Home Backup Power System: 15 kWh of Battery, a 12 kW Hybrid Inverter, and the Transfer Switch I Did Not Need

This is the battery and inverter wired into the house: the system that runs the well, the furnace blower, the freezers and the kitchen when the grid is gone, and that will do it without anyone home to flip anything. It is also the page where I show the thing I got wrong. I put a manual transfer switch between a hybrid inverter and the panel, which is the right part for a generator and the wrong part for this inverter. It worked, which is why it took a year to see. Below is what I built, why it ran anyway, what it cost me, and the whole-home wiring that replaces it. No prices on this page; the parts list says what each piece is and does.

Four things plug into one box. Learn the box and the rest of the page is easy.

This is the part people miss about a hybrid: it is not sitting there waiting for an outage. Every clear day the array runs the house and the grid barely gets a vote. The band below is an ordinary day with the grid up, read from the inverter screen, and the numbers under it are the arithmetic.

The inverter app shows this split for every day: PV, battery, grid import and consumption. A clear day in any month reads a few hundred watt-hours of import against 7 kWh used. The grid earns its keep on the gray week, and that is also where the generator section comes in. The numbers and the January-versus-June math are on the basics page.

The word hybrid is doing real work. An off-grid inverter has no idea the grid exists; if the house also has a grid feed, something external has to keep the two apart, and that something is a transfer switch. A grid-tied string inverter is the opposite: it lives on the grid, has no battery, and by law shuts off the moment the grid does. A hybrid sits between them. It has a GRID port and a LOAD port with a relay between, it passes grid power through to the house, it charges and drains a battery on rules you set, and when the grid drops it opens that relay and runs the house as an island. The transfer switch is inside it.

Side by side, the table is the same story in rows.

The sizing changes with the topology, and this is the part that surprises people who assume off-grid means the same system minus the bill.

Four things people say about these three that are not true.

Those three words were my mistake in one sentence. I bought a hybrid, then wired it the way an off-grid inverter has to be wired.

This is the mechanism that makes an external switch unnecessary, and it is worth seeing as three states.

The first version ran the inverter's LOAD output into a Reliance Pro/Tran 310D, a ten-circuit manual transfer switch, and fed the inverter's GRID port from a 50 amp breaker in the main panel. Eight circuits went on the switch: the well pump, the furnace, the stove, the kitchen and garage outlets, the desk and the basement, with the compressor pair left off. Every toggle sat on GEN so the inverter fed them all the time, which gave me automatic backup for those eight.

There is a detail that hid the problem for a year. With the grid up, the inverter was not only feeding the eight circuits. It was pushing sun and battery power backward through that 50 amp breaker into the main panel, with the CTs holding zero export at the meter, so the whole house ran on the array every clear day. The bill said the system worked. Only an outage would have shown that most of the house was on the wrong side of the switch.

The inverter has a 200 amp pass-through rating for exactly this: the service feeds it, and it feeds the whole house.

1. The service disconnect feeds the GRID port. The 125 A main outside becomes the only thing between the meter and the inverter. The manual asks for a 200 A main on a whole-home job; a 125 A service is inside the 200 A pass-through rating with room to spare. The 50 A breaker goes away.
2. A grid-side box with two breakers. The inverter has its own breakers on the LOAD output (200 A) and the battery (two at 200 A) but none on the GRID input, so protection comes from outside. The feed from the outside main lands in an Eaton CH 125 A main-breaker load center on the wall where the transfer switch hung. Breaker A feeds the inverter GRID terminals and doubles as the inverter disconnect; breaker B feeds the bypass leg.
3. A house-side box with an interlock. A second, identical 125 A load center below it. Its main breaker is fed by the inverter LOAD output, which is the normal path. A backfed breaker takes the bypass leg, and an interlock plate between the two means only one can ever be on, so the grid and the inverter output can never touch. An output breaker feeds the existing main panel, which stays exactly as it is. The next section is why.
4. The LOAD port feeds the whole house. Through the bottom box, every breaker in the main panel is now on the far side of the inverter and backed up, the well and the furnace and the kitchen included, with no list to maintain.
5. One neutral-to-ground bond, at the service disconnect. The neutral runs through the inverter unbroken, so the bond outside stays the bond for the whole system, grid up or down. The bonding screw in the main panel comes out and its neutrals and grounds move to separate bars, exactly as in any sub-panel. This is the item an inspector looks for first and the one a DIY install most often gets wrong.
6. CTs on both grid legs at the service entry, arrows toward the inverter. They tell the inverter what the house is drawing from the grid so it can hold Zero Export and size its own output. If the arrows face the wrong way the inverter reads the house backwards; the fix is a setting (CT direction) rather than a rewire.
7. Wire sized to what protects it. Every AC run is protected by a 125 A breaker, the outside main or one of the box mains, so every run is #1 copper with a #6 copper ground. The manual would want 3/0 on the LOAD side if it landed on main lugs; because it lands on the bottom box's own 125 A main, #1 is correct, and that is one of the reasons the second box earns its place.
8. The generator gets its own port. A 240 V inlet outside, a 100 A breaker, and a cord to the GEN terminals. The inverter treats it as a third source: it carries the house and charges the battery while the generator runs, then drops it. The old manual switch is not part of this.
9. The car gets a plan, not a panel. Everything is behind the inverter, so the EV charger counts against its 12 kW in an outage and can pull on the battery any evening. The plan is one slider in the charger app: charge at noon, at the amps the array can give, slow in an outage, never from the reserve.
10. The electrician makes the service-side connections and the inspection is on the paperwork. I ran the conduit, mounted the gear and wired the DC side. The disconnect, the feeder, the bond and the final checks are the electrician's, and the utility gets its interconnection form even though nothing exports.

What changes for the house: nothing, until the grid drops, and then still nothing. Every circuit is backed up, the transfer is automatic, and the only switch anyone ever touches is the service disconnect during a test. What changes for the inverter: it finally sees the loads it was rated for, which is why the next section exists.

Putting the inverter in series fixes the backup problem and creates a new one. Every watt the house uses now passes through one machine. If it ever faults, or has to come off the wall for a warranty swap or a firmware session, the whole house is dark, and the generator cannot help because it feeds through the same machine. The fix is a bypass, and the cleanest way to build one is two identical load centers and an interlock plate.

Which of all this is required and which is my choice is worth being honest about, because a reader building the same thing can skip the second column and still have a safe, inspectable system.

Every run in this system is a different voltage and a different current, and the wire for each is chosen by one rule and one check. The map is the runs in order, the ladder is the sizes, and the three boxes are what happens when the size is wrong in either direction.

The sequence the electrician and I are following. Most of it is undoing the first version one wire at a time; the order matters because two of the sources cannot be made safe by the outside main.

1. Shut down in the manual order. Inverter grid breaker off, inverter load breaker off, the DC isolator at the array off, then the battery breaker and the battery modules off.
2. Kill the service. Open the outside 125 A main and confirm zero volts with a meter at the main panel, at the inverter terminals and at the transfer switch. The utility side of the outside main stays live until the utility pulls the meter; nothing on that side gets touched.
3. Put each circuit back. In the main panel, take each transfer switch wire off its breaker, undo the splice to the original circuit wire, and land the original wire on its own breaker. One circuit at a time, labeled as you go.
4. Remove the switch. Disconnect its neutral and ground leads and its feed from the inverter LOAD terminals, then the switch and its conduit come off the wall.
5. Remove the backfeed. The 50 A breaker and its conductors between the main panel and the inverter GRID terminals come out.
6. Hang the two boxes and rewire in series. Outside main to the top box; breaker A to the GRID terminals; LOAD terminals to the bottom box main; breaker B to the backfed breaker with the interlock plate fitted; the bottom box output breaker to the main panel. Neutral and ground bars stay separate in all three panels, because the only bond belongs at the outside service disconnect.
7. Move the CTs to the feed between the outside main and the top box, arrows toward the inverter.
8. Power up in the manual order. Battery first, then PV, then the inverter load breaker, and grid last. Confirm the house is live from the inverter, then confirm it reconnects to the grid.
9. Test. Open the outside main and confirm the whole house stays on without a flicker. The full test is the section below.

Whole-home means the car charger is behind the inverter too. Mine is an Emporia charger on a 14-50 plug, and the one feature that matters more than any other is in its app: the charge rate, anywhere from 6 to 40 amps, changed from the phone. That single slider is how the car charges on sun, how it stays out of the home battery, and how it keeps charging through an outage without taking the house down. With the grid up the pass-through carries it, and the only question is where the energy comes from: the array, the grid, or the home battery. With the grid down the inverter has 12 kW continuous, and a car at 40 A is most of that. The table is the ceiling; the six rules under it are the slider, used well.

Seven settings decide how the system behaves, and the defaults are wrong for at least three of them. These are mine, with the reason for each.

The old setup had the generator and the inverter competing for the same switch. In the whole-home layout the generator has its own door. A 240 volt inlet outside, a 100 amp breaker, and the GEN terminals on the inverter. The inverter treats the generator as a source it can draw up to 15 kW from: it carries the house on it and charges the battery with what is left, then drops it when the battery reaches the stop setting. The generator never touches the panel directly, so there is nothing to isolate and nothing to backfeed.

The Predator 13000 makes 9,100 watts running on propane, which is less than its gasoline number and the one to plan around, because propane is the fuel that stores. Half load, which is where the house will sit while the battery charges, runs about 16 hours on a 100 pound tank. The inverter manual wants a generator that is neutral-bonded and 240 volt split-phase; the electrician checks the bonding at the inlet, because a floating neutral on a portable generator is common and it matters here.

15.36 kWh is the label. With the off-grid cutoff at 10 percent, 13.8 kWh is what the house can use, and the hours depend entirely on what is switched on. The bars are that arithmetic for five evenings.

Three sizes of house, and the three numbers each one needs. Find the column that sounds like yours, then put your own loads into the calculator to get the exact figures.

The sizing calculator on the basics page does all three sums from a list of your own appliances, with your sun hours and the gray days you want to ride out.

An hour on a sunny afternoon, with the family warned. It finds every problem on this page in daylight.

1. Pick a weekday afternoon with sun and tell the house. Note the battery percentage on the screen.
2. Open the service disconnect. That is the grid going away, the same as a real outage. Watch the inverter screen: it should show off-grid or EPS within a second, and nothing in the house should blink.
3. Run the well pump. Open a tap until the pressure switch kicks it on. The surge is the hardest thing this system will ever be asked for; the screen will show the spike and settle.
4. Run the furnace, the microwave and the kitchen at once. Watch the load number. It should sit well under 12,000 W. Then, deliberately, set the charger to 40 A for one minute and watch what 9.6 kW does to the battery percentage. That is the lesson, in person; drop it back to 8 A after the minute and watch the house stop noticing.
5. Leave it for an hour. Note the percentage again; the drop against the hour is your real overnight rate, which the chart above only estimates.
6. Close the disconnect. The inverter should wait, sync and go back to pass-through on its own. If anything in the house rebooted at either end, the transfer is not working and the installer needs to hear about it.
7. Do it again with the generator. Start it on the inlet, confirm the inverter picks it up and the battery shows charging, then shut it down and confirm the inverter lets go.