SizeMyGenerator

Generator Sizing Calculator

Find the perfect backup generator for your home

Takes about 60 seconds • No email required

Generator Sizing Calculator
What do you want to power?
Step 1 of 5
Home details
Step 2 of 5
Used to estimate elevation & typical summer high. Leave blank to assume sea level & 60 °F.
NEC general lighting load = 3 watts per sq ft × this number. Include garage & basement.
Appliances & circuits
✅ Preset applied. 0 items selected.
Step 3 of 5
HVAC & EV loads
Step 4 of 5
❄️ Air conditioning
Add one row per outdoor condenser (e.g., 3-ton up, 2-ton down).
🔥 Heating
Per NEC, we use the larger of heating or A/C. For heat pumps, we add 65% of supplemental heat.
🔌 EV chargers
Included at 100% like A/C. Enter charging/breaker amps at 240 V.
⚡ Max starting load
Enter details for the appliance with the highest starting surge (often an A/C or well pump). Use the largest starting amps (LRA) you see on any appliance tag. The yellow guide below shows where to find it.
Used to adjust peak (surge) amps for the recommendation. Soft starters typically cut start amps ~50%.
If enabled, we count the sum of unmanaged + the largest managed item for each category (general, HVAC, EV).
LMS items — No LMS items selected
Select items to manage (shed/sequence). Only the largest managed item per category is counted along with unmanaged loads.
Managed count
0
Largest managed (Appl.)
0 W
Largest managed (AC/EV)
0 W
Settings & safety margin
Step 5 of 5
?
Default is 20%. Most homes use 10–30% headroom.

How to use this calculator

1

Choose your coverage level

Select whether you want to power essentials only, most of your house, or everything during an outage.

2

Enter your home details

Provide your ZIP code (for elevation/temperature adjustments) and square footage for accurate calculations.

3

Review & adjust appliances

We'll pre-select common appliances based on your coverage level. You can customize the list and wattages as needed.

4

Add HVAC, heating & EV chargers

Include any air conditioning units, heating systems, or electric vehicle chargers you want to power.

5

Get your recommendation

Use the summary card to see planning load, needed amps and peak surge, then talk to a licensed electrician.

Finding appliance wattage & starting amps

Look for the metal nameplate or data tag on your appliance (usually on the back or bottom).
Find "Watts" or "W" — this is the running wattage you'll enter here.
If only Amps and Volts are listed: Watts = Amps × Volts (e.g., 5A × 120V = 600W).
For motors/compressors, look for "LRA" or "Starting Amps" to use in the Max Starting Load section.
📸 Video tutorial and photo examples coming soon!
Heads up: This calculator is for general information only and does not replace a site visit or detailed load study. Always consult a licensed electrician and follow local codes before purchasing or installing a generator.