Solar & battery
Solar Energy Calculator
Works energy over time, which is what bills and sizing are both measured in.
Result
The escalation rate does most of the work in any long solar projection and nobody knows it. Rates have risen faster than general inflation in some markets and fallen in others. Run it at a low figure as well as an optimistic one before treating the total as real.
Get a Free Business AuditHow the calculation works
Power multiplied by hours gives energy, aggregated to daily, monthly and annual figures. Because both consumption and production are energy quantities, this is the common unit in which a system and a household can be compared, and the comparison should be made monthly rather than only annually.
What moves the number
- Daily and seasonal variation in production.
- Consumption pattern through the day and year.
- Whether production coincides with consumption.
- Tariff structure, where time of use applies.
Where this one goes wrong
Matching annual energy is not the same as matching consumption, because production peaks in the middle of the day and in summer while a household's use frequently peaks in the evening and in winter. Under a time of use tariff or a tariff that credits exports below retail, that timing mismatch is the whole economics, and an annual energy match can leave a bill far larger than expected.
Before you quote
- Compare production and consumption monthly, not annually.
- Check the timing of use within the day under a time of use tariff.
- Read what exported energy is actually credited at.
Questions
About this calculator
Does matching annual energy zero my bill?
Not necessarily. Production peaks midday and in summer while household use often peaks in the evening and in winter, and under a tariff that credits exports below retail that mismatch leaves a meaningful bill despite the annual totals matching.
Why compare monthly rather than annually?
Because the seasonal shape is what determines how much is exported and how much is imported. Two systems with the same annual production can produce very different bills depending on how well their monthly shape matches the household's.
Does a time of use tariff change the design?
It can, since the value of production depends on when it occurs. A west-facing array produces less annually and more in the late afternoon, which under some tariffs is worth more, and that is a design conversation rather than an afterthought.
The formula
each year: production x rate, with production degrading 0.5% and the rate escalating
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How to use this calculator
- System size (kW DC). Enter your own figure; the page starts at 7.2.
- Peak sun hours per day. Enter your own figure; the page starts at Mid-Atlantic, Ohio Valley (4.5).
- System derate (%). Enter your own figure; the page starts at 80%.
- Electricity rate per kWh ($). Enter your own figure; the page starts at $0.16.
- Annual electricity price rise (%). Enter your own figure; the page starts at 3%.
- Years. Enter your own figure; the page starts at 25.
The result updates as you type, so you can try a figure, see what it does to value of the energy, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| System size (kW DC) | 7.2 |
| Peak sun hours per day | Mid-Atlantic, Ohio Valley (4.5) |
| System derate | 80% |
| Electricity rate per kWh | $0.16 |
| Annual electricity price rise | 3% |
| Years | 25 |
What comes out
| Result | Value |
|---|---|
| Value of the energy main result | $51,605 |
| Annual production | 9,461 |
| Value in year one | $1,514 |
| Energy over the period | 222,858 |
| Electricity rate in the final year | $0.34 |
The system generates 222,858 kWh over 25 years, worth $51,605 at an electricity price rising 3 a year.
How sensitive the answer is
Holding everything else steady and changing only electricity rate per kwh, this is what happens to value of the energy. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Electricity rate per kWh ($) | Value of the energy |
|---|---|
| $0.08 | $25,803 |
| $0.12 | $38,704 |
| $0.16 example above | $51,605 |
| $0.24 | $77,408 |
| $0.32 | $103,210 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- System size (kW DC)
- A number, between 0 and 5000, starting at 7.2.
- Peak sun hours per day
- Choose from Pacific Northwest, northern UK (3.5), Northeast, Midwest (4.0), Mid-Atlantic, Ohio Valley (4.5), Southeast, coastal California (5.0), Texas, inland California (5.5), Arizona, Nevada, New Mexico (6.0).
- System derate (%)
- A percentage, between 40 and 100, starting at 80%.
- Electricity rate per kWh ($)
- A money amount, starting at $0.16.
- Annual electricity price rise (%)
- A percentage, between 0 and 20, starting at 3%.
- Years
- A number, between 0 and 40, starting at 25.
What the results tell you
| Result | What it is |
|---|---|
| Value of the energy | The headline result. A money figure. |
| Annual production | A number. |
| Value in year one | A money figure. |
| Energy over the period | A number. |
| Electricity rate in the final year | A money figure. |
A design still needs a site visit
Output and sizing figures use the assumptions you enter, not measured irradiance, shading or roof orientation at your address. Real production depends on all three, plus inverter behaviour and local interconnection rules.
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