Solar Panel Comparison Calculator

How to compare solar panels

When you compare solar panels, it's tempting to fixate on efficiency or wattage alone — but the panel that looks best on a spec sheet isn't always the best value for your roof. The right comparison weighs efficiency, cost per watt, warranty, degradation rate and how much roof space you have. For most homes with adequate roof area, the cheapest path to your target output wins; only when space is tight does paying up for high efficiency pay off. Knowing which situation you're in is the key to a sensible choice.

Choosing between solar panels is where a lot of buyers get steered toward spending more than they need to, because the specification that’s easiest to market — efficiency — is not the specification that matters most for value. A salesperson can always point to a glossier efficiency figure, but whether that figure is worth paying for depends entirely on your roof. The disciplined way to compare is to reduce every panel to a few objective numbers — cost per watt, efficiency, warranty terms and degradation rate — and judge them against your own constraints rather than the brochure’s emphasis. Do that and the right choice usually becomes obvious, and is often not the most expensive panel on the table.

This calculator lets you put competing panels side by side on the metrics that actually matter, rather than the one number a brochure emphasises.

Efficiency: when it matters, when it doesn't

Panel efficiency is the share of sunlight a panel converts to electricity — typically around 19–23% for modern residential panels. Higher efficiency means more watts from the same physical area. That's valuable only when your roof space is limited: if you can't fit enough standard panels to hit your target, higher-efficiency panels let you reach it in less space. But if you have ample roof, lower-efficiency panels that cost less per watt usually deliver the same total output for less money — the extra space costs you nothing. So efficiency is really a proxy for "how much output per square metre," which matters most when square metres are scarce.

Cost per watt: the great equaliser

The most useful single comparison metric is cost per watt (price ÷ rated watts), because it normalises panels of different sizes and powers. A 450 W panel at $135 and a 400 W panel at $120 are both $0.30/W — equivalent value per unit of capacity. Comparing cost per watt cuts through marketing and tells you which panel delivers capacity most cheaply. Combine it with efficiency only when roof space forces the issue.

Warranty and degradation

• Product warranty covers defects — typically 12–25 years; longer suggests manufacturer confidence.
• Performance warranty guarantees the panel still produces a stated percentage (often ~85–90%) after 25 years.
• Degradation rate — how fast output declines, usually ~0.4–0.5% a year. A lower rate means more lifetime energy. Premium panels often degrade more slowly, which can justify a higher price over 25 years.

A cheaper panel that degrades faster or has a weaker warranty may cost more over its life — which is exactly what the lifetime cost-per-kWh figure above captures — so weigh these against the upfront price rather than chasing the lowest sticker.

Other factors

Temperature coefficient (how much output drops as the panel heats up) matters in hot climates; a lower coefficient holds output better on a scorching roof. Build quality, the manufacturer's track record and whether they'll honour a 25-year warranty also count — a warranty is only as good as the company behind it. We deliberately don't name brands; the point is to compare on these objective metrics rather than reputation alone.

The lifetime cost-per-kWh view

The single most honest way to compare panels is to divide each option’s total installed cost by the total energy it will produce over its life — its lifetime cost per kWh. This rolls every factor that matters into one number: a panel’s price, its efficiency (via how much it generates in your space), its degradation rate and its expected lifespan. A premium panel that costs more upfront but degrades slowly and lasts longer can deliver cheaper energy over 25 years than a bargain panel that fades faster. Sticker price and even cost per watt can mislead; lifetime cost per kWh rarely does, which is why the calculator above reports it and why it’s the figure worth anchoring your decision on.

Frequently asked questions

Only when roof space is limited. If you can't fit enough standard panels to meet your needs, high-efficiency panels pay off by fitting more capacity into less area. With ample roof, cheaper panels at a lower cost per watt usually give the same output for less money.

Use cost per watt — price divided by rated watts — which normalises panels of different sizes. It tells you which delivers capacity most cheaply. Then factor in efficiency (if space-constrained), warranty and degradation for the full picture.

Around 0.4–0.5% per year or lower is good; it means the panel retains more output over its 25-year life. Premium panels sometimes degrade more slowly, which can justify a higher price through greater lifetime energy production.

For total output, what matters is your array's total watts and your sun-hours — wattage and panel count together. Efficiency matters mainly for fitting that capacity into limited space. On a large roof, a cluster of cheaper, lower-efficiency panels can match a few premium ones for less.