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g/solar • Posted by u/battery_backup_brad •

Battery sizing when the customer wants "whole home backup" but has a 5 ton AC Storage

Recurring conversation and I want to compare approaches.

Customer wants whole home backup. House has a 5 ton single stage AC with an LRA of 88A, an electric range, an electric dryer and a well pump.

The surge is the whole problem. A single Powerwall 3 does 10kW continuous and about 30A of motor start assist. That 5 ton compressor wants 88A locked rotor for a few hundred milliseconds.

Options I present:

  1. Multiple batteries in parallel to cover surge
  2. Soft starter on the compressor — about $400 installed, drops LRA to around 25A
  3. Managed loads panel, drop the AC on battery
  4. All three

Almost every time, option 2 does more than option 1 at a tenth of the cost, and yet I still get pushback because customers understand "more batteries" and do not understand "soft starter."

How do you frame this?

💬 6 replies

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u/hvac_hero •

HVAC side: put in the soft starter regardless of the battery conversation. It reduces mechanical shock on the compressor, reduces the voltage sag that dims the customer's lights on every cycle, and extends contactor life.

Frame it as an AC upgrade that happens to enable backup, not as a battery accessory. The customer is not being asked to accept a compromise on their battery, they are getting a better air conditioner. Same part, same $400, completely different conversation.

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u/battery_backup_brad OP •

That is the reframe I was missing. I have been selling it as a workaround for a battery limitation, which makes it sound like a downgrade. Selling it as an AC improvement is both more accurate and easier.

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u/admin_jake ADMIN •

Worth adding one thing to the framing: "whole home backup" is almost never what the customer actually wants. What they want is to not think about it.

A managed loads panel that keeps the fridge, the well, the lights and the network up for three days beats a heroic system that runs the whole house for eight hours. Ask them how long they want to last, not what they want to run. The answer reframes the whole design and usually costs less.

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u/battery_backup_brad OP •

"How long do you want to last" is a better opening question than anything I have been asking. Every one of these conversations starts with the customer listing loads and I have been letting them.

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u/solar_steve •

I do the load math in front of the customer, on paper, and let them watch the numbers. Something about seeing 88A written down next to 30A does more than any amount of explaining.

Then show them the second battery price next to the soft starter price. Most people choose correctly when they can see both numbers at the same time.

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u/string_inverter_sarah •

Do not skip the well pump. A 1HP submersible has its own LRA and it starts on demand with no warning, which means it can coincide with the compressor start. Two motors starting together is the condition that actually trips the system, and it is the one nobody models.

Soft start both, or at minimum stagger them with a controller.

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