You can get surprisingly close to the right system size from one bill and a little arithmetic. Here is how we do it, so you can check anybody's figures including ours.
Step one: find your monthly units
Your bill shows units consumed, in kWh. Take an average across a few months rather than one, and make sure you include a summer month — sizing from a mild month guarantees a system that disappoints in July.
Say the average is 600 units a month. That is about 20 kWh a day.
Step two: decide how much of it you want solar to cover
This is a decision, not a calculation, and it is where honest advice matters.
Covering your whole consumption means a system large enough for the worst day of the year, which is expensive and idle most of the time. Covering the daytime portion is usually where the sense is.
If roughly half your use is in daylight, you are targeting about 10 kWh a day.
Step three: convert to panel capacity
In this region a well-oriented, unshaded array yields roughly four kilowatt-hours per installed kilowatt per day, averaged across the year. It is more in spring, less in a cloudy December.
So:
10 kWh a day ÷ 4 = 2.5 kW of panels
With 590 W modules that is five panels. Round up rather than down: panels are the cheapest part per unit of output, and an array sized exactly to today's use has no room for tomorrow's.
Step four: size the inverter separately
The inverter is not sized from your bill. It is sized from the load you run at the same time, with headroom for motor startup. A house using 600 units a month might need a 5 kVA inverter or a 7 kVA one depending entirely on whether the air conditioners run together.
We cover that arithmetic in what size inverter do you need.
Step five: size the battery, if you are having one
From the energy you want overnight, not from the panels.
Suppose you want lights, fans, a fridge and a television for five evening hours: roughly 500 W for five hours, about 2.5 kWh. One 25.6 V 100 Ah lithium unit holds 2.56 kWh, so one unit covers it with nothing to spare. Two gives you a comfortable margin and a slower, gentler cycle, which the battery repays in years.
Add an air conditioner overnight and the figure changes completely. This is the step where wishful thinking costs the most.
The assumptions, written down
Every figure above rests on something:
- 4 kWh per kW per day assumes good orientation, no meaningful shading, and panels that get cleaned.
- Half your use in daylight is a guess until you look at your actual habits.
- Appliance wattages vary; an inverter air conditioner and a conventional one of the same tonnage are not comparable.
A survey replaces each of these with a measurement. That is what it is for, and it is why we do one before ordering anything rather than after.
What we would rather you did
Use the load calculator. It works from the appliances you own rather than from an average, and it takes a few minutes. Then a sales engineer prepares an itemised quotation from it, at no charge.
If you would rather talk it through, call us — we would rather spend twenty minutes getting it right than sell you a system that disappoints.