This is an excellent and complex question, Phil! You're right to be thinking about both the load side and the potential "generation" side effects of your project. Let's break it down

I. Solar and Battery Backup - Normal Load Reduction Scenario:
Your Thinking is Correct (Mostly)

In most cases, solar and battery systems

reduce the load on the utility transformer. When your solar is generating, it's directly powering your loads, reducing the amount of power you draw from the grid. When the battery discharges (during peak load or at night, depending on your settings), it also supplements or replaces grid power. Load Calculations

Your load calculations should account for this reduction, but it's tricky. You can't simply subtract the potential solar output from your peak demand. A realistic approach involves estimating the

minimum load your solar and battery can realistically offset during your peak demand periods (usually late afternoon/early evening in summer). Example: If your historical peak load is 200 amps and you reasonably expect your solar to offset at least 50 amps during that time, you might use a net load of 150 amps for some transformer sizing considerations. Transformer Benefit

This reduction in load is generally beneficial to the utility transformer. It runs cooler, lasts longer, and has more capacity available for other customers.

II. The "Sending Power to the Grid" Scenario (Reverse Power Flow)

This is where things get more interesting and where your concerns about "burning up" the transformer come into play.

Potential for Transformer Overload (but not in the way you're thinking)

The primary concern isn't that the voltage is being stepped up instead of down. Transformers are bidirectional devices and don't care which direction the power flows. The issue is

overloading the transformer beyond its rated capacity due to increased current flowing through the winding of the transformer. This would cause excessive temperature rise within the transformer, and could shorten it's lifespan or lead to catastrophic failure if not properly protected against. Factors Affecting Reverse Power Flow

Several things influence how much power you send back to the grid:

Solar Generation

Obviously, the more your solar panels produce, the more likely you are to have excess power.

Battery Charging

If your batteries are full and your solar is still producing, that's excess power going back to the grid (unless your inverter has a way to curtail production).

EV Charging

A charged EV can also backfeed to the grid if the EV charger is equipped and configured for it.

Local Loads

The extent to which any excess generation is being consumed on-site determines how much energy makes it to the grid.

Utility Protection

The utility should have protections in place to disconnect the source of backfeed in various circumstances.

Anti-Islanding Protection

Required by the NEC (National Electrical Code) to shut down your solar system during a grid outage to prevent energizing a section of the grid that is supposed to be de-energized.

Overcurrent Protection

Fuses, breakers, and relays on the transformer primary protect it against overload, whether the current is flowing "normally" or in reverse.

Why Overload is Still Possible



Simultaneous Backfeed

If many customers in your neighborhood are sending power back to the grid at the same time, the transformer (and the grid wiring itself) can become overloaded. This is becoming a more common concern for utilities.

Poor Grid Management

In some areas, the grid infrastructure may not be adequately upgraded to handle the increasing amount of distributed generation.

Unintentional Feed

If there is a voltage drop/anomaly, then the inverter could respond by sourcing voltage, attempting to compensate.

III. Load Calculations - Accounting for Backfeed Potential



NEC Perspective

The NEC (National Electrical Code) primarily focuses on protecting the system from overcurrent, not necessarily on optimizing transformer loading. It requires your system to be designed so that it cannot overload the service equipment (your 400A service) and the grid. This typically involves current limiting inverters or additional circuit breakers.

Utility Perspective

The utility will be very interested in how much power you could potentially send back to the grid. They will likely require:

Interconnection Study

The utility will conduct a study to assess the impact of your system on the grid. This will involve analyzing voltage rise, fault current contribution, and the capacity of the local distribution network.

Power Export Limit

The utility may impose a limit on the amount of power you're allowed to export to the grid. This is often based on the size and characteristics of the local transformer.

Load Calculation Recommendation



Separate Load and Generation Analysis

Instead of trying to fold the generation into your load calculations, treat them as separate analyses.

Worst-Case Load Calculation

Perform a worst-case load calculation for your "bugout" shelter, assuming minimal solar/battery contribution. This ensures your service equipment is adequately sized for maximum demand.

Maximum Generation Analysis

Calculate the maximum potential power you could send back to the grid. This is essentially the sum of your solar capacity plus the maximum discharge rate of your batteries plus any EV capacity (40kW + 85kWh rating translated to amperage). Share this information with your utility.

IV. Your Specific Project



40kW Solar and 85kWh Battery

This is a significant amount of generation and storage.

400A Service

A 400A service is likely appropriate for your loads, but you absolutely need to consider the backfeed potential.

Geothermal Systems

These are energy-intensive and will be a significant load.

Four 14-50 EVSE

These represent a substantial potential load, especially if multiple vehicles are charging simultaneously.

Communication is Key



Contact Your Utility Immediately

Before you proceed with installation, contact your utility company. Explain your plans in detail and ask about their interconnection requirements, grid capacity, and any limitations on power export. This is essential.

Work with a Qualified Contractor

Hire a licensed and experienced solar installer who is familiar with grid-tied systems and the NEC requirements in your area. They should be able to guide you through the permitting and interconnection process.

In Summary: Solar and battery backups typically reduce the load on the utility transformer. However, the potential for reverse power flow must be carefully considered. Overload is possible if multiple customers are exporting power simultaneously or if the grid is not adequately equipped to handle distributed generation. Detailed load calculations, separate generation analysis, and close coordination with your utility are critical to ensure a safe and reliable system. The NEC is not necessarily concerned about the transformer itself, but with the safety of your wiring from overload, which is why the utility is the main partner to address the safety and capacity of the transformer in question.Good luck with your project! It sounds like an exciting and well-planned endeavor. Please let me know if you have any further questions. Flag for review