Key takeaways
- Size for continuous watts and surge watts.
- Oversizing can increase idle losses and cost.
- Inverter choice affects battery draw and wiring requirements.
Sizing guide
How to size an inverter for solar
Inverter sizing has two jobs: (1) handle your maximum AC load (plus surges), and (2) avoid wasting energy with an oversized unit. This guide shows a simple method and the numbers that matter.
Step 1: List your AC loads and peak watts
Add up the AC devices you may run at the same time. For each device, use nameplate watts or a measured value (many appliances vary during operation).
Peak watts ≈ sum of simultaneous AC watts
Step 2: Account for surge (starting) power
Some loads require a high startup surge (motors, compressors). Inverter specs typically list a surge rating for a short time window.
Surge headroom = inverter surge rating − expected surge load
If you’re near the limit, the system may trip or fail to start the device reliably.
Step 2a: Know which loads have surges
Surge loads are usually motors or compressors: fridges, pumps, fans, and power tools. Electronics like laptops and TVs generally do not need a large surge margin.
- Motor loads: high surge for a short time, then lower running watts.
- Heating loads: mostly steady and predictable.
- Electronics: often close to nameplate watts.
Step 3: Check battery-side current draw
Inverters draw significant current from the battery, especially at lower system voltages. A rough estimate:
Battery amps ≈ AC watts ÷ (battery volts × efficiency)
Example: 1,000W ÷ (12V × 0.9) ≈ 93A. High currents impact wiring size, fusing, and heat.
Step 4: Choose inverter type and waveform
For many off-grid and RV use cases, waveform matters for compatibility.
Step 5: Consider efficiency and idle draw
Inverters consume power even when nothing is plugged in. This idle draw matters for off-grid systems because it can drain batteries overnight.
Higher-quality inverters often have better efficiency and lower standby losses, which can reduce the size of the battery bank you need.
Step 6: Match inverter size to system voltage
Large inverters draw high current at 12V, which can require very thick cables and larger fuses. Moving to 24V or 48V reduces current and can simplify wiring.
Use the system voltage comparison to decide if a higher voltage is appropriate.
Step 7: Plan for wiring and protection
Every inverter needs properly sized DC cables, fuses or breakers, and a disconnect. Undersized cables can overheat and reduce performance.
Use the battery cable size guide and fuse sizing guide to keep the system safe.
Step 8: Consider AC output type and wiring
Some inverters provide standard household AC, while others support split-phase or multi-output wiring. Make sure the inverter output matches your loads and any transfer switch or panel you plan to use.
If you are unsure about AC wiring, consult a licensed electrician.
Example sizing walk-through
Suppose your cabin runs a 700W microwave, 120W laptop, and 100W lighting at the same time. Your peak load is roughly 920W before losses. If the microwave has a 1,500W surge, you want an inverter with enough surge headroom to handle that burst.
Now check battery current. At 12V and 90% efficiency, 920W ÷ (12V × 0.9) is about 85A. That current level drives thicker cables and larger fuses. If you moved to 24V, that current is closer to 42A, which can simplify wiring.
This is why voltage choice and inverter sizing should be considered together.
Inverter features that affect sizing
- Built-in charger: useful for generator or shore power charging.
- Transfer switch: lets you switch between inverter power and an external source.
- Search or sleep mode: reduces idle draw when loads are small.
- Power factor tolerance: some tools and motors need extra surge headroom.
These features can change how large an inverter you need and how much energy it uses.
Planning checklist
- List peak loads: include devices that run together.
- Account for surge: motors and compressors often need extra headroom.
- Check battery voltage: higher voltage reduces current and cable size.
- Confirm waveform: sensitive electronics prefer pure sine wave.
- Verify wiring and protection: fuses, disconnects, and cable sizes must match.
Grid-tied vs off-grid sizing notes
Grid-tied inverters often size to array output and grid rules, while off-grid inverters must handle your peak loads directly from batteries. Off-grid sizing usually prioritizes surge capability and battery draw.
Idle draw and daily energy use
Large inverters can consume meaningful power even with no loads. If you are off-grid, that idle draw should be part of your daily energy budget.
Noise and location
Inverters can produce fan noise when under load. Plan the mounting location so sound and heat are manageable.
Checklist recap
Confirm load list, surge needs, battery voltage, and wiring before purchase.
Common inverter sizing mistakes
- Ignoring surge loads: motor starts can trip an otherwise “big enough” inverter.
- Oversizing by default: large inverters waste energy at idle and cost more.
- Underestimating DC current: high current can overheat cables and connectors.
- Using modified sine with sensitive loads: can cause noise or poor performance.
Safety and code notes
Inverters connect high-current DC to AC loads. Use proper disconnects, DC-rated protection devices, and follow local code requirements. If you are unsure about wiring or bonding, consult a licensed electrician.
Common sizing examples (quick ranges)
| Use case | Typical inverter size | Common notes |
|---|---|---|
| Charging + small appliances | 300–800W | Lower surge needs |
| Microwave / mixed RV loads | 1,000–2,000W | Surge and wiring matter |
| Heavy loads | 2,000–4,000W+ | Battery bank and voltage become critical |
Plan for future loads
If you expect to add a microwave, power tools, or AC later, note those loads now. A small headroom is fine, but avoid doubling size without a reason. Upgrades also change cable size and battery cost.
Write down likely upgrades so you can choose a size with intent.
Moving small loads to DC can also reduce the inverter size you need without major rewiring later on.
FAQ
What happens if my inverter is too small?
It may trip under load, fail to start surge devices, or run hot near its limit.
Is a bigger inverter always better?
No. Bigger units cost more and can waste energy at idle. Size to realistic peak and surge needs.
Does inverter size change battery size?
Indirectly. Higher AC loads require more battery energy, and inverter losses add to demand.
Should I choose pure sine wave for solar?
If you run a mix of electronics and appliances, pure sine wave is usually the safest default.
Can I run AC loads without an inverter?
Only if those loads are DC or you have a different power source. Most AC appliances require an inverter in off-grid systems.