Every solar system design starts from the same question: how much energy does this place actually use? Get this number right and every other calculation — panels, battery bank, inverter, even the charge controller — follows naturally. Get it wrong, and you're sizing an expensive system around a guess.
Why track every appliance individually
A single "average daily usage" number from a utility bill tells you the past, but not the shape of your actual load — and shape matters for solar sizing. Two homes using the same total daily energy can need very different systems: one running everything in daylight hours is easy to serve directly from panels, while one running most of its load at night needs a battery bank sized around it.
Daily energy vs peak load — two different numbers, two different jobs
| Number | What it drives |
|---|---|
| Daily energy (kWh/day) | Panel array size and battery bank capacity — how much energy you need to generate and store |
| Peak simultaneous load (W) | Inverter sizing — how much instantaneous power the system needs to deliver at once |
It's common to have a modest daily energy total but a surprisingly high peak load, if a few high-wattage appliances (AC, water heater, power tools) happen to run at the same time as everything else. This calculator tracks both so neither gets overlooked.
Where to find accurate wattage numbers
- Rating label. Most appliances list rated wattage (or amps × volts) on a label, usually on the back or bottom.
- Actual measurement. A plug-in power meter gives real-world wattage, which is often lower than the rated "maximum" figure for things like fridges and AC units that cycle on and off.
- Manufacturer specs. If the label is worn or missing, the manufacturer's website or manual usually lists power consumption.
Common mistakes when estimating daily load
- Using nameplate max instead of typical running watts for appliances that cycle (fridges, AC units) — this overestimates daily energy, though the nameplate figure is still useful for peak load.
- Forgetting small, always-on loads — routers, security cameras, standby power on electronics. These run 24 hours a day and add up more than their small wattage suggests.
- Guessing hours of use instead of tracking actual habits for a few days first, especially for seasonal loads like AC or heating.
Your daily load figure is the starting point for sizing the rest of the system: feed it into the inverter calculator to size your inverter, the battery calculator to size your bank, or the off-grid system wizard to size everything at once.
Frequently asked questions
Should I add a safety margin on top of this total?
Yes — most system designers add 20-25% to the calculated daily energy total to account for inverter/battery losses, future load growth, and days of lower-than-expected sun. Our other calculators already build in their own safety margins where relevant.
How does this relate to my utility bill?
Your utility bill's monthly kWh figure is a good sanity check — if this calculator's monthly total is wildly different from your bill, double-check appliance wattage or hours of use for errors.
What if my usage varies a lot by season?
Build this list for your highest-usage season (typically summer with AC, or winter with heating) if you're sizing an off-grid system — designing around the peak season avoids under-sizing for the months that matter most.