- The cap exists because a solar device feeds current from the “wrong” end of a circuit, where the breaker cannot see it.
- Europe and the UK use 800 W on 230 V circuits; most US laws use 1,200 W on 120 V circuits.
- The limit is AC output at the inverter. Panels can be larger, and the inverter clips the peak.
- Staying well below the cap on a circuit with no heavy loads is the safest practice.
If you are new to plug-in solar, the limits can seem arbitrary. Why 800 W in Germany but 1,200 W in Virginia and 1,920 W in Colorado? The answer lies in how your home’s wiring is protected.
In this article
How a breaker protects a circuit
Every circuit in your home is protected by a breaker (or fuse) at the panel. In the US, common sizes are 15 A and 20 A at 120 V, which means 1,800 W and 2,400 W. The breaker measures the current flowing from the panel into the circuit. If it exceeds the rating for long enough, the breaker trips before the wire overheats.
The problem with feeding power in
A plug-in solar device injects current into the circuit at the outlet. Imagine a 15 A circuit with the solar kit plugged into the last outlet and a 1,500 W space heater plugged into an outlet in the middle. The heater draws 12.5 A. If the solar kit supplies 10 A of that from one side, the breaker only sees 2.5 A. Now add a hair dryer on the same circuit: the breaker may still be comfortable, while the stretch of wire closest to the heater is carrying current from both directions, beyond its safe rating. The breaker cannot protect what it cannot measure.
Limiting solar input keeps the worst case within the safety margins built into wiring standards. Regulators chose limits they considered safe for typical residential circuits.
Why the numbers differ by country
| Region | Limit | Reasoning in short |
|---|---|---|
| Germany / EU | 800 W inverter output (Germany also caps panels at 2,000 Wp) | 230 V circuits on 16 A breakers; 800 W is about 3.5 A |
| United Kingdom | 800 W | Ring-main circuits; safety study found the risk manageable at this level |
| Most US states | 1,200 W | About 10 A at 120 V, leaving margin on 15-20 A circuits |
| Colorado | 1,920 W | 80% of a 20 A, 120 V circuit |
| Maine (simplified tier) | 420 W | Low enough for any outlet without changes |
| Maryland / Colorado (lighter rules) | 391 W | Treated as low-risk for existing wiring |
The 391 W figure comes from calculations of how much extra current a standard circuit can absorb even in unfavorable conditions, which is why some kits are deliberately sized at 360-390 W.
AC limit vs panel wattage
The cap is almost always on the inverter’s AC output, not the panels. A kit with two 450 W panels (900 W DC) and an 800 W inverter is compliant in Germany. On a perfect day, the inverter simply clips the peak at 800 W. Most of the year panels rarely hit their rated output, so slightly oversized panels increase total yearly energy with very little clipping loss, especially for vertical or east-west setups. Germany allows up to 2,000 Wp of panels behind an 800 W inverter for exactly this reason.
Practical rules for your home
- Plug into a dedicated outdoor outlet if you can, or one on a lightly loaded circuit.
- Do not put a space heater, kettle, hair dryer or window AC on the same circuit as a large kit.
- Never use extension cords or power strips between the kit and the outlet.
- If you want more than about 800 W in the US, ask an electrician to check the circuit, or split capacity across circuits only if your state’s law allows it (most set a per-household or per-meter limit).
For older homes, see plug-in solar and old wiring.
Is the limit a problem for savings?
Rarely. Because most plug-in laws do not pay for exports, the useful size of a kit is set by how much your home uses during the day. For an apartment with a baseload of 150-300 W, anything beyond 800 W mostly exports for free unless you add a battery. See should you add a second kit?