Deep-dive guide
Rooftop Solar & EV Charging vs Grid: 10-Year Inflation Stress Test
Executive industry analysis
Rooftop solar is sold as a way to beat electricity inflation, and for many homes it does. A system bought today fixes the cost of a large part of your electricity for twenty-five years, while the grid price is set by fuel markets, networks and taxes you cannot control. The catch is that the saving arrives slowly, month after month, while the cost arrives at once. Most households finance the purchase, so the real question is not whether solar saves money in total, but whether the savings each month exceed the loan payment and upkeep, and how that changes if the assumptions about prices turn out wrong.
That is why a ten-year stress test is more useful than a single payback number. A payback of five years sounds safe, but it hides three bets: that grid prices will keep rising, that the panels will produce what the brochure says and that you will use most of the energy yourself. If the grid price stays flat, the loan rate is higher than expected or the roof is shaded in winter, the same system can cost more than it saves every month for a decade. The simulator lets you break each of these assumptions one at a time and see how much room you really have.
The domino model follows one kilowatt-hour through five stages. First, how much of your own use is covered by solar, because energy you export is worth far less than energy you consume. Second, the monthly value of the electricity saved, including the effect of rising prices. Third, how much of that saving the loan, maintenance and inverter reserve consume. Fourth, whether the savings cover every cost with room to spare. Fifth, the average net benefit per month. Adding an electric vehicle raises consumption and therefore self-use, which is why EV charging often makes a solar system better than the household alone would.
The iceberg view shows what a simple payback hides. Above the waterline sit the loan principal you repay, which builds an asset, and the electricity you still buy from the grid. Below it sit four costs people forget: loan interest, maintenance and insurance, the reserve for replacing the inverter in about ten years, and the output lost as panels age. Together they often take a third of the gross savings. Use the simulator to find the electricity price, interest rate or sunshine level at which your plan breaks, and then decide whether to resize, add storage or wait.
Formula and methodology blueprint
The model averages ten years into one monthly figure. Output is system size times yearly yield divided by twelve, reduced by panel degradation at its mid-point. On-site use is the lower of the self-use share of output and your consumption. Exported energy is paid at thirty percent of the retail price. The grid price is multiplied by the average of ten years of compounding. Costs are the loan payment, maintenance and an inverter reserve of eight percent of the system cost.
- Average monthly output
- Q = kW × yield ÷ 12 × (1 − d)^4.5
- Production per month averaged over ten years, after panel degradation d.
- Savings
- S = (self-used × price + exported × price × 0.3) × F
- Energy used on site is worth the retail price and exports are worth thirty percent, scaled by the price-growth factor F.
- Loan payment
- M = P × r ÷ (1 − (1 + r)^−120)
- The fixed monthly payment on the system cost P at monthly interest rate r over 120 months.
- Net benefit
- Net = S − M − maintenance − inverter reserve
- The average monthly result. The breaking point is where it reaches zero.
- Price growth factor
- F = ((1 + g)^10 − 1) ÷ (10 × g)
- The average multiplier on today's grid price over ten years of compounding growth g.
- Cost cover
- C = S ÷ (M + maintenance + reserve)
- Savings divided by everything you pay for the system each month; below 1.0 means a monthly loss.
Cascading elasticity
The grid price and the share used on site are the strongest levers because they scale the value of every kilowatt-hour. Installed cost and loan rate scale the payment instead, so they matter most when the margin is already thin. Yearly price growth compounds, so it matters more the longer the horizon. Maintenance and degradation are small individually but add up over ten years.
Three real-world case studies
Bootstrapped starter
A small system for an apartment roof share
A household with modest consumption installs three kilowatts and uses half of it on site. The loan is small, so the monthly benefit is close to break-even. Load it to see how little margin a small system has when prices stop rising.
Scaling mid-market
A large system with two electric vehicles
A family with a heat pump and two electric vehicles installs fourteen kilowatts and charges the cars in daylight. Self-use is high and the grid price is high, so the saving is large. Load it to see how EV charging improves the economics.
Black-swan crisis
An oversized system in a shaded location
A large system is installed on a partly shaded roof for a low-consumption household, financed at a high interest rate with flat grid prices. Output is low, most energy is exported and the loan outweighs the saving. Load it to see which lever restores the case.
Industry benchmark matrix
| Metric | Danger zone | Industry average | Top 10% elite |
|---|---|---|---|
| Share of own use covered by solar | Below 30% | 45–65% | Above 75% |
| Loan and upkeep as share of savings | Above 85% | 55–70% | Below 45% |
| Installed cost per kW (residential) | Above 1,800 | 1,000–1,400 | Below 900 |
| Share of output used on site | Below 30% | 45–60% | Above 75% |
| Savings cover of financing and upkeep | Below 1.0× | 1.2–1.6× | Above 2.0× |
Variables and levers
System size
Installed solar capacity in kilowatts peak.
Lever: Size the system to your own use, since exported energy earns only a fraction of the retail price.
Installed cost per kW
Total price of panels, inverter, mounting and installation per kilowatt.
Lever: Collect several quotes and compare installed price per kilowatt, because it is the largest single driver of the loan.
Home electricity use
Average monthly electricity used by the household, excluding the car.
Lever: Shift flexible loads such as laundry, heating and hot water into daylight hours so more of the output is used on site.
EV charging at home
Average monthly electricity used to charge an electric vehicle at home.
Lever: Charge the car when the sun is up, because daytime charging turns exported energy into a saving.
Yearly output per kW
Kilowatt-hours a kilowatt of panels produces in a year at your location.
Lever: Check roof orientation, tilt and shading, as these change yearly output more than the choice of panel brand.
Grid price per kWh
What you pay your utility for one kilowatt-hour today, all taxes included.
Lever: Review your tariff; the higher your grid price, the more each solar kilowatt-hour is worth.
Yearly grid price increase
Assumed average annual rise in the electricity price over the next ten years.
Lever: Test a flat-price case as well as a rising one, so the plan works even if electricity prices stop rising.
Share used on site
Share of solar output you use directly or store, instead of exporting.
Lever: Add a battery or a smart charging schedule to raise the share of output you use on site.
Loan interest rate
Annual interest rate on the ten-year loan that finances the system.
Lever: Shop for the loan or pay a larger deposit, because interest is the biggest hidden cost of financing.
Maintenance per year
Cleaning, inspection, insurance and monitoring each year.
Lever: Compare service contracts and keep panels clean, and choose equipment with a long warranty.
Yearly panel degradation
Share of output the panels lose each year as they age.
Lever: Choose panels with a low guaranteed degradation rate and a long performance warranty.
How to use this simulator
- 1Start from the realistic preset or enter your quote, bills and loan terms.
- 2Read the five domino boxes to see whether self-supply, savings or financing turns red first.
- 3Press Find my breaking point to learn which price, interest rate or sunshine level pushes the benefit to zero.
- 4Compare the current plan with the smaller system with battery over 24 months in the Parallel universe tab.
- 5Copy the share link so an installer or family member can open the same scenario.
Mistakes to avoid
- Assuming the grid price will keep rising at the recent rate. Test a flat case as well, so the plan still works if electricity gets no more expensive.
- Sizing the system to the roof instead of to your use. Extra panels mostly produce exports that earn a fraction of the retail price.
- Comparing only the loan payment with today's bill. Maintenance, the inverter replacement and panel ageing arrive later and reduce the real benefit.
- Using the maximum output from the brochure. Shading, orientation and dirt reduce real yield, so use a local estimate and keep a margin.
Frequently asked questions
How does EV charging change the case?
It raises your consumption, so more of the solar output is used on site at the full retail price. Daytime charging is the best match for solar production.
Why is exported energy worth less?
Utilities usually pay a wholesale or feed-in rate that is far below the retail price. This model assumes thirty percent of the retail price for exports.
What happens if electricity prices do not rise?
Set yearly grid increase to zero. Savings fall, and the loan payment stays fixed, which shows how much of the benefit depends on rising prices.
Why is a ten-year view used?
It matches a typical loan term and averages out price growth and degradation into one monthly number. Panels last longer, so the years after the loan add benefit beyond this model.
Should I add a battery?
Only if the extra self-use it creates is worth more than its cost. Compare the two plans in the Parallel universe tab to see the answer for your numbers.
How accurate is this?
It is an educational planning model with stated assumptions, not engineering or financial advice. Use your own quotes, bills and local sunshine data.
This is an educational planning model with stated assumptions. It is not financial, tax or legal advice. Replace the inputs with your own figures and verify important decisions with a qualified professional.