You can buy a $4,000 home energy monitor that streams real-time per-circuit consumption to a phone app. For most people that is overkill. The same useful information, for the narrow purpose of generator sizing and outage sequencing, fits in a spreadsheet with eight columns. This walkthrough builds that spreadsheet from scratch and shows how to use it the way an electrician would.
The goal: be able to answer, in under thirty seconds, "if the power goes out right now, what is the worst single-instant load my generator has to support?"
Step 1: pick a tool
Anything with formulas works. Google Sheets is free, has revision history, and you can share it with a spouse who can update it on their phone. Excel works the same. LibreOffice Calc works the same. Even a Notion table or an Airtable view with a couple of formulas will do the job. Pick whatever you already use.
Step 2: the columns
The minimum schema:
| Appliance | Circuit | Watts (run) | Watts (surge) | Always on? | Cycle (min) | Priority | Notes |
- Appliance is the human name (fridge, well pump, garage door, etc).
- Circuit is the breaker number, once you have your panel labeled. This matters during the outage so you know what to flip off if the generator trips.
- Watts (run) comes from the nameplate. Multiply amps × volts if the label gives amps. A 1.2A appliance at 120V is 144 running watts.
- Watts (surge) is the inrush. For resistive loads (lights, kettles, toasters), this equals running watts. For variable-speed inverter motors, multiply running by 1.5. For older single-speed induction motors (well pumps, sump pumps, classic fridges), multiply running by 3. The Wikipedia article on locked-rotor current covers the underlying physics if you want to verify.
- Always on? is a yes/no for outage planning: would this stay plugged in continuously during an outage, or only when in active use?
- Cycle (min) is rough: how often does this thing turn on per hour, if it cycles? Fridge: every 20 minutes. Well pump: variable. Microwave: only when in use.
- Priority is 1 (critical, must run) to 3 (nice to have).
- Notes is free-text for things like "starts hard in cold weather" or "soft-start kit installed 2025".
Step 3: the summary formulas
At the bottom or in a separate tab, add four cells:
-
Total running watts (always-on only): SUMIF on
Always on? = yesof the running-watts column. - Largest single surge: MAX of the surge-watts column, filtered to priority 1 and 2.
- Worst-case overlap: largest surge + sum of running watts of all other always-on appliances. This is your worst-case single-instant load.
- Headroom on a 7,500 watt generator: 7500 minus worst-case overlap. Negative means your generator will trip during overlaps.
That last formula is the entire point of the spreadsheet. It tells you, in one cell, whether your current generator can actually do the job.
Step 4: validation step
Before you trust the numbers, validate them with a clamp meter. A $40 Fluke or AstroAI clamp meter will read the current on any individual circuit. Flip on the appliance, watch the inrush, watch it settle. Update the spreadsheet with the real numbers.
Most appliances behave close to the rule-of-thumb estimates, but some surprise you. Modern inverter fridges often draw less surge than the formula predicts. Older deep freezers often draw more. The thirty minutes of measurement removes the guesswork.
Step 5: feed it back into your outage plan
Once the spreadsheet is populated and validated, two things become useful:
- Sort by surge watts descending. That is your startup order during an outage: biggest motor first, smallest last, never two big motors in the same ten-second window. The longer walkthrough at How to Sequence Appliance Startups on a Portable Generator During an Outage explains the why.
- Sort by priority + always-on. That is your "what to plug in first" list, which can differ from the surge-ordered list if some big-surge items are low-priority.
Print both sorts. Tape them inside the panel door.
Why not just use a calculator?
For most people, the spreadsheet is overkill. If you do not already work with spreadsheets daily, building one from scratch is a barrier. A purpose-built tool like the one at EvvyTools has the appliance database, the surge multipliers, and the sequencing logic already wired in. You input the appliances you care about, and it produces the load total and the suggested order.
The spreadsheet wins if:
- You have unusual appliances not in any standard database (commercial-grade equipment, specialty pumps, three-phase tooling).
- You want to track multiple scenarios (summer outage with AC, winter outage with furnace).
- You want version history of how your home's load profile changes as you add or remove appliances.
The calculator wins if:
- You want it done in five minutes.
- You do not care about historical tracking.
- You want the surge multipliers to be expert-tuned per appliance category instead of relying on rule-of-thumb 3x estimates.
Both approaches use the same underlying logic. The calculator is just a faster path through it.
Useful references
- Ready.gov power outage prep
- Wikipedia: Locked rotor current
- ENERGY STAR appliance database for running-watt nameplate data when the physical appliance is hard to read.
- CDC generator carbon monoxide safety
A more advanced version: time-series scenarios
Once the basic spreadsheet is working, the next upgrade is to add scenario columns. Instead of a single "always-on?" column, you have one column per scenario: "Summer outage", "Winter outage", "Brief outage under 4 hours", "Multi-day outage". Each cell is yes or no per appliance per scenario.
This sounds like overkill but it actually changes the planning. A window AC unit is essential in a summer outage and irrelevant in a winter one. The furnace blower is the opposite. A multi-day outage might justify running the well pump's pressure tank fully every night so you have water for the morning even if the pump is off most of the time.
Each scenario column gets its own summary formulas at the bottom. You end up with four worst-case-overlap numbers, one per scenario. The largest of the four is what you size the generator for. The others tell you how much headroom you have in less-demanding situations.
Why a spreadsheet beats memory
The strongest argument for the spreadsheet (over a one-time mental calculation) is that load profiles change. You buy a new fridge. You add a freezer. You install an electric vehicle charger. You replace a single-speed pool pump with a variable-speed one. Each change shifts the worst-case overlap, sometimes by a lot.
A spreadsheet you actually maintain is a few cells of update per change. A mental calculation requires you to redo the whole thing from scratch, and most people simply do not. The result is a load profile that has drifted from what your generator was sized for, often for years, until an outage reveals the gap.
Quick checklist for the spreadsheet build
- [ ] Schema with appliance, circuit, run watts, surge watts, always-on, cycle, priority, notes
- [ ] At least one row per always-on appliance during an outage
- [ ] Surge watts populated using a sane multiplier (1x resistive, 1.5x inverter motor, 3x induction motor)
- [ ] Summary cell for total always-on running watts
- [ ] Summary cell for largest single surge
- [ ] Summary cell for worst-case overlap (largest surge plus all other always-on running)
- [ ] Summary cell for generator headroom (generator surge rating minus worst-case overlap)
- [ ] Optional: scenario columns for summer, winter, brief, multi-day
- [ ] One round of clamp-meter validation against the rule-of-thumb estimates
The spreadsheet is a one-Saturday project. After that it pays you back every time you add an appliance, replace a fridge, or wonder whether you can run the espresso machine and the microwave at the same time during the next ice storm.
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