Usually yes, up to a point, and the reason is about costs rather than generation.
Why bigger is usually better value
A large share of an installation is fixed cost: scaffolding, certification, network paperwork, travel, setting up, making good. Those barely change between eight panels and sixteen.
So the extra panels cost mostly just the panels and their mounting. Per watt, the marginal capacity is much cheaper than the average.
This is why installers usually suggest filling the usable roof, and on this point the advice is generally sound rather than a sales tactic. See comparing system sizes.
Where it stops being better
When the extra generation is mostly exported.
A unit used at home saves you about 26.32p. A unit exported earns roughly half. As the system grows relative to your usage, a larger share of each additional panel’s output goes out at the lower value.
The extra panels are still cheap. They are just earning at half rate.
When it crosses the grid connection threshold Going above 3.68kW per phase moves you from a simple notification to an application, with delay and the possibility of a restriction. See grid connection.
When it forces a larger inverter The step up in inverter cost can make an increment less cheap than it looks.
When the extra panels go on a poor roof face The same mounting cost for much less generation.
When the roof or the budget says no Obviously.
The things that push the answer towards bigger
- A battery. Stores what you would have exported.
- An electric car. A large flexible load.
- A heat pump. A large load across most of the year.
- Someone home during the day.
- A time-of-use export tariff, which changes what exported units are worth.
- Expecting to electrify later. A system sized for today may be undersized in five years, and adding to it later is more expensive than building it bigger now. See adding to a system.
That last point is worth weighing seriously. Most households’ electricity use is going up, not down.
How to settle it
Ask for two quotes at different sizes, then:
Extra cost = larger quote - smaller quote
Extra generation = larger estimate - smaller estimateThen ask what share of the extra generation the installer expects you to use rather than export. Value the used portion at your import price and the exported portion at your export rate. That gives you the annual value of the increment, and you can compare it against the extra cost directly.
If the marginal panels pay for themselves comfortably within their warranty period, fit them.
The short version
Fill the roof if you have, or plan to have, something that uses a lot of electricity: a battery, a car, a heat pump, or somebody at home. If none of those applies, size closer to your usage and put the difference into a battery instead.