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For retailers

Free tool

Commercial solar calculator

Work out the size, savings, payback and 25-year return of solar for your business. Every assumption is shown and can be changed, and you can download the working. No email needed.
Engineer comparing a solar roof layout drawing with a cost spreadsheet
1. Where and how much power

Sets the solar zone, which sets how much the system is assumed to produce.

kWh

Add up 12 months of the kWh figure on your bills, or ask your retailer for 12 months of data.

c/kWh ex GST

Victorian Default Offer 2026-27 small business flat usage rates are 24.77 to 34.23 c/kWh including GST (about 22.5 to 31 c excluding GST; Essential Services Commission, from 1 July 2026). We use 25 c excluding GST, a cautious figure from the low-to-middle of that range. Other states and larger users pay different rates: use your own bill. Include the per-kWh network charges on your bill; leave out demand and supply charges.

2. When the business uses power
days

Solar produces every day, but only days you’re using power can use it directly.

3. The system
kW

Enter a postcode and usage to get a suggested size, or type your own.

$/kW ex GST

$ ex GST

Agents usually pay $33 to $38; the clearing house pays $40. Ask your installer what they use.

c/kWh

Assumption, deliberately low: many business plans pay little for exports. Use your own plan.

4. Prices over time
% a year

Assumption: the middle of the Reserve Bank of Australia’s 2 to 3% inflation target. Prices can rise faster or fall.

% a year

Assumption: a typical business hurdle rate. Use your own cost of capital. It’s the return you’d want from the money otherwise.

Advanced assumptions
% of output

Leave blank to use our model: an average day, with output between 6am and 6pm and usage spread over your opening hours. It errs high, so use interval data from your retailer if you have it.

% of usage

Assumption: the share of yearly usage that runs outside operating hours. Check your interval data.

% a year

Assumption: panel makers commonly warrant output declining by about half a percent a year.

$/kW a year

Assumption: cleaning, inspections and monitoring, $ per kW per year, rising with inflation. Excludes inverter replacement.

Your estimate

Add a postcode and your usage

The results appear here as you fill in the form. Nothing is sent anywhere: the sums run in your browser.

What this doesn’t include

  • Demand charges. Solar often saves less on these than on energy charges, because a cloudy afternoon can still set your monthly peak.
  • Batteries, which add cost and change when you use power.
  • Finance costs: interest, fees or lease payments if you don’t pay cash.
  • Tax and depreciation, and GST. Prices here exclude GST, which most GST-registered businesses claim back. Ask your accountant.
  • Network connection costs and studies (see grid connection), roof repairs, switchboard upgrades, ground-mounting, and inverter replacement (usually once in 10 to 15 years).
  • Day-to-day variation in your load, cloud and seasons. The self-consumption figure is a smooth average and errs high.

This is an estimate to help you decide what to ask installers, not a quote or financial advice. Check certificates with the STC calculator and read is commercial solar worth it? for when the answer is no.

How the calculator works

How does the calculator work out how much solar I use myself?
It takes an average day. Output follows a smooth curve from 6am to 6pm. Your usage is a baseload that runs all day plus the rest spread evenly across your opening hours. In each hour you use the smaller of the two; the rest is exported. It ignores cloud, seasons and lumpy loads, so it errs high. If you have interval data from your retailer, type your own figure under Advanced assumptions.
Why is the payback longer than what installers quote?
Quotes often use a higher electricity rate, assume every kWh is used on site, or leave out maintenance and falling output. This calculator shows each of those and lets you change them. Use the same inputs when you compare quotes.
What do NPV and IRR mean?
Net present value (NPV) adds up all the future savings, each reduced by your discount rate to reflect that money later is worth less than money now, then subtracts the net cost. Above zero means the system beats your discount rate. The internal rate of return (IRR) is the yearly return that makes the NPV exactly zero.
Does it include demand charges?
No. If your bill has a demand charge, solar usually reduces it by less than it reduces the energy charge, and sometimes not at all. Read our guide to demand charges before you rely on the savings figure.
Which certificates does it use?
Up to 100 kW it uses small-scale STCs: size × zone rating × the years left until 2030. From 100 kW up to 1 MW, installed on or after 1 October 2026, it uses a fixed 5 years. Above 1 MW there are no STCs. See the STC calculator for the detail.
Where do the default prices come from?
The installed price comes from Solar Choice’s September 2026 average commercial prices, with GST taken out and the STCs added back; above 100 kW we hold the 100 kW level because no public table covers it. The electricity rate is from the Victorian Default Offer for small business. Every default is editable and its source is shown beside it.

Sources

Checked 31 October 2026. This page is general information, not financial, tax or legal advice. Rules, rebates and prices change, so check the sources before you decide.

Ready to compare real numbers?

Run your own figures first, then ask at least three accredited installers to quote on the same brief. Quotes are free and you’re under no obligation.

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