Buyers are offered solar systems by size, as if size were the decision. It is the result of a decision. This page compares the four common ways a size gets chosen — by roof, by bill, by load list and by backup list — and shows what each one gets right and wrong.
Sizing to the roof fills every available metre and hopes consumption agrees; it suits a building whose daytime demand plainly exceeds anything the roof can generate, and disappoints elsewhere. Sizing to the bill works backwards from spending, which confuses price with energy and breaks whenever tariffs or habits change. Sizing to the load list starts from what the building actually runs — slower to do, and the only approach that survives contact with the delivered system.
Sizing to the backup list is a special case of load-list sizing, applied to the circuits that must survive an outage. Most real projects combine approaches: the load list sets the target, the roof sets the ceiling, and the backup list sets the storage.
| Approach | Appeal | Weakness |
|---|---|
| By roof | Fast | Generates without reference to use |
| By bill | Familiar figures | Confuses money with energy |
| By load list | Matches the building | Needs an honest load list |
| By backup list | Protects what matters | Covers listed circuits only |
Before ordering, confirm the minimum order quantity and lead time for your configuration, and ask which documents apply to the exact system you are quoted.
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