Solar power can slash electricity bills and emissions, but a system that is too small disappoints and one too large wastes money. Sizing it correctly, and understanding how it connects to the grid, is the key to a system that pays off. This guide walks through the calculations and the decisions behind them.
Start with your electricity usage
Before sizing anything, look at your bills to find your daily and annual energy use in kilowatt-hours, and notice when you use power — a household that is out all day and uses electricity in the evening has different needs from one that runs appliances at midday. This usage profile drives every other decision, because there is no point generating power you cannot use or store. Knowing your numbers first prevents both over- and under-sizing.
How many panels do you need?
Panel count depends on your daily energy use, the peak sun hours your location gets, and each panel's wattage. More usage or fewer sun hours means more panels. The solar panel calculator turns these inputs into the number of panels and the total system size in kilowatts, which is the headline figure installers quote.
Sizing the inverter and battery
The inverter converts the panels' DC output into the AC your home uses, and it must handle your peak load. Any battery must store enough energy for the backup or night-time use you want. An undersized inverter trips under load; an undersized battery runs flat overnight. The inverter size calculator and battery bank calculator size both correctly. If you also want a backup generator for long outages, the generator size calculator helps.
Grid-tie, net metering and off-grid
Most homes use a grid-tied system: solar offsets your usage and surplus power is exported. Where net metering applies, you are credited for that exported energy, which improves the economics and can reduce or remove the need for a battery. An off-grid system, by contrast, must be sized with enough panels and battery to cover the worst days entirely, making it larger and costlier. Which model applies in your area strongly affects both sizing and savings.
Orientation, shading and losses
A panel's output depends on facing the sun and being free of shade. Even partial shading from a tree or chimney can disproportionately cut a string's output, and a poor orientation reduces yield year-round. Real systems also lose some energy in wiring, the inverter and dust. This is why you size with headroom and use conservative sun-hour figures rather than ideal-lab numbers.
What will you save?
The financial case rests on two numbers: your annual savings and how long the system takes to pay for itself. Savings depend on how much of your bill solar offsets and any export credits; payback divides the net system cost by those savings. The solar savings calculator and solar payback calculator show both, often revealing a payback of several years followed by largely free electricity for the rest of the panels' 25-year-plus life.
A long-term investment
Solar is best viewed as a long-term investment rather than a gadget. Panels degrade slightly each year but typically still produce strongly after two decades, and they need little maintenance beyond occasional cleaning and an eventual inverter replacement. Size with about 20% headroom, use conservative assumptions, and the system that comfortably meets your needs on an average day will keep delivering savings — and avoided carbon — for many years. The kWh to CO2 calculator shows how much CO2 your generation avoids each year, the environmental return on top of the financial one.
Maximising self-consumption
With a grid-tied system, the savings are often largest when you use your own solar power directly rather than exporting it cheaply and buying it back expensively later. Shifting heavy loads — washing, dishwashing, water heating, EV charging — to daylight hours can dramatically improve the payback, sometimes more than adding panels would. A timer or smart plug makes this automatic. Where electricity tariffs vary by time of day, a battery becomes more attractive because it lets you store cheap or self-generated power for use at peak-price times. The cheapest unit of energy is always the one you do not have to buy, so before sizing a bigger system, look at how and when you use power — small changes in habits and timing can lift the return on a modest system above that of a larger, dearer one used carelessly.