Solar & battery
Solar Replacement Calculator
Works replacement cost and timing for the components that do not last the system's life.
Result
Panels rarely fail outright; they fade slowly and usually still work at 25 years. The case for replacing is normally about fitting more capacity on the same roof with better modules, or a roof that needs replacing anyway, rather than about degradation. Removing and refitting an array during roof work is a real cost that belongs in this comparison.
Get a Free Business AuditHow the calculation works
Each component's replacement cost at its expected replacement year, discounted or simply scheduled, giving the reserve or the cost to anticipate. Inverters and batteries are the usual items; modules occasionally fail but are generally warranted for most of the system's life.
What moves the number
- Expected life of each component.
- Replacement cost at the time, not today's price.
- Warranty coverage and what it actually covers.
- Whether the replacement part will still be available.
Where this one goes wrong
Warranty coverage frequently excludes the labour to install the replacement, which on a roof-mounted component is a meaningful share of the cost, so a warranted failure is not a free one. Read what the warranty covers. Availability is the other issue, since a system installed with equipment from a manufacturer that has since left the market can be difficult to repair compatibly, which is an argument for established equipment rather than the cheapest option.
Before you quote
- Check whether warranties cover labour as well as parts.
- Reserve for replacement at future cost, not today's.
- Prefer equipment likely to remain supported.
Questions
About this calculator
Does a warranty make a replacement free?
Rarely. Many warranties cover the part and not the labour to install it, which on a roof-mounted component is a meaningful share of the cost. Reading what is actually covered is worth doing before the failure rather than after.
Which components need replacing?
Inverters and batteries, generally, since modules usually outlast them and carry long warranties. Planning for at least one inverter replacement over a system's life is realistic rather than pessimistic.
Does equipment availability matter?
Yes. A system using equipment from a manufacturer that has left the market can be difficult to repair compatibly, which is a real argument for established equipment over the cheapest available at installation.
The formula
current output = new output x (1 - degradation)^age; payback = cost / (extra kWh x rate)
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How to use this calculator
- System age (years). Enter your own figure; the page starts at 18.
- Original system size (kW). Enter your own figure; the page starts at 6.
- Annual degradation (%). Enter your own figure; the page starts at 0.6%.
- Annual production when new (kWh). Enter your own figure; the page starts at 7,800.
- Cost to replace ($). Enter your own figure; the page starts at $16,000.
- Electricity rate per kWh ($). Enter your own figure; the page starts at $0.18.
The result updates as you type, so you can try a figure, see what it does to current annual production, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| System age (years) | 18 |
| Original system size (kW) | 6 |
| Annual degradation | 0.6% |
| Annual production when new (kWh) | 7,800 |
| Cost to replace | $16,000 |
| Electricity rate per kWh | $0.18 |
| New system size (kW) | 9 |
What comes out
| Result | Value |
|---|---|
| Current annual production main result | 6,999 |
| Lost to degradation | 801 |
| New system production | 11,700 |
| Extra production if replaced | 4,701 |
| Payback on the replacement | 18.9 |
After 18 years the array makes 6,999 kWh, down 801 kWh from new. Replacing with 9 kW adds 4,701 kWh a year and pays back in about 18.9 years.
How sensitive the answer is
Holding everything else steady and changing only cost to replace, this is what happens to current annual production. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost to replace ($) | Current annual production |
|---|---|
| $8,000 | 6,999 |
| $12,000 | 6,999 |
| $16,000 example above | 6,999 |
| $24,000 | 6,999 |
| $32,000 | 6,999 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- System age (years)
- A number, between 0 and 50, starting at 18.
- Original system size (kW)
- A number, between 0 and 5000, starting at 6.
- Annual degradation (%)
- A percentage, between 0 and 5, starting at 0.6%.
- Annual production when new (kWh)
- A number, between 0 and 5000000, starting at 7,800.
- Cost to replace ($)
- A money amount, starting at $16,000.
- Electricity rate per kWh ($)
- A money amount, starting at $0.18.
- New system size (kW)
- A number, between 0 and 5000, starting at 9.
What the results tell you
| Result | What it is |
|---|---|
| Current annual production | The headline result. A number. |
| Lost to degradation | A number. |
| New system production | A number. |
| Extra production if replaced | A number. |
| Payback on the replacement | A decimal figure. |
A design still needs a site visit
Output and sizing figures use the assumptions you enter, not measured irradiance, shading or roof orientation at your address. Real production depends on all three, plus inverter behaviour and local interconnection rules.
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