Buying: spend on the parts that cannot be changed later

Budgets on this shelf tend to be allocated to whichever component has the most impressive number, and that is rarely where the money does most good.

Spend first on what is buried

Cable, fusing, terminations and the routes they take. These are chosen once, installed behind panelling, and changed only by dismantling. A system that has been sized generously here accepts a bigger battery later without a single modification.

Size the inverter for what runs, not what might

The inverter is one of the easiest components to replace and one of the easiest to over-buy, and over-buying has a running cost in idle consumption. Match it to the loads that actually run; a second, small inverter for the steady loads is cheaper than paying for the big one to idle.

Buy the battery on its qualifiers

Usable capacity rather than nominal, the charge and discharge current limits, the cycle rating with its depth of discharge and its end-of-life percentage, and whether the management system blocks low-temperature charging. Those are what separate two batteries with the same headline figure.

Let the panels wait if anything must

Panels are the easiest component to add to, and the mounting is the part that constrains how many will fit. Deciding the mounting early and adding modules later is a sequence that works; deciding the battery late rarely is.

Count the parts nobody quotes

A monitor, an isolator, connectors, crimping tools, a meter to check the result. They are unglamorous, they are not in anyone’s comparison table, and leaving them out of the budget is why the last stage of an installation always costs more than expected.