Solar and Batteries in a Conversion Plan

Why this matters

Solar and a battery usually arrive in a conversion plan from a different salesperson, at a different time, with a different idea of what the house is. The solar installer sized the interconnection to the panel as it stood. The heat pump contractor sized the load calculation to the house as it will be. The battery was sold on keeping the furnace running in an outage. Then the furnace comes out, the panel turns out to have no room on its busbar for the battery inverter, the main breaker cannot be reduced to make room because the new load calculation needs every amp of it, and the backup that was supposed to keep the house warm can run the heat pump for about three hours.

None of that is a component problem. It is a sequencing problem, and this card covers the two places it bites: the busbar rule for connecting inverters to a panel, and what a battery can actually carry once heating is electric. Battery safety, siting and the listing behind it belong to the Residential Battery Storage Reference; the choice between load-side and supply-side interconnection in depth belongs to NEC 705 Interconnection Point Selection.

The busbar rule, briefly

An inverter connected on the load side of the service, through a breaker in the panel, back-feeds the panel's busbar. The busbar can then be fed from two directions at once, from the utility through the main breaker and from the inverter through its breaker, and the main breaker no longer protects it against the sum. NEC 705.12, as adopted by your jurisdiction, limits how much inverter current a busbar may take. The subsection numbering moved between the 2017 and 2020 editions, so cite it from the edition your jurisdiction uses.

The option most residential installs rely on is the 120 percent rule: 125 percent of the inverters' combined rated output current, plus the rating of the main breaker protecting the busbar, may not exceed 120 percent of the busbar's ampacity. It applies under the conditions your edition lists, which in recent editions include the inverter breaker sitting at the opposite end of the busbar from the main supply and a permanent label warning that it must not be relocated.

Three things trip people up:

• Every inverter counts. An AC-coupled battery has its own inverter, and its rated output current adds to the solar inverter's in the same sum. A battery added to an existing solar system is a second source on the same busbar.
• The 125 percent is applied to the inverter's rated output current, from its nameplate, not to the breaker it lands on.
• The other options are real. The edition also allows other arrangements: the sum of all breakers on the busbar, loads and sources but not the main, within the busbar's rating, a supply-side connection that does not touch the busbar at all, and a listed power control system that limits the current the inverters can deliver, which can then be used in the calculation. Each has its own conditions.

Worked example: the battery that did not fit

The house: a 200-ampere panel with a 200-ampere busbar and a 200-ampere main breaker, a 7.6 kW solar inverter on the load side, and a conversion plan that adds a heat pump, a heat pump water heater, induction and an EV charger. The customer now wants an AC-coupled battery with a 5 kW inverter.

120 PERCENT CHECK, 200 A BUSBAR (nameplate figures)
limit       = 1.20 x 200 A busbar      = 240 A
main breaker                           = 200 A
room left for inverters  240 - 200     =  40 A

PV inverter, rated output 32 A x 1.25  =  40.0 A
battery inverter, 21 A x 1.25          =  26.25 A
sources total                          =  66.25 A
over the 40 A allowance by             =  26.25 A

main reduced to 175 A: 240 - 175 =  65 A, still short
main reduced to 150 A: 240 - 150 =  90 A, fits
post-conversion calculated load        = 158 A

The solar alone used the whole allowance: 32 amperes times 1.25 is 40.0, exactly the 40 amperes the 120 percent rule leaves. The battery inverter adds 26.25 amperes to a busbar with nothing left.

The usual fix for a full busbar is to reduce the main breaker. Dropping to 175 amperes leaves 240 minus 175, which is 65 amperes, still short of 66.25. Dropping to 150 leaves 90, which fits. But the electrician's load calculation for the house after conversion, done by the existing-dwelling method in NEC Article 220, came to 158 amperes. A 150-ampere main on a 158-ampere calculated load fails the load calculation. The conversion plan has used up the headroom that a main-breaker derate would have spent.

That leaves four real options, each with a different cost and schedule:

• A DC-coupled (hybrid) inverter. Replace the solar inverter with one that also charges and discharges the battery on its DC side. One inverter, one rated output current. If its rated output is the same 32 amperes, the sources total stays at 40.0 and fits.
• A new panel with a larger busbar. A 225-ampere busbar with a 200-ampere main gives 1.20 times 225, which is 270, minus 200, leaving 70 amperes for 66.25. It fits, and the panel replacement may already be on the conversion plan for space reasons. See related: Panel Space Versus Panel Capacity.
• A supply-side connection. Connecting the inverters between the meter and the main breaker leaves the busbar out of the calculation entirely. Whether it is possible depends on the meter and service equipment and on the utility's requirements.
• A power control system. A listed system that limits the combined inverter output current to a set value can let the calculation use that value instead of the nameplate sum.

Which one is right depends on the equipment and the utility, which is the electrician's call. The point for the conversion plan is that the interconnection has to be checked against the post-conversion load calculation, not the current one. See related: Measuring Existing Load Before You Sell a Service Upgrade.

Backup panels and what a battery can carry

A battery's spec sheet carries two numbers that answer different questions:

• Continuous power output, in kW. How much it can supply at once. Everything running at the same moment has to fit under it.
• Usable energy, in kWh. How long it can supply it. Average draw in kW, divided into usable kWh, gives hours.

Most also list a surge or peak rating, for a few seconds, which decides whether a motor can start. A fixed-speed compressor or well pump draws several times its running current for a fraction of a second at start, and if that exceeds the surge rating, the battery inverter trips or refuses to start it. Inverter-driven heat pumps ramp up gently and rarely have that problem. Fixed-speed equipment may need a soft starter.

There are two ways to wire backup:

• A backup (critical loads) subpanel. Chosen circuits move to a separate panel the battery supplies in an outage. Simple, and it guarantees the battery is never asked for a load it cannot carry, provided someone chose the circuits with the battery's numbers in hand.
• Whole-home backup. The battery supplies the main panel through a transfer arrangement, and a listed load controller or energy management system sheds the large loads (strips, EV charger, range) so the battery is not overloaded.

How electrification changes the backup plan

With a gas furnace, heating in an outage costs the battery only the blower and controls. Once heating is electric, the heat pump becomes the largest load the battery will see, and it is largest exactly when outages are most dangerous.

Take the same house on a 15 F night. The 3-ton heat pump's extended data shows about 3.7 kW of input at maximum capacity at that temperature. The fridge, lights, internet and the sump pump add about 0.5 kW on average. The 10 kW backup strips cannot run on a 5 kW inverter at all.

• Before conversion, the backup load on a winter night was about 1.0 kW: the furnace blower and controls, plus the same 0.5 kW of other loads. A 13.5 kWh usable battery lasts 13.5 divided by 1.0, about 13.5 hours.
• After conversion, with the heat pump at full input, the load is 3.7 plus 0.5, which is 4.2 kW. That fits under the 5 kW continuous rating, but the battery lasts 13.5 divided by 4.2, about 3.2 hours.

The plan that fixes it has several parts. Keep the strips off the backup panel entirely, or let the load controller lock them out. Set an outage heating setpoint several degrees below normal so the heat pump runs at part load, where its input is far lower. Leave induction and the EV charger off the backup circuits or shed them. And be honest with the customer: one battery will carry an electrified house through a short winter outage, not a multi-day one. Longer coverage means more batteries, a generator, or a non-electric heat source kept for emergencies, and solar recharges little on short, overcast winter days.

Sequence it so nothing is undone

Run the post-conversion load calculation first. Size the service, the panel and the interconnection to it. Then size solar and storage against the load the house will have, not the load it had. A battery chosen before the heat pump was sized was chosen against the wrong house. See related: The Whole-House Electrification Sequence.

The utility's interconnection agreement also has to match what is installed. Adding a battery or changing the inverter on an existing solar system usually means an amended interconnection application and a new permission to operate before the system may run. Check with the utility before the install date is set.

How to verify the plan holds together

Before any work starts, put three documents on the table at once: the post-conversion load calculation, the interconnection calculation showing every inverter's rated output current, and the backup load list with each circuit's running draw against the battery's continuous and surge ratings. If any one was done against the pre-conversion house, redo it.

References

• NFPA 70, National Electrical Code, 705.12 (load-side source connections), 705.11 (supply-side connections), 705.13 (power control systems), Article 706 (energy storage systems) and Article 220 (load calculations), in the edition adopted by your jurisdiction.
• Inverter and battery manufacturer spec sheets (rated output current, continuous and surge power, usable energy).
• The utility's interconnection requirements and application process.
• See related: Residential Battery Storage Reference; NEC 705 Interconnection Point Selection (Load Side vs Supply Side).
• See related: Measuring Existing Load Before You Sell a Service Upgrade; Panel Space Versus Panel Capacity; The Whole-House Electrification Sequence.