Solar Panel Savings, Explained

After reading this you will know how a rooftop solar system turns sunlight into money, why self-consumption matters more than raw production, and how to read a 25-year payback curve without fooling yourself.

What the simulator does and one example

A rooftop solar system converts sunlight into electricity. The value of that electricity depends on where each kilowatt-hour goes. If you use it in your house, it replaces electricity you would have bought at the retail price. If you cannot use it, it flows back to the grid and earns a smaller feed-in payment. The Solar Panel Savings Simulator builds a full year of production and consumption hour by hour, decides where every kilowatt-hour goes, and then projects that year forward 25 years with price inflation and panel aging.

Here is the hook. Suppose a 5 kWp system produces 5,000 kWh in its first year. You consume 4,500 kWh across the whole year, but only 1,750 kWh of the solar output lines up with the moments you actually need power. At a retail price of 0.30 per kWh and a feed-in rate of 0.08 per kWh, the direct-use energy is worth 1,750 × 0.30 = 525, while the exported 3,250 kWh earns only 3,250 × 0.08 = 260. Same panels, but the first bucket is worth more than twice as much per unit. That gap is the whole story.

When to use it, and when not

Use the simulator when you want a realistic first estimate of whether solar pays for you, how much a battery adds, and roughly which year you break even. It is good for comparing scenarios: south versus east-west roof, 4 kWp versus 8 kWp, battery versus no battery. Change one input, rerun, and read the difference.

Do not use it as a final decision. The tool models a seasonal average sun profile by latitude, not your local weather, shading from your neighbor's chimney, or your utility's exact tariff rules. Net metering, time-of-use pricing and subsidies vary by country and even by city. Treat the output as a shape you understand, then get real installer quotes and check your local rules before signing anything.

The single largest source of error is the retail price and the feed-in rate. If your utility pays close to the retail price for exports (full net metering), a battery saves you almost nothing. If exports earn a tenth of retail, a battery can be the difference between a 12-year and an 18-year payback.

The formula and the intuition behind it

Start with first-year production. A common rule of thumb is the specific yield: kilowatt-hours produced per kilowatt-peak of installed panels per year.

E_1 = P_{kWp} \times Y \times f_{orient} \times f_{tilt}

Here E_1 is first-year energy in kWh, P_{kWp} is system size in kWp, Y is the base specific yield for your latitude (about 1000 for central Europe on an ideal roof), and f_{orient} and f_{tilt} are factors near 1.0 that fall as the roof turns away from the equator or leaves the ideal tilt.

Panels lose output slowly. Year n production is

E_n = E_1 \times (1 - d)^{n-1}

with d the annual degradation rate, typically 0.005 (half a percent). After 25 years that leaves (1 - 0.005)^{24} \approx 0.887, so about 88.7% of the first-year output.

Now the money. Split each year's production into self-consumed energy S_n and exported energy X_n. The annual saving is

B_n = S_n \times p_1 (1+i)^{n-1} + X_n \times r

p_1 is today's retail price, i is annual electricity-price inflation, and r is the feed-in rate. Self-consumption is multiplied by a rising retail price, which is why the self-consumed share grows in value over time. The payback year is the first n where the running total of B_n exceeds the upfront cost C.

A worked example using the demo data

A 5 kWp south-facing roof in central Europe

The demo defaults describe a 5 kWp system on a south-facing roof at 35° tilt, central-European sun, a household using 4,500 kWh a year, no battery, retail price 0.30, feed-in 0.08, price inflation 3%, degradation 0.5%, and an installed cost of 9,000.

First-year production: 5 × 1000 × 1.0 × 1.0 = 5,000 kWh.
Without a battery, roughly 35% of production is used directly: S_1 = 0.35 \times 5000 = 1750 kWh. The rest, 3,250 kWh, is exported.
Year-1 saving: 1750 \times 0.30 + 3250 \times 0.08 = 525 + 260 = 785.
Each later year, retail-priced savings grow by 3% while production shrinks by 0.5%. Year 2 self-consumption is about 1741 kWh at 0.309, giving 538, plus export of 3234 at 0.08 = 259, so 797.
Summing all 25 years gives roughly 26,400 in total savings. The running total passes the 9,000 cost partway through year 11, so the payback year is 11.

The effective return compares lifetime savings to the upfront cost. One simple measure is 26400 / 9000 - 1 \approx 1.93, meaning the system returns about 193% of its cost over 25 years, on top of returning the cost itself.

The exported bucket is nearly twice the size in kilowatt-hours but worth half as much in money. That mismatch is what a battery attacks.

How a battery changes the picture

A battery stores midday surplus so you can use it in the evening. That moves kilowatt-hours from the low-value export bucket into the high-value self-consumption bucket. Its economic worth per stored kilowatt-hour is the price gap p_1 - r, here 0.30 - 0.08 = 0.22.

Adding a battery might lift self-consumption from 35% to 65%, so an extra 0.30 × 5000 = 1,500 kWh is now used at home instead of exported. That is worth about 1500 \times 0.22 = 330 in the first year. If the battery costs 6,000, it pays back on its own only if 330 a year, growing with inflation, covers that outlay inside the battery's useful life. Over 15 years at 3% inflation, 330 grows to a total near 6,100, which is barely break-even. Batteries are often a comfort-and-independence choice as much as a financial one.

Before you pay for a battery, look at what the no-battery run does to self-consumption. If your household already uses most of its production directly, because someone is home during the day or you run appliances on timers, the battery has little left to capture.

Reading and interpreting the results

Three numbers carry most of the meaning. The payback year tells you when cumulative savings first exceed the upfront cost. Total lifetime savings is the height of the cash-flow curve at year 25. The effective return restates that height as a percentage of the cost, so you can compare it to other uses of the same money.

Watch the shape of the cumulative curve, not just its endpoints. It starts below zero at the moment you pay, rises each year by that year's saving, and crosses zero at payback. Because retail prices inflate, the curve gets steeper over time, so the second half of the 25 years usually contributes more than the first.

The curve crosses zero during year 11, the marked payback year, then climbs faster as inflated retail prices raise each later saving.

Common mistakes

The first mistake is reading annual production as if it were annual saving. Only the self-consumed part earns the full retail price, so a big south roof that dumps most of its output to the grid at a low feed-in rate can pay back slower than a smaller, well-matched system.

The second is assuming a flat electricity price. At 3% inflation, a price of 0.30 becomes about 0.30 \times (1.03)^{24} \approx 0.61 by year 25. Setting inflation to zero can push the payback year out by several years and understate lifetime savings by a third or more.

The third is ignoring degradation the other way. It is real but small: half a percent a year removes only about 11% of output over 25 years. Do not let it scare you out of a system, and do not forget it when you compare quotes that claim zero decline.

The fourth is trusting a battery payback that leans on a feed-in rate you have not confirmed. Under full net metering the battery's per-kWh value collapses toward zero, because exports already earn nearly the retail price.

Related tools

Solar sits inside the larger cost of owning a home. If you are buying, the Mortgage Calculator shows the payment and interest, and the Home Affordability Calculator estimates the price your income supports. To decide whether owning makes sense at all, use the Rent vs Buy Calculator. If you already own and have spare cash, compare putting it into panels against overpaying the loan with the Mortgage Extra Payment Calculator. For financing details, the Down Payment & LTV Calculator and the Mortgage Refinance Calculator cover the loan side.

Frequently asked questions

Does the simulator include subsidies or tax credits?

No. Subsidies, grants and tax credits differ so much by location that the tool leaves them out. If you receive a subsidy, subtract it from the installed cost you enter, and the payback year will move earlier.

Why is my payback longer than a friend's with the same panels?

Most likely your self-consumption is lower, your retail price is lower, or your feed-in rate is higher relative to retail. Two identical 5 kWp systems can differ by years of payback purely from how well production lines up with usage.

Should I oversize the system to cover future needs?

Only if you expect to consume more during daylight, for example an electric car charged at midday or a heat pump. Otherwise extra panels mostly add exports at the low feed-in rate, which lengthens the payback per kWp.

How accurate is the 1,000 kWh per kWp figure?

It is a central-European, ideal-roof average. Sunnier southern regions can reach 1,300 to 1,600, while cloudier northern ones sit closer to 800. The orientation and tilt factors then scale it down for real roofs.

Is the effective return the same as an interest rate?

Not exactly. The effective return here is total lifetime savings expressed against the upfront cost, spread over 25 years. It ignores the time value of money, so it reads higher than an annualized rate would. Use it to compare scenarios, not as a bank interest quote.

Try it yourself in the tool