To my understanding, my main service panel is 125A with a breaker of 100A. My subpanel is 100A.

I'm in the design stage of installing solar but I'm planning later in the year to install a heat pump and maybe in a couple of years an EV charger. Much later I might consider a heat pump water heater.

Per the solar installer, I don't need a panel upgrade which is good because trenching for new underground wires would cost more than $10,000 and a panel upgrade and relocation (due to currently being above the gas meter) would cost $4,500.

The current solar design is for a 9kW solar system with SolarEdge SE6000H-US. According to its specs, its maximum continuous output current @240V is 25A. The solar design says: max breaker calculation: 125A * 1.2 - 100A = 50A and they plan on installing a 35A breaker.


  1. Should the breaker for solar be equal to or larger than the maximum continuous output current of the inverter? I'm considering increasing the solar system size which would require a larger inverter. The SolarEdge SE7600H-US has an output current of 32A while the SE10000H-US has 42A. What breakers would each one of these bigger inverters need? Should I not increase the solar system size to allow capacity for the heat pump? Or should I even consider decreasing the solar system size?

  2. How many and what breakers would a heat pump need? I don't have AC. I only have a gas furnace that will be replaced with the heat pump. Currently, the furnace is in a shared 15A circuit with lights and outlets (labeled lights,outlets entry,stairs below). Can the air handler be in that shared circuit? What if I have to install two air handlers if I want two zones HVAC? What about the condenser? Will there be capacity left in the main service panel after going solar?

  3. Will there be capacity left in the main service panel to add an EV charger? Or will my only option, other than the very costly 200A upgrade, be to use the dryer outlet with one of those shared devices?

  4. How about replacing my gas water heater with a heat pump water heater? How much current would that need? Will my panel have capacity?


Pictures of my panels:

main panel main panel label enter image description here subpanel enter image description here

  • Is the solar installer with okay with your plans of using it for a heatpump ? In NZ, they are not happy about it, I guess because of the inrush/peak current. Feb 10, 2023 at 12:19
  • They know I'm planning on installing a heat pump this year which is why my solar offset is close to 200% and they haven't raised any concerns. I'm in California in USA and many have both solar and heat pump. But then most people I know have 200A panels while mine is 125A...
    – user162793
    Feb 10, 2023 at 12:36
  • How many square feet is your house @user162793? Feb 10, 2023 at 12:43
  • It's 1940 sq. ft. Curious, how is it relevant? My understanding is there are some recommendations for panel sizes based on square feet but I'm not sure how accurate they are. By the way, based on NEC section 220.87, which unfortunately I cannot use with solar, and my last year peak usage of 1.45 kWh in a 15min window (or 1.45 * 4 kWh in 1h) I can do the following calculations: panel-rated wattage = 125A*240V/1000 = 30 kW, highest load last year = 5.8 kW, so load = 5.8 * 1.25 = 7.25 kW. So my residence could accommodate another 22.75 kW of nameplate rated devices which is plenty.
    – user162793
    Feb 10, 2023 at 13:00
  • 1
    Square footage is part of the standard load calculation that needs to be done to determine if there's enough headroom in an existing panel.
    – FreeMan
    Feb 10, 2023 at 18:27

3 Answers 3


Averting a "heavy-up" is possible, but requires us to pull out a few tricks

Everything you want to do, namely grid-tied solar, heat pump space heat, heat pump water heat, and EV charging, is possible on your existing 100A service, but we have to do some trickery to get there.

Trick #1: a bit of consolidation

The first slick trick we can pull is we can cut your load calculation by almost two kilowatts simply by consolidating your oven and your cooktop onto the same circuit. This just requires turning off the subpanel for a bit, then taking the wires off the oven breaker and splicing them to the wires from the cooktop breaker and a pair of 8AWG THHN pigtails, then landing those pigtails on the existing range breaker. As far as Code goes, this trick relies on NEC Table 220.55 Note 4, which permits a cooktop and up to two ovens to be combined and treated as a single range load for load calculation purposes.

While you're in there, you'll want to swap the top right and bottom right breakers with each other. This moves the tandem breaker in the top right to a spot labeled for accepting tandems.

Trick #2: (heat) pumping away your problems, part 1

The next trick is a fair bit trickier than the first, but lets us get both space and water heat via heat pumps without having to do a full 200A heavy-up. The best part is that it'd even work were you not in a mild climate that is considered favorable to traditional heat pumps without auxiliary heat, as modern mini-split heat pumps and reverse cycle chillers can run well below 0°F without issue, and it is the latter we will employ here as it lets us get space heat and hot water through the same mechanism.

In particular, a Chiltrix CX34 reverse cycle (heat pump) chiller can supply most of the heating and cooling load your house needs (2-2.5 tons of cooling and up to 2.8 tons of heating capacity, assuming the 3 tons is closer to a Manual J size than a "rule of thumb" which tends to overestimate) while also providing domestic hot water service via an indirect tank, and doing so with efficiency values comparable to modern mini-split heat pumps in the process, while only pulling 15A (or less!) at 230VAC. Because it's a hydronic device, it also supports niceties like radiant heat, as well; however, the installation is a bit complicated (especially if you're using its automatic anti-legionella support, which does periodically draw a bit more power), so I won't describe the setup needed here.

Trick #3: (load) shedding the EV

The third trick is to not only commission your EV charger for a lower power setting (such as 16 or 24A for a 20 or 30A circuit, respectively), but to use a dedicated energy management system with current transformers to shed the EV load if it becomes too much for your service to handle. There are a few suppliers you can choose from for this: DCC Electric makes the DCC-10, BlackBox Electrical makes an EVEMS, and there's also the SimpleSwitch 240M out there.

This also permits, and may require, your EV charger circuit to be run from your subpanel instead of the main panel due to the need to get CTs around the supply wires, something that'd be tricky to do at best in your meter-main. That said, with trick #1 above, putting the charger on the subpanel is possible to do, even if you want more than 20A at 240V, as your panel accepts Eaton BRD tandems natively (as a function of the cross-labeling) and also should work with Siemens QT (as you see in it right now), simply by replacing the existing quadplex serving the oven and kitchen outlets.

Trick #4: (heat) pumping away your problems, part 2

The final trick we can do to shrink your electrical service footprint is a bit heftier of an ask than the remainder, but also fairly simple: replacing your existing electric dryer with a Miele heat pump dryer would slash a good few kVA off the load calculation, as the Miele heat pump dryers are low-power enough to run on 120V isntead of 240V. That said, I put this last because heat pump dryers aren't practical for everyone: they take somewhat longer to dry clothes than conventional dryers do, and also require a bit more maintenance to keep them running efficiently, with extra lint traps to clean. (They're also not available in XXL sizes for those who are used to washing massive comforters and other such bulky items at home.)

Once all that's done, we can turn to the sunny side

Now that we have done all that to cram your load desires onto 100A, we can turn to addressing the solar system. The good news is that there's really no difference in electrical requirements between the two inverter sizes beyond the breaker (which'll have to go at the top of your main panel, given its current configuration) and wire sizes for the inverter feed, so feel free to go for the larger inverter if you have the panels to drive it to its fullest.

  • For consolidating cooktop and oven, do I keep the oven breaker? i.e. the oven wires go to both the existing oven breaker and to the existing cooktop breaker?
    – user162793
    Jun 1, 2023 at 21:32
  • I'm looking for a way to make space for a 240V/30A breaker in the sub panel. Can you clarify what you mean "replacing the existing quadplex serving the oven and kitchen outlets"?
    – user162793
    Jun 1, 2023 at 21:48
  • @user162793 no, you take the wires off both the oven and cooktop breakers, then splice them to pigtails that lead back to the existing cooktop breaker. You can then replace the quadplex the oven was on with a 30/20/20/30 to get your 240V/30A breaker spaces Jun 2, 2023 at 2:13
  • But the oven wires are thinner i.stack.imgur.com/TnT6N.jpg Are they going to be protected with the 40A cooktop breaker?
    – user162793
    Jun 2, 2023 at 5:09
  • @user162793 -- they'd be tap conductors under the 210.19(A)(3) Ex 1 cooking appliance tap rule. (not sure how one'd apply the "no longer than necessary" part of that verbiage to existing wiring that's being reconfigured, though) Jun 2, 2023 at 11:46

The EV charging unit is called an EVSE. It is not a charger, it's a smart GFCI that tells the car how much current it can draw. The EV world is full of costly misconceptions about EV charging. Those commercials with the unicorn and tooth fairy don't state the half of it. I want the one with the tentacle monster who think they need cord-and-plug connection, or the faeries who think they need a 50A circuit. To correct many of these I always link this place in this video.

The reason for the rule

To my understanding, my main service panel is 125A with a breaker of 100A. My subpanel is 100A. .... The solar design says: max breaker calculation: 125A * 1.2 - 100A = 50A and they plan on installing a 35A breaker.

Yes, that checks out because of your 100A main breaker.

Why is that rule that way? Because without a limit, you might have 100A of utility power and 100A of solar power, feeding 200A of loads on a 125A bussed panel. NEC says if the solar and utility breaker are on opposite ends of the bus with the loads between them, you can push the bus limit by 20%. The "main breaker in the center" is granted an exception but still, the major loads need to be between main and solar breaker! If you can't do that, then you're stuck with bus limit and main+solar must <= 125A.

Since loads must be in the middle, I'd expect your solar to be placed at space 15-16 and your 100A feed breaker to be moved to spaces 13-14. I hope you have the feeder length for that; this is the folly of cutting the wires as short as possible. The surge should go to 7-8, with the 15A breaker to 6, and the heat pump and EVSE should go in 9 through 12. Or be double-stuffed if that panel supports quadplex and the loads don't need GFCI. A hard-wired EVSE never needs GFCI, it is a GFCI.

Alternately, you could replace that interior subpanel with a much larger panel, and move a lot of stuff there. What's most important to be in the all-in-one is daytime loads that would benefit most from solar.

Fitting more loads

First, a +30A service upgrade is cheaper than you think: all it'll cost you is a gas stove and a bit of plumbing lol.

If you watched that video you know charging an EV off a 15-20A 240V circuit isn't a problem. Some people might prefer more but they don't need more. (let's face it, if you're driving 200 miles you'll pass a bunch of DC fast chargers, and you'll be stopping anyway for bathroom/snack; simply coordinate them. They take about the same amount of time.)

Another option is that 30A general-use, receptacle circuits are regulated like 15-20A general-use receptacle circuits: no limit to number of receptacles, and you're left to your own recognizance to not overload them. Thus a dryer and a plug-in EVSE can be on a 30A general-use circuit.

They also make "Load shed sensing EVSEs" that will put a clamp meter on a circuit or the service, and back off EV charging to keep a circuit or the service from going over. Really we're going to see A LOT of tech like that in the near future become very affordable. Power companies are going to force it to happen because it then enables incentive-based demand side management, which is cheaper than peaking plants.

On the heat pump, I can't guess at sizing. The original gas furnace takes a minor amount of power for the air handler, and you'll still need that if you reuse your duct system.

Heat pump water heaters are something of a swindle. They pump heat from the utility room into the water, thus causing the utility room to become an icebox (which then hurts their efficiency). So you need to run the furnace harder, which means you're really heating the water with gas. Or resistance electric if you stick baseboard or space heaters in the utility room, only now your efficiency is worse! It's only a benefit if you can scavenge that cold blast to air condition your home.

Also since they air condition the utility room, a heat pump water heater needs a condensate drain. In theory they will have an emergency drain for the overpressure safety valve, but an emergency drain can route uphill. A condensate drain cannot.

Much more interesting to me is the fact that split heat pump systems can support multiple heads in multiple rooms. One of those heads could be a hot water tank! So now your heat pump water heater has the outdoor unit actually outdoors where it needs to be. That makes sense.

  • Thanks for your detailed response. I'm using a 15A 120V circuit for EV charging and it has been meeting my current needs. Good to know there are options for my future needs. I attached the inside picture of my main panel. The great news is I have the feeder length to move the 100A feed breaker. But not the 15A breaker from space 14 to 6. What options do I have? So adding a 35A breaker for solar, how much amperage do I have left at 240V for heat pump (3-ton) and EV charger? What's the formula for calculating that?
    – user162793
    Feb 10, 2023 at 23:54
  1. The inverter will have its own protection against overcurrent. The breaker will be there to protect the wire between the panel and the inverter. You can exceed the current capacity that the inverter can supply in that case.

  2. A heatpump will likely need a single 240 circuit for the outside unit. Some installations will have a connection between the inside and outside units (common in minisplits). Though otherwise the existing circuit will be enough. You can have multiple zones with a single air handler using zone dampers.

  3. You can splurge on a smart charger that includes current sensing probes and will dynamically throttle charging when all the other loads reduce the headroom available in the house. With that you can ignore the EV charger from load calculations.

  4. A waterheater heatpump is going to need another 240V circuit.

    Though there are options where the air heatpump and the water heatpump share an outside unit and thus doesn't need an extra breaker. And in the summer it will heat your water by moving the heat from the air inside straight to the water instead of moving first to the outside and then back to the water saving you on electricity.

With 2 heatpumps (separate air and water), electric cooktop, oven and a electric dryer, you are going to exceed the capacity of a 125 A service if you ever turn them all on at the same time.

  • For the forced air heat pump, unless the OP is going hybrid and keeping gas as backup heat, they will need another 240 volt breaker for in indoor air handler and backup heat strip.
    – DoxyLover
    Feb 10, 2023 at 15:12
  • Thanks @DoxyLover for bringing up the backup heat strip however it rarely drops below 40°F so I'm not going hybrid and no need for a backup heat strip in my area.
    – user162793
    Feb 10, 2023 at 21:02
  • Thanks @ratchet. So it sounds like without a waterheater heatpump I should be fine. What's the exact headroom I have in my main service panel? Can I apply NEC 220.83(B) or is this for the subpanel? How do I use the solar system in the calculation? Does the size of the inverter affect the headroom?
    – user162793
    Feb 10, 2023 at 21:21

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