Use the calculator above to compare a generator, battery or combined backup option using home size, estimated critical load and backup hours. Keep the equipment, access and installation options consistent when you compare estimates.
Before choosing equipment, ask a licensed electrical contractor to confirm the loads, starting demand, transfer equipment, placement and permit requirements for your property.
What information should you gather first?
- The appliances and circuits you want to keep running.
- Their running demand and any motor starting demand.
- The backup hours you want to plan around.
- Your preferred backup type and available fuel or recharge options.
- Known panel, space or access constraints.
If backup power is part of a larger renovation, include those requirements in your home remodeling plan.
Which inputs does the calculator use?
Enter the property address and ZIP code, home size, estimated critical load in kW and backup hours. Choose the backup type, fuel type, access and quantity, then select the listed transfer switch, load management, permit, pad or mount, and removal options.
Critical Load (kW) is one estimated essential running-load value. Keep that value and the selected options consistent when comparing planning estimates.
What should a licensed electrical contractor review?
Keep appliance and circuit information for the electrical review, including running demand, motor starting demand and which loads may operate together. The points below describe that separate review.
- Critical loads: refrigerator, lighting, internet, medical devices, garage door, and similar essentials.
- Motor start-up: surge watts for HVAC, pumps, and compressors.
- Runtime goal: a few hours, overnight, or multi-day coverage.
- Comfort vs essentials: decide whether HVAC, cooking, or laundry stays on during outages.
- Backup type: portable generator, standby generator, battery backup, or a hybrid setup.
- Connection method: interlock kit, manual transfer switch, or automatic transfer switch (ATS).
- Panel constraints: available breaker spaces and service capacity.
- Fuel/recharge plan: gasoline storage, propane, natural gas, or solar/utility recharge.
- Placement: clearances, ventilation, noise considerations, and service access.
- Future changes: planned remodels, added circuits, or an EV charger.
What else affects project planning?
| Decision | Why it matters | Quick check |
|---|---|---|
| Scope | Defines what is included and prevents surprises. | Write a line-item scope and allowances list. |
| Site constraints | Access and hidden conditions can change the plan. | Check access and existing surfaces before work starts. |
| Selections | Long-lead items can drive the schedule. | Confirm availability before starting. |
| Permits | Some scopes require approvals and inspections. | Ask your city/county what applies to your project. |
Equipment availability, site conditions and permit review affect how the project can be scheduled.
What should be checked before choosing equipment?
Even with a calculator, generator battery backup sizing needs a field check. The goal is simple: your system starts reliably, runs the loads you care about, and connects to the home safely and legally.
Which loads and connections need review?
- Surge validation: confirm start-up loads for HVAC, pumps, and compressors (this is where many setups fail).
- Connection safety: choose an interlock/transfer switch/ATS that prevents backfeed and keeps circuits clearly separated.
- Load prioritization: verify which circuits run together and which ones should stay mutually exclusive.
- Runtime reality check: align fuel storage or battery capacity with your actual outage plan.
- Placement review: confirm clearances, ventilation/exhaust routing, and service access before equipment is ordered.
- Documentation: request a written load list and the assumptions used to size the system.
Planning Mistakes That Skew Sizing
- Using only running watts: ignoring surge watts leads to tripping and hard starts.
- Counting “everything” as critical: oversizing often comes from not prioritizing.
- Mixing watts and amps: mismatched units (or missing voltage) throws results off fast.
- Forgetting seasonal loads: heating or cooling demands can change the plan.
- Skipping the connection method: the safest setup needs the right hardware, not just a bigger generator.
- Assuming battery = whole-home: batteries excel at quiet essentials and short-to-medium runtime; whole-home coverage depends on capacity and recharge plan.
What should the electrical contractor confirm?
- Which loads can run together and which need priority.
- The transfer equipment, placement and service access.
- Permit and inspection responsibilities.
- The operating instructions and commissioning checks.
Which equipment and maintenance details matter?
The “materials” for backup power are less about finishes and more about dependable components: transfer equipment, correctly sized conductors, proper disconnects, and manufacturer-approved installation details. A clean install also makes future service simpler.
Core Components To Discuss
- Transfer equipment: interlock kits, manual transfer switches, or ATS hardware matched to your panel and loads.
- Subpanel strategy: a dedicated critical-load subpanel can simplify load management.
- Generator type: portable vs standby, inverter vs conventional, plus the fuel source plan.
- Battery system: inverter rating (kW), capacity (kWh), and how you plan to recharge.
- Protection and labeling: breakers, disconnects, and clear circuit labeling that matches the load list.
Maintenance That Prevents Surprises
- Generators: run periodic test starts, keep up with oil/filters per the manual, and manage fuel quality/storage.
- Battery backups: keep the area clear, check app/monitoring alerts, and apply manufacturer-recommended firmware updates.
- Transfer gear: test the transfer method during a planned outage drill and confirm labels stay accurate after any electrical work.
How can space and access affect the plan?
- Use a dedicated critical-load subpanel to keep the backup footprint focused on essentials.
- Plan wall space and service clearances early so equipment doesn’t block garage storage or access paths.
- Consider a hybrid approach: battery for quiet essentials, generator for extended runtime, sized to your actual priorities.
- Route conduit and cabling with future access in mind, especially in tight utility corners.
What access will future service need?
- Keep transfer equipment close to the main panel when possible to reduce complexity and long conduit runs.
- Confirm safe generator placement and exhaust management; never plan indoor operation.
- Leave room for shutoffs, service access, and future electrical upgrades.
- Match the layout to your outage behavior: quick manual transfer for essentials or automatic transfer for convenience.
What decisions come before installation?
Most homeowners move from “calculator results” to a working system by converting numbers into decisions: loads, runtime, connection method, equipment placement, and (when applicable) permits/inspections. Equipment lead times and jurisdiction review can influence the calendar as much as the install itself.
Typical Flow From Numbers To Install
- Build the load list and run the calculator.
- Confirm the backup strategy (generator, battery, or hybrid) and the runtime goal.
- Review the electrical panel and choose the transfer method.
- Finalize equipment placement and the wiring route.
- Handle permits/inspections if the scope requires it.
- Install, commission, label, and run an outage drill.
Key Decisions At A Glance
| Decision | What drives size | What it affects |
|---|---|---|
| Critical loads list | Which circuits you include | Continuous watts and inverter rating |
| Motor loads (HVAC/pumps) | Start-up surge watts | Generator kW and battery surge capability |
| Runtime target | Hours of coverage | Fuel storage or battery kWh capacity |
| Connection method | Interlock vs transfer switch vs ATS | Parts, labor, and permit scope |
| Placement | Distance to panel and clearances | Conduit runs, noise, and service access |
What can affect the installation schedule?
- Simple portable setups: can move quickly once the load list and connection method are confirmed.
- Standby generators and whole-home battery systems: may take longer due to equipment lead time and plan review.
- Permits/inspections: timing varies by city/county and scope, so it can be the main schedule driver.
- Work at the property: panel changes, equipment placement and access affect the installation sequence.
What should you take to the electrical review?
Bring the calculator estimate, appliance information, your load list, desired backup hours and photos of the existing panel and proposed equipment area. Discuss equipment selection after the site conditions and connection method have been reviewed.
Talk Through Your Options
If you’re comparing portable vs standby vs battery backup, a short call can clarify what fits your outage plan and how quickly it can be installed.
How should you read the calculator output?
The output usually boils down to two questions: how much power you need at once (watts/kW), and how long you need it (hours or kWh). Use the results to spot the “big drivers” in your load list, then decide whether you want to reduce peaks (load management) or increase capacity (bigger equipment).
- Continuous load: what runs steadily (lights, fridge, internet).
- Peak/surge load: what spikes when motors start (HVAC, pumps).
- Generator rating: running watts matter, but surge capability often decides usability.
- Battery capacity (kWh): ties directly to runtime assumptions and what you consider “critical.”
- Inverter power (kW): sets the ceiling for how many loads can run at once.
- Recharge plan: utility, solar, or generator recharge changes the practical runtime.
Planning backup power alongside a remodel?
Share the calculator estimate, your essential-load list and the other changes planned for your home in a project inquiry. US Construction Remodeling Corp. can discuss how that scope fits the renovation. Electrical design, equipment selection and electrical installation need review by a licensed electrical contractor.
For the wider project, review our home remodeling service. You can also return to the planning tools to compare other parts of the scope.
Sacramento Permitting Note (When It Applies)
For Sacramento addresses, many residential permit applications and plan-review status updates run through the city’s online Community Development portal (Accela Citizen Access). Permit needs vary by scope, so it helps to confirm early if your project includes panel modifications, transfer equipment, or permanent standby units.
At commissioning, expect a supervised test, a homeowner walkthrough, and a simple shutoff and startup plan.
Helpful Links
Frequently Asked Questions
Use the estimate for early planning with the home size, estimated critical load in kW, backup hours and options you enter. Before choosing equipment, ask a licensed electrical contractor to review appliance and motor starting demand, loads that may run together, transfer equipment and placement.
The calculator asks for these planning inputs:
- Property address, ZIP code and home size.
- One estimated Critical Load (kW) value and Backup Hours.
- Backup type, fuel type, access and quantity.
- Transfer switch, load management, permit, pad or mount, and old-equipment removal options.
Separately, prepare appliance and circuit details, running demand and motor starting demand for review by a licensed electrical contractor.
To connect backup power to a home safely, you typically need approved transfer equipment (interlock, manual transfer switch, or ATS). This prevents dangerous backfeed and keeps circuits clearly managed.
Costs vary by scope and equipment, but the biggest drivers are usually:
- Transfer method (interlock vs transfer switch vs ATS)
- Electrical panel work (space, upgrades, subpanels)
- Generator vs battery (and the runtime target)
- Fuel/recharge plan and equipment placement
- Permits and inspections (when required)
Permit needs depend on the jurisdiction and the exact scope. Permanent equipment, panel modifications, and transfer gear often trigger permit/inspection requirements, so confirm early with your contractor for your address.
It depends on equipment availability and whether permits apply. Portable generator connection solutions can move faster, while standby generators or whole-home battery systems may take longer due to lead times and plan review.
Look for a contractor who will:
- Provide a written load list and sizing assumptions
- Explain transfer options and safety protections clearly
- Define permit and inspection responsibilities in writing
- Commission the system and label circuits
- Offer a simple operating walkthrough for your household