---
title: "When solar pays back and when it doesn’t"
source: https://www.taim.io/sustainable-living/when-solar-pays-back-and-when-it-doesnt
published: Sun May 10 2026 19:30:49 GMT+0000 (Coordinated Universal Time)
updated: Sun May 10 2026 19:31:53 GMT+0000 (Coordinated Universal Time)
description: "Solar pays back when five numbers line up: usage, tariff, roof yield, system cost, and self-consumption. This guide shows you how to run the maths yourself."
---

# When solar pays back and when it doesn’t

Solar pays back when five numbers line up. If you can estimate those five inputs from your own bills and roof, you can answer “is solar worth it?” with arithmetic instead of marketing.

Solar pays back when five numbers line up. If you can estimate those five inputs from your own bills and roof, you can answer “is solar worth it?” with arithmetic instead of marketing.

## Key takeaways

- Your entire rooftop solar decision fits into five inputs: annual usage, tariff, roof yield, system cost, and self-consumption ratio.
- Simple payback is annual savings divided into system cost; with realistic inputs, good roofs pay back in roughly 7-12 years and bad roofs often never do.
- Batteries are a separate economic decision: they usually have longer payback than panels and only make sense when tariffs and usage patterns line up.
- If your roof is shaded, cheap power, low usage, and low self-consumption, rooftop solar is a financial décor piece, not an investment.

## 1. Who this guide is for (and how to use it)

This is for homeowners looking at rooftop solar PV and wondering, in plain terms, **is solar worth it here, at these prices, on this roof?**

You do not need to love spreadsheets. You do need last year’s electricity bills and the ability to write down five numbers.

The focus here is financial payback. Solar also cuts CO2e, but that is not a substitute for the maths. Policy and grid mix data (IEA, IPCC, national grid operators) confirm solar helps, yet a bad-roof install can still be a poor use of your money.

You’ll:

1. Collect five inputs from your own home.
2. Run a first-pass payback for a notional system.
3. Check your result against two worked examples.
4. Refine the weakest inputs and rerun.

By the end, you should either have a **rough payback range in years** you trust, or a clear reason to drop the idea.

## 2. The five-input model: your whole decision on one page

All rooftop solar economics reduce to five inputs:

  **1. Annual usage (kWh)**
  How much grid electricity your home buys in a year. From bills or meter data.
  **2. Tariff (per kWh)**
  What you actually pay for each kWh, including taxes and fixed per-kWh charges.
  **3. Roof yield (kWh per kW per year)**
  How much energy 1 kW of panels would produce on your roof, over a year.
  **4. System cost (per kW installed)**
  Total installed cost divided by system size, including scaffolding, inverter, and VAT/sales tax.
  **5. Self-consumption ratio**
  Share of your solar generation you actually use in the home, rather than exporting.

> Most solar decisions get lost in gadget talk: panel brands, inverter types, smart meters. None of that matters as much as these five numbers. Once you can write them down for your roof, everything else, panels, batteries, tariffs, becomes an optimisation problem rather than a leap of faith. The physics is simple; the discipline is in getting clean inputs.

If a quote or salesperson can’t give you credible values or ranges for all five, you’re not ready to decide.

## 3. Your first pass: collect your five numbers

This is your **first attempt**. Do it quickly and accept that some numbers will be rough. The goal is to get a working model, not perfection.

**1. Annual usage (kWh)**

Open your last 12 months of bills. Find total kWh, not just the cost. If you only have cost, divide by your tariff per kWh. Note seasonal variation if it’s obvious.

**2. Tariff (per kWh)**

Look at your latest bill. Take total variable electricity charges and divide by total kWh for that period. Ignore standing charges or fixed daily fees for now; they barely move with solar.

**3. Roof yield (kWh/kW/year)**

Use a reputable online PV calculator (national grid operators and universities often host one). Enter your address, roof tilt and orientation, and system size. Write down annual kWh per kW. Typical ranges:

- Very good, unshaded, roughly south-facing: **1,000-1,300 kWh/kW/year**.
- East–west or mild shading: **800-1,000 kWh/kW/year**.
- Heavy shading or poor orientation: **600-800 kWh/kW/year**.

**4. System cost (per kW)**

Take at least one real quote. Divide total installed price by the quoted kW. If you only have online estimates, pick a mid-range number but flag it as weak. Typical variation is wide; costs can easily swing ±20-30%.

**5. Self-consumption ratio**

Without a battery, many homes consume **20-50%** of their solar output directly. If you’re home most days, lean higher. If the house is empty 9-5, lean lower. For a first pass, choose:

- 30% if you’re out most days.
- 40% if there’s usually someone home.
- 50% if you can shift loads (washing, EV charging) to sunny hours.

Write your five numbers down. You now have enough to run version 1 of your payback.

## 4. The simple payback formula (and what it hides)

We’ll start with simple payback: **how many years until savings equal cost**, ignoring inflation and degradation. It’s blunt but useful.

Let:

- (P) = system size (kW)
- (Y) = roof yield (kWh per kW per year)
- (C_{sys}) = system cost per kW (your quote)
- (T) = grid tariff (per kWh)
- (S) = self-consumption ratio (0-1)
- (E) = export value as a fraction of tariff (0-1). If you don’t know, assume 0.2-0.4.

Annual generation is:

Text

`Annual_kWh = P × Y
`
Value of each kWh:

Text

`Average_value_per_kWh = S × T + (1 – S) × (E × T)
`
Annual £ (or $) savings:

Text

`Annual_saving = Annual_kWh × Average_value_per_kWh
`
System cost:

Text

`Total_cost = P × C_sys
`
Simple payback (years):

Text

`Payback_years = Total_cost ÷ Annual_saving
`
What this hides:

- Panel degradation (roughly 0.3-0.7% per year).
- Tariff changes (up or down, depending on your grid and policy).
- Inverter replacement around year 10-15.
- Any subsidies or tax credits.

For a first cut, that’s acceptable. If your simple payback is already close to or beyond panel life (25-30 years), refinements won’t rescue it.

## 5. Worked example 1: high-yield roof, high tariff (good case)

Let’s anchor a **good scenario**.

Assume:

- System size (P = 4) kW.
- Roof yield (Y = 1{,}150) kWh/kW/year (sunny, unshaded, near-ideal orientation).
- System cost (C_{sys} = 1{,}200) per kW (moderately competitive market).
- Tariff (T = 0.30) per kWh.
- Self-consumption (S = 0.45).
- Export value fraction (E = 0.3) (export tariff ~30% of import price).

**Step 1: Annual generation**

(Annual_kWh = 4 × 1{,}150 = 4{,}600) kWh/year.

**Step 2: Average value per kWh**

(Average_value = 0.45 × 0.30 + (1-0.45) × (0.3 × 0.30)).

That’s:

Direct savings: (0.45 × 0.30 = 0.135)

Export savings: (0.55 × 0.09 ≈ 0.0495)

Total ≈ **0.1845 per kWh**.

**Step 3: Annual saving**

(Annual_saving ≈ 4{,}600 × 0.1845 ≈ 849) per year.

**Step 4: System cost**

(Total_cost = 4 × 1{,}200 = 4{,}800).

**Step 5: Payback**

(Payback_years ≈ 4{,}800 ÷ 849 ≈ 5.7) years.

That is unusually strong. Even if tariffs soften and yield is 10% lower, you’re still under 8-9 years.

This is the kind of roof and tariff combination where solar is hard to argue against, assuming you have the capital and plan to stay in the house.

## 6. Worked example 2: low-yield roof, low tariff (bad case)

Now a **weak scenario**.

Assume:

- System size (P = 4) kW (same size).
- Roof yield (Y = 750) kWh/kW/year (north-east orientation, some shading).
- System cost (C_{sys} = 1{,}600) per kW (smaller market, higher labour costs).
- Tariff (T = 0.18) per kWh (cheap grid electricity).
- Self-consumption (S = 0.25) (house empty weekdays).
- Export value fraction (E = 0.2) (poor export rate).

**Step 1: Annual generation**

(Annual_kWh = 4 × 750 = 3{,}000) kWh/year.

**Step 2: Average value per kWh**

Direct savings: (0.25 × 0.18 = 0.045)

Export savings: (0.75 × (0.2 × 0.18) = 0.75 × 0.036 = 0.027)

Total ≈ **0.072 per kWh**.

**Step 3: Annual saving**

(Annual_saving ≈ 3{,}000 × 0.072 = 216) per year.

**Step 4: System cost**

(Total_cost = 4 × 1{,}600 = 6{,}400).

**Step 5: Payback**

(Payback_years ≈ 6{,}400 ÷ 216 ≈ 29.6) years.

That is **longer than the typical economic life** of the system, especially once you include an inverter replacement. Any risk (panel damage, moving house earlier than planned, policy change) pushes this from marginal to unattractive.

The same panels, slightly different numbers, completely different outcome. That’s why these five inputs matter more than slogans like “solar always pays”.

Here’s the contrast side by side:

Input / Output
Good roof, high tariff
Bad roof, low tariff

Roof yield (kWh/kW/year)
1,150
750

Tariff per kWh
0.30
0.18

System cost per kW
1,200
1,600

Self-consumption ratio
0.45
0.25

Export value (fraction)
0.30
0.20

Annual generation (kWh)
4,600
3,000

Annual saving
~849
~216

Simple payback (years)
~5.7
~29.6

## 7. Live practice: run your own first calculation

Now it’s your turn. This is the **core practice loop**.

### Step 1: Choose a provisional system size

Pick a sensible size for your roof and usage. A common starting point is **roughly your annual kWh divided by 900-1,000**, then capped by roof area. If you use 4,000 kWh/year, that suggests ~4 kW, assuming good yield.

Write down (P) in kW.

### Step 2: Plug in your five inputs

Using the numbers you gathered:

- Compute annual generation: `Annual_kWh = P × Y`.
- Estimate your export value fraction (E) based on local export tariffs vs import price.

Then calculate:

Text

`Average_value_per_kWh = S × T + (1 – S) × (E × T)
Annual_saving         = Annual_KWh × Average_value_per_kWh
Total_cost            = P × C_sys
Payback_years         = Total_cost ÷ Annual_saving
``

Do this in a spreadsheet or on paper. Round to whole numbers; precision beyond the nearest year is fake comfort.

### Step 3: Check your feedback signals

You now have a first answer. How do you know if it’s useful?

**Strong signals:**

- Your payback falls clearly into one band: **under 10 years**, **10-20 years**, or **over 20 years**, and it’s stable when you nudge inputs by ±10%.
- You can explain which input dominates the result (e.g., “my low tariff is killing it” or “roof yield is excellent, so it works even with mediocre export rates”).

**Weak signals:**

- Changing one guessed input (like \(S\) from 0.3 to 0.5) moves payback by a decade.
- Your cost per kW is based on a single online calculator, not a quote.  
- Roof yield estimates differ wildly between tools.

If your signals are weak, that’s not failure. It tells you where to refine.
`

## 8. Retry: tightening your inputs the smart way

Don’t throw more complexity at the model. Improve the **fewest** inputs that move the result the most.

Start by sensitivity-checking:

- Increase and decrease each of (Y, C_{sys}, T, S) by 10-20% and see how payback shifts. Note which one changes it most.

Then refine in this order:

**1. System cost per kW**

Get at least one real quote, preferably two or three. Ensure they include scaffolding, inverter, monitoring hardware, and any switchgear. Recompute (C_{sys}) from the best realistic quote, not the cheapest headline.

**2. Roof yield**

Use a second reputable calculator. Cross-check by adjusting for shading carefully; tools from grid operators or academic sources tend to be conservative, which is good. If one tool is an outlier, discard it.

**3. Self-consumption ratio**

Look at your daily pattern. If you can access half-hourly data from your meter or utility app, approximate how much of your load sits between 9am–5pm on weekdays and 9am–4pm on weekends. Re-estimate (S) to match.

Run the payback again. If your range collapses into something like “likely 9-12 years”, you now have a decision-grade number. If it still spans 8-25 years, the project is too uncertain or too marginal to call attractive.

## 9. Battery economics: run it as a separate project

Batteries complicate solar maths because they **change only one input**: self-consumption. They don’t increase your roof yield, they just shift when you use it.

Treat a battery as a separate investment:

1. **Recalculate solar-only self-consumption** (no battery).
2. Estimate battery-boosted self-consumption. Many households go from ~30-40% up to **60-80%**. Treat extra export as now being stored and used later.
3. Compute the **extra annual savings** due to that increase in (S).
4. Divide battery cost by those extra savings.

Example, continuing from the good-roof case:

- Solar-only (S = 0.45). With a battery, you get to (S = 0.75).
- Tariff (T = 0.30), export fraction (E = 0.3).
- Annual_kWh = 4,600.

Solar-only average value per kWh (we already had): ~0.1845.

With battery:

Direct savings: (0.75 × 0.30 = 0.225)

Export savings: (0.25 × 0.09 = 0.0225)

Total ≈ **0.2475 per kWh**.

Extra value per kWh from battery: 0.2475-0.1845 = 0.063.

Extra annual saving: 4,600 × 0.063 ≈ **290290** per year.

If a usable 8 kWh battery costs **4,0004,000** fully installed, battery payback is about **4,000 ÷ 290 ≈ 13.8 years**.

That may be acceptable or not, depending on your risk tolerance and battery warranty length. But the key is: **don’t let battery economics hide inside your PV payback**. Make it pass its own test.

## 10. When solar never pays back (financially)

There are conditions where, even with optimistic assumptions, rooftop solar is financial décor:

- **Low roof yield** ((

## 11. Interpreting your result: keep or kill the project

At this point you should have a refined payback estimate, plus a sense of uncertainty.

A simple way to interpret:

- **Under 10 years, with realistic inputs**: strong candidate. Proceed to detailed quotes, roof survey, and finer-grain modelling (panel layout, shading, inverter choice).
- **10-15 years**: borderline but plausible, especially if tariffs might rise or you value CO2 savings. Compare with alternative uses for the capital (debt paydown, other efficiency upgrades).
- **15-20 years**: only consider if you have unusually strong confidence in high future tariffs, low degradation, and plan to stay in the house long-term.
- **20+ years**: treat as a lifestyle or climate choice, not an investment.

Regardless of band, sanity-check against your own roof:

- Does your annual generation estimate look reasonable compared to neighbours with similar systems?
- Are you assuming heroic behaviour changes to hit your self-consumption ratio?
- Have you accounted in your head for at least one inverter replacement in a 25-year life?

If you’re still unsure, deliberately try to kill the project: push inputs towards the pessimistic end (higher cost, lower yield, lower self-consumption). If it still lands under ~15 years, that’s strong evidence it’s robust.

## 12. Cheatsheet: quick-reference numbers and rules of thumb

Use this section as a compact field guide while you run your own numbers.

(There’s a more structured cheatsheet below, but here are the essentials in prose.)

- Treat **five inputs** as non-negotiable: annual usage, tariff, roof yield, system cost per kW, self-consumption. Don’t decide without them.
- A typical, decent residential system will often land between **7-15 years** simple payback on a good roof in a medium-to-high tariff environment.
- Roof yield under **800 kWh/kW/year** or tariffs under **0.15-0.18 per kWh** need very low system costs or unusually high self-consumption to be viable.
- Always compute **battery payback separately** from PV, based purely on the incremental increase in self-consumption and savings.
- When in doubt, compare solar with other measures: insulation rarely has marketing, but it frequently has better maths.

## 13. FAQ: real-world questions homeowners actually have

#### Are batteries worth adding to a rooftop solar system?

Often, batteries have **longer payback** than panels themselves. The right way to evaluate them is by asking: *how much do they increase my self-consumption, and what are those extra kWh worth at my tariff?* If the incremental savings give you a payback shorter than the warranted life of the battery (for both years and cycle count), it’s reasonable. If not, you’re effectively pre-paying many years of electricity at a high effective price.

Batteries make the most financial sense where there is a large spread between peak and off-peak tariffs, or where export payments are very poor. They also add resilience, which is hard to price but may matter to you if your grid is unreliable. Always run a separate battery payback alongside your solar calculation rather than letting marketing blend them.

#### What if I’m likely to sell the house in five years?

Then your horizon is shorter than a typical solar payback, so you need to think in terms of **resale value uplift plus bill savings**. Some markets recognise solar in property valuations; others barely do. Look for local data or talk to agents about sold prices of comparable homes with and without PV.

If your calculated payback is around 6-8 years and you expect to realise half of that value in bill savings before selling, you only need the remainder to show up in the sale price to break even. On the other hand, if your payback is 15-20 years, the odds of fully recovering your investment through a five-year stay plus a modest price uplift are low. In that case, only proceed if non-financial benefits matter more to you.

#### How do feed-in tariffs or export payments change the maths?

Export payments effectively set the value of the **(1 – S)** part of your generation. A higher export price pushes up the average value per kWh and shortens payback. In the formula we used, this is the (E × T) term. If export is close to your import tariff, the penalty for low self-consumption is small; if export is tiny, high self-consumption becomes crucial.

To see the impact, rerun your calculation with export at zero (worst case) and at a generous level (say 50-80% of import). If solar only looks good under the most optimistic export assumptions, treat that as a warning signal. Also remember that policies can change; be conservative rather than counting on today’s best offer lasting 25 years.

#### ⚖️ Is leasing or a ‘free solar’ deal better than buying outright?

Leasing or so-called “free solar” deals usually swap your upfront capex for a **long-term contract**: either you pay a fixed monthly fee, or a third party owns the panels and sells you power under a specific tariff. Economically, you should compare the net present cost of that contract with owning the system and buying less from the grid.

Watch for escalators (built-in annual price rises), limits on roof work, and what happens when you sell the house. If the lease payments plus any remaining grid usage are similar to or higher than your current bills, you’re effectively financing someone else’s asset. In many cases, buying outright (or via a standard loan) gives you clearer control and better upside, but you need to run the numbers on the specific contract, not on marketing examples.

#### How accurate does my self-consumption estimate need to be?

Self-consumption is one of the fuzzier inputs, but you don’t need perfection to make a good decision. Aim for a **realistic band** instead of a single number: for example, “without behaviour change I’m 25-35%; with some load shifting I might reach 40-50%.” Then compute payback at the low and high ends of that band.

If solar only looks attractive at the most optimistic self-consumption level, that’s a problem. Either you need a battery to lift it, or the economics are weak. If it still works even at the conservative end of your range, you’ve got a robust case and any extra optimisation is upside rather than a requirement.

#### Can solar still be worth it on a shaded or east–west roof?

Yes, but the bar is higher, because shading and suboptimal orientation mostly hurt **roof yield**. An east–west roof can still do well if your yield is in the 850-1,000 kWh/kW/year range and other inputs (tariff, cost per kW, self-consumption) are strong. Deep shading that drags yield below ~700-800 kWh/kW/year is harder to overcome economically.

For a compromised roof, focus on controlling the variables you can: negotiate a good installed cost per kW, improve daytime self-consumption (including with flexible loads like EV charging), and ensure you get a realistic yield estimate using shading-aware tools. If, after all that, payback still lands beyond 20 years in your simple model, solar is probably not your best first upgrade for the property.

### Solar payback field cheatsheet

#### ⚡ Five-input quick-check

Before taking any quote seriously, write down:

1. Annual usage: last 12 months’ kWh from bills.
2. Tariff: total variable cost ÷ kWh on latest bill.
3. Roof yield: kWh/kW/year from a reputable calculator (aim for 1,000-1,300 good, 800-1,000 okay, <800 weak).
4. System cost: quote total ÷ kW size (compare to local norms).
5. Self-consumption: 25-35% if house is empty in daytime, 35-50% if someone is usually home, higher only with batteries or large shiftable loads.

#### Payback formula snapshot

Use:
Annual_kWh = System_size_kW × Roof_yield.
Average_value_per_kWh = S × Tariff + (1 – S) × (E × Tariff), where S is self-consumption and E is export value fraction.
Annual_saving = Annual_kWh × Average_value_per_kWh.
Total_cost = System_size_kW × Cost_per_kW.
Simple payback (years) = Total_cost ÷ Annual_saving.
Check payback at conservative and optimistic ends of S and Y to get a band, not a fake-precise single year.

#### Roof-yield rules of thumb

Unshaded, near-south, typical tilt: ~1,000-1,300 kWh/kW/year.
East–west split with mild shading: ~850-1,050 kWh/kW/year.
North-east / north-west or noticeable shading: ~650-850 kWh/kW/year.
If your calculator shows <700 kWh/kW/year, you need especially good tariffs and low costs to make the maths work; treat payback estimates with skepticism.

#### ⚠️ Bad-case warning signs

Be cautious or walk away if you see:

- Roof yield <800 kWh/kW/year and no easy shading fixes.
- Tariff <0.15-0.18 per kWh plus low export rates.
- Installed cost per kW significantly above local market averages.
- Self-consumption stuck below ~25% with no realistic way to shift loads. Any two of these together almost always push simple payback beyond 20 years.

#### Battery add-on snapshot

Evaluate batteries on their own merits: calculate solar-only self-consumption (S1) and battery-boosted self-consumption (S2). Extra value per kWh = (S2 – S1) × Tariff + (S1 – S2) × (E × Tariff). Multiply by annual_kWh to get extra annual saving. Battery payback = Battery_cost ÷ Extra_annual_saving. Only attractive if this is comfortably shorter than both the warranty period and your expected time in the property.

### Solar economics: deeper questions

#### Are batteries worth adding to a rooftop solar system?

Often, batteries have longer payback than panels themselves. The right way to evaluate them is by asking: how much do they increase my self-consumption, and what are those extra kWh worth at my tariff? If the incremental savings give you a payback shorter than the warranted life of the battery (for both years and cycle count), it’s reasonable. If not, you’re effectively pre-paying many years of electricity at a high effective price.

Batteries make the most financial sense where there is a large spread between peak and off-peak tariffs, or where export payments are very poor. They also add resilience, which is hard to price but may matter to you if your grid is unreliable. Always run a separate battery payback alongside your solar calculation rather than letting marketing blend them.

#### What if I sell the house in five years?

Then your horizon is shorter than a typical solar payback, so you need to think in terms of resale value uplift plus bill savings. Some markets recognise solar in property valuations; others barely do, so don’t assume a 1:1 recovery of your install cost.

If your calculated payback is around 6-8 years, you can reasonably expect to recover a chunk of the value through five years of lower bills and some uplift at sale. If payback is 15-20 years, the odds of fully recovering your investment via a five-year stay plus modest value gain are low. In that case, treat the project as partly non-financial (CO2, comfort, interest) or redirect the capital to upgrades with shorter payback.

#### What about feed-in tariffs or export payments?

Export payments effectively determine the value of surplus solar you cannot use directly. In the payback formula, this shows up as the E × Tariff term, where E is the export value as a fraction of your import price. A higher E softens the penalty of low self-consumption; a very low E makes self-consumption crucial.

When evaluating a system, compute your payback under three export scenarios: zero export value (worst case), current advertised rate, and a slightly reduced rate to allow for policy changes. If the system only looks attractive with a very generous export assumption, it is sensitive to policy risk. Ideally, you want the economics to still work even with mediocre export payments.

#### ⚖️ Is leasing or a ‘free solar’ deal better than buying?

Leasing or third-party-owned systems replace upfront cost with a contract: you either pay a fixed monthly amount or a tariff for the solar power. To compare with buying, estimate your total payments over the term (including any escalators) and add your remaining grid bills, then compare that to owning the system and just paying residual grid usage.

Key checks: how long the contract runs, what happens on sale of the house, who pays for maintenance, and whether you are locked into specific tariffs. If the lease leaves you with only modest savings compared to doing nothing, then the third party is capturing most of the economics. Buying outright, or with transparent financing, usually gives you better control, provided the payback maths works in the first place.

#### Is leasing better if I can’t afford upfront costs?

If cash upfront is the constraint, leasing may still make sense **only if** the contract gives you real bill savings compared with doing nothing, and those savings don’t come at the cost of major restrictions on the property. You’re effectively trading equity in the system for lower upfront risk.

Before signing, model your all-in cost per kWh under the lease: total payments (including escalators) plus any remaining grid charges, divided by the energy you expect to use. Compare that with your current cost per kWh. If the difference is small, you may be better off reducing consumption, improving efficiency, or waiting until you can fund a system you own.

#### What if my roof is shaded or oddly shaped?

Shading and awkward layouts mainly hurt yield, raise installation complexity, and push up cost per kW. The first task is to get a shading-aware yield estimate; tools that account for nearby trees, chimneys, and other buildings are more realistic than simple orientation-based calculators.

Odd shapes can mean multiple small arrays, more wiring, and perhaps multiple inverters or optimisers, all of which raise C_sys (cost per kW). If, after using a realistic yield and an honest installed cost, your simple payback is still under ~15 years, it may still be viable. If it stretches past 20 years with conservative inputs, your capital probably works harder in insulation, air sealing, or an efficient heating system instead.

### Closing the loop: from marketing claims to your own numbers

You now have everything you need to stop arguing about whether solar is “good” in the abstract and start asking whether it is good **for this roof, at these prices, on this tariff**.

Five inputs and a short calculation get you to a payback range that is honest enough to act on. A good roof, decent tariffs, and reasonable costs produce obviously solid numbers; a shaded roof and cheap power do not. Batteries can help, but only when they fix a self-consumption problem at a cost the extra savings can support.

The value of this process isn’t the single number you get; it’s the ability to **stress-test assumptions** and see what actually drives the outcome. That’s the same discipline you need for every other big energy decision—insulation, heating systems, even vehicles.

Use this guide as a template. Next time a glossy brochure or enthusiastic installer makes a claim, translate it into the five inputs, run the maths, and see if the story survives contact with your numbers.

### Next steps: put your roof into the model

- Gather your last 12 months of electricity bills and write down total kWh and total variable cost to get annual usage and tariff.
- Use a reputable PV yield calculator to estimate kWh/kW/year for your specific roof orientation, tilt, and shading; note low, medium, and high estimates if given.
- Request at least one real, itemised quote and compute cost per kW, making sure scaffolding and inverter are included.
- Estimate your self-consumption realistically based on current routines, then separately estimate what you could reach with some deliberate load shifting.
- Run your first simple payback calculation for a plausible system size, then do a sensitivity check by nudging each major input ±10-20%.
- Identify the one or two inputs that move payback most and refine those with better data or additional quotes; then rerun and decide whether to proceed, resize, or pause the project.