the engines compute.
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