How to read a commercial electricity bill
The charges on a commercial bill respond to different fixes. Learn which is which, and five checks to run on your last 12 months before you spend anything.
In short
- Most shortfalls have ordinary causes: faults, dirt, shading, settings and billing errors.
- You cannot see them without comparing actual output to a weather-adjusted expectation.
- A useful report names the cause, the owner and the date the fix was verified.
Common reasons output falls short
- Equipment faults
- A failed inverter, string or combiner can take part of the array offline while the rest keeps running, so the loss is easy to miss.
- Soiling
- Dust and dirt reduce output, and how much depends on the site and the season. Cleaning is only worth paying for if you know how much it recovers.
- Shading
- Vegetation grows and new structures go up. A small shaded area can cost more than its size suggests.
- Settings and limits
- Export limits, curtailment settings and control changes can quietly cap output.
- Lost monitoring
- If communications drop, the system keeps generating but nobody is watching.
- Metering and billing errors
- Your savings only show up if the bill reflects what the system produced.
- Maintenance gaps
- Inspections skipped, alarms ignored or parts not replaced.
How to know
Compare actual output with what the system should have produced given the sunshine it actually received. A common measure is performance ratio: actual output divided by the output an ideal system would have produced under the same conditions. Track it month by month. A sudden step down usually points to a fault or a changed setting. A slow decline points to soiling or degradation.
Then follow the money. Compare the saving on your bill with the saving in your business case. If output looks fine but the bill does not fall, the problem may be the tariff, the metering or your load rather than the system.
What good reporting looks like
- Output against forecast and baseline, for every site, every month
- The cause of any shortfall, not just the fact of it
- An owner and a date for each action
- The financial value lost and recovered
- Evidence that each fix worked
Keep your warranty rights alive
Warranty claims need evidence and often have notice periods. Record faults when they happen, keep the monitoring history and know when each warranty and guarantee expires. Finding a fault while a defects period is still running is a very different conversation from finding it after it has ended.
Our performance management work does this measuring every month, for systems we designed and for systems we did not.