Collect usage, not just the amount you paid
Start with approximately twelve consecutive months of billed electricity use. Add the kWh, not the dollar amounts. Record the dates and whether readings were measured or estimated. A single unusually hot or cold month is a weak substitute for an annual picture.
If you only have a short history, keep the calculator result labeled as a provisional scenario. Multiplying one month by twelve is an assumption; it does not reveal seasonal use or changes in occupancy.
Separate variable and fixed charges
The flat-price calculator needs the cost associated with one fewer kWh bought from the grid. A fixed monthly connection or customer charge generally does not disappear when your small solar setup reduces consumption.
- Energy used
- 320 kWh
- Variable electricity charges
- $96
- Fixed monthly charge
- $15
Working unit price: $96 ÷ 320 = $0.30/kWh. Dividing the full $111 by 320 would give about $0.347/kWh and include a charge that the example assumes solar does not avoid.
Your bill may split variable charges across several lines. Check which change with consumption, and include applicable variable taxes consistently. Ask your utility if the bill is unclear.
Annual use is only half the story
Write down a normal weekday and weekend: when people are home, when cooking happens and whether large loads tend to run during sunny hours. Choose the closest calculator profile rather than pretending you have hourly measurements.
If interval data is available, use it to question your assumptions. Otherwise, a seven-day diary is a practical starting point. Record time periods and notable loads; do not add appliance nameplate watts and treat the result as continuous household use.
Keep watts and kilowatt-hours distinct
Watts describe power at a moment; kWh describe energy over time. As a unit example, a constant 100 W load for 24 hours uses 2.4 kWh. It is not a measured estimate of your home, and many appliances cycle on and off.
When a bill reports total kWh, enter that energy figure in the annual-use field. Enter panel Wp and the inverter’s AC limit in their separate system fields. Mixing those units can produce a precise-looking but meaningless result.
Recognize when a flat rate is not enough
Time-of-use rates, demand charges and credits for exported energy can materially change the comparison. The current tool uses one electricity unit price and no export payment. It does not optimize charging against a variable tariff.
Before you calculate, keep three notes beside you: the dates behind your annual kWh, how you derived the unit price, and why you chose a usage profile. Those notes make a saved scenario much easier to revise when a bill or routine changes.
Sources & further reading
Worked examples use assumed values and are not product tests. A question or correction? Contact the publisher.
Make it personal
Turn a useful guide into your own comparison.
Put your notes into a comparison with and without storage. Change an assumption and see what it affects.
Compare solar & storageCheck the rules first