Solar & battery
Solar Backup Calculator
Sizes a backup system, where power and energy are separate requirements.
Result
Outages rarely happen in good weather. Sizing on a sunny-day solar figure is optimistic, because the storm that took the grid down is the same storm shading the array. Run this with the solar figure set low as well.
Get a Free Business AuditHow the calculation works
Two separate requirements: the power the inverter must deliver to run the backed-up loads simultaneously, including motor starting surges, and the energy the battery must store to run them for the intended duration. A system can satisfy one and fail the other, which is why both are calculated.
What moves the number
- Simultaneous load in watts, including surge.
- Energy required over the intended duration.
- Motor starting currents for pumps, compressors and wells.
- Whether loads can be managed rather than all run at once.
Where this one goes wrong
Motor starting surge is the requirement that catches backup designs, because a well pump or an air conditioning compressor draws several times its running current briefly on start, and an inverter sized to running loads trips instead of starting it. Check surge requirements for every motor load. Load management is the cheaper alternative, since preventing two large loads starting simultaneously can halve the inverter requirement.
Before you quote
- Size the inverter for motor starting surge, not running load.
- Size the battery for energy over the intended duration.
- Consider load management instead of larger equipment.
Questions
About this calculator
Why does my backup inverter trip when the pump starts?
Motor starting surge. A well pump or compressor draws several times its running current briefly on start, and an inverter sized to the running load cannot supply it. Surge capability has to be checked for every motor load.
Is power or energy the binding constraint?
They are separate requirements and a system can meet one and fail the other. Power determines what can run at once; energy determines for how long. Both have to be calculated, and the design is set by whichever is harder to meet.
Can load management reduce the cost?
Considerably, since preventing two large loads from starting simultaneously can halve the inverter requirement. Managing the loads is almost always cheaper than sizing equipment to handle every possible coincidence.
The formula
battery = (daily backup load - daily solar) x days / usable fraction
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How to use this calculator
- What to back up. Enter your own figure; the page starts at Essential loads only.
- Normal monthly use (kWh). Enter your own figure; the page starts at 950.
- Outage to plan for (days). Enter your own figure; the page starts at 2.
- Solar production per day (kWh). Enter your own figure; the page starts at 25.
- Battery cost per kWh installed ($). Enter your own figure; the page starts at $900.
The result updates as you type, so you can try a figure, see what it does to battery needed, 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 |
|---|---|
| What to back up | Essential loads only |
| Normal monthly use (kWh) | 950 |
| Outage to plan for (days) | 2 |
| Solar production per day (kWh) | 25 |
| Battery cost per kWh installed | $900 |
What comes out
| Result | Value |
|---|---|
| Battery needed main result | 0.0 |
| Daily backup load | 9.4 |
| Net daily draw after solar | 0.0 |
| Battery cost | $0 |
| Share of normal use backed up | 30.0% |
Solar production exceeds the backup load, so the battery only has to carry overnight. A modest bank is enough in this weather.
How sensitive the answer is
Holding everything else steady and changing only battery cost per kwh installed, this is what happens to battery needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Battery cost per kWh installed ($) | Battery needed |
|---|---|
| $450 | 0.0 |
| $675 | 0.0 |
| $900 example above | 0.0 |
| $1,350 | 0.0 |
| $1,800 | 0.0 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- What to back up
- Choose from Essential loads only, Essentials plus some comfort, Whole home.
- Normal monthly use (kWh)
- A number, between 0 and 200000, starting at 950.
- Outage to plan for (days)
- A number, between 0 and 30, starting at 2.
- Solar production per day (kWh)
- A number, between 0 and 2000, starting at 25.
- Battery cost per kWh installed ($)
- A money amount, starting at $900.
What the results tell you
| Result | What it is |
|---|---|
| Battery needed | The headline result. A decimal figure. |
| Daily backup load | A decimal figure. |
| Net daily draw after solar | A decimal figure. |
| Battery cost | A money figure. |
| Share of normal use backed up | A percentage. |
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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