Two numbers that matter for every load
- Continuous power (kW): what the appliance draws while running. The motor in a 3-ton AC compressor draws ~3.5 kW continuous when actively cooling.
- Surge / starting power (kW): what it draws for ~1–10 seconds at startup. That same 3-ton AC may surge 8–14 kW for 5 seconds during compressor start. Your inverter must deliver this surge or the appliance won't start (you'll get a "compressor start fault" or the inverter trips offline).
Typical residential heavy loads
| Load | Continuous kW | Surge kW | Notes |
|---|---|---|---|
| Refrigerator (residential) | 0.15 | 0.6–1.2 | Easy — almost any inverter handles |
| Window AC (12k BTU) | 0.9 | 2.2 | Easy |
| Sump pump (1/2 HP) | 0.8 | 2.5–3 | Low-medium surge |
| Standard well pump (1 HP) | 1.5 | 5–7 | Medium surge — check inverter spec |
| Submersible deep well (2 HP) | 3.0 | 10–15 | HIGH surge — many inverters can't start it |
| Central AC compressor (3-ton) | 3.5 | 10–14 | HIGH surge |
| Central AC compressor (4-ton) | 4.5 | 13–18 | HIGH surge — needs soft starter or hybrid inverter |
| Heat pump (3-ton, heating) | 3.5 | 10–14 | HIGH surge |
| Heat pump (3-ton, with backup resistance) | 8–10 | 15+ | Backup heat strips add huge continuous draw |
| Electric water heater (resistance) | 4.5 | 4.5 | No surge but high continuous; deplete battery fast |
| Heat pump water heater | 0.5–1 | 2 | Easy if no resistance backup activates |
| EV charger (32A Level 2) | 7.2 | 7.2 | No surge but continuous 7.2 kW for hours |
| EV charger (48A Level 2) | 11.5 | 11.5 | Same; even higher continuous |
| Induction range (single burner) | 1.8–3.5 | 3.5 | Per burner; up to 7–12 kW for full range |
| Electric oven (resistance) | 3.5 | 3.5 | Continuous |
| Electric dryer (resistance) | 3.0 | 3.5 | Continuous |
What inverters can deliver
| Inverter / battery | Continuous kW | Surge kW (10s) | Notes |
|---|---|---|---|
| Tesla Powerwall 3 | 11.5 | 185 amps | Strong surge; handles most residential |
| Powerwall 2 (legacy) | 5.0 | 7 | Cannot start a 3-ton AC; cannot start most deep wells |
| Enphase IQ Battery 5P | 3.84 | 7.68 | Cannot start a 3-ton AC |
| Enphase IQ Battery 10C | 7.68 | 10 | Handles 3-ton AC; marginal on 4-ton |
| FranklinWH aPower 2 | 10.5 | 15 | Strong |
| Sol-Ark 12K | 9 | 16 | Strong; hybrid inverter |
| Sol-Ark 15K | 12 | 22 | Strong; can run typical home + AC |
| EG4 18kPV | 12 | 21 | Strong |
| Generac PWRcell | 9 | 12 | NMC older; LFP transition underway |
The "soft starter" hack
If your AC compressor surge exceeds your inverter's surge rating, a soft starter (e.g., Micro-Air EasyStart, SureStart) reduces the AC compressor's startup surge by 60-80%. Cost: $300–$700 plus install. Often the cheapest way to make a marginal inverter start a marginal AC.
How to verify before signing a solar + battery contract
- List every load you want to back up (use our battery sizing tool).
- Get the actual nameplate data from each appliance — running watts AND starting watts. Most are on a sticker on the back.
- Sum continuous kW: must be less than inverter's continuous rating with 20% headroom.
- Identify the highest single surge kW: must be less than inverter's surge rating with 10% headroom.
- Ask the installer: “Will this inverter start my [highest-surge load] when the grid is down?” Get answer in writing.
- If marginal, ask if a soft starter will solve it OR if you need a different inverter / additional battery.
What happens if the load exceeds the inverter
Behavior varies by inverter type:
- Tesla Powerwall: trips the inverter offline; backup loads lose power until you reset.
- Sol-Ark / EG4: trip overload; some models shed loads via Generator port or external smart panel.
- Enphase: distributed micro-inverters handle export, but battery in backup mode trips offline if combined load exceeds rating.
- All: the appliance you're trying to start either makes a "stalling" hum or simply doesn't start. The most common symptom is "lights flicker, AC compressor doesn't engage."
Smart load management as alternative
Instead of sizing inverter to handle peak load, install a smart panel that prioritizes loads:
- Span Drive: per-circuit on/off control; load shedding by priority during backup; supports surge-detection.
- Lumin Smart Panel: similar; less expensive than Span; less granular.
- Schneider Square D Energy Center: integrated panel + battery management.
- Sol-Ark Generator port + external relays: DIY-friendly; less polished but works.
Frequently asked questions
How do I know if my well pump will work?
Read the nameplate on the pump or pump controller. Look for "starting watts" or "LRA" (Locked Rotor Amps). Multiply LRA × 240 to get surge watts. Compare to inverter surge rating with 10-20% margin. If marginal, add a soft starter.
Can I run my whole house on a Powerwall 3?
One Powerwall 3 (11.5 kW continuous) can run most of a typical 2,000-3,000 sq ft home if you don't run the AC, EV charger, electric dryer, and oven simultaneously. Whole-home continuous backup typically requires 2 Powerwalls or larger hybrid inverter for most US homes with central AC.
What if I want to add a heat pump later?
Plan now. Heat pumps with electric resistance backup heat can draw 8-12 kW continuous in cold weather. Most single-battery systems can't sustain that for whole-home backup. Either size the inverter/battery for it now, or plan to keep gas backup heat for outage scenarios.