800 VA explained: why a 1,260 W kit is still within the limit

Every plug-in solar kit is described with two figures, and confusing them is the single most common misunderstanding in the category.

DC watts describe the panels. It is a laboratory rating measured at standard test conditions: 1,000 watts of irradiance per square metre, a cell temperature of 25°C, and a defined light spectrum. A British garden delivers that combination for a handful of hours a year, if at all.

AC volt-amperes describe the inverter — the box that converts panel output into mains-compatible electricity and pushes it through your plug. This is the number the framework regulates, because it is the only number the electricity network ever sees.

If panels reliably produced their rated output, matching 800 W of panel to an 800 VA inverter would be correct. They do not. A south-facing British array spends most of its generating hours well below its rating — cloud, low sun angle, temperature and dirt all take their share.

Oversizing the DC array fixes the shape of the day rather than the height of the peak. More panel area means the inverter reaches useful output earlier in the morning, holds it later into the evening, and produces something worthwhile on grey days that would otherwise be near-worthless. The trade is that on a handful of bright, cool afternoons the panels could have produced more than 799 VA — and that surplus is simply not converted.

What clipping actually costs

Our savings model applies a cautious flat annual derate wherever DC capacity exceeds the AC ceiling, rather than pretending oversizing is free.

Even after that derate, the larger kits generate more each year than the smaller ones. What they do not do is produce a higher peak — the ceiling is identical across the range.

Watts measure real power — the part that does work. Volt-amperes measure apparent power, the product of voltage and current regardless of how well they line up in phase. Network operators regulate apparent power because current is what heats cables, whether or not it is doing useful work.

A modern grid-tied microinverter runs at a power factor close to unity, so 799 VA and roughly 799 W amount to much the same thing in practice. The framework is written in VA because that is the engineering unit that matters at the connection point.

A seller advertising a "1,000 W plug-in solar kit" is describing panels. That is fine, provided the inverter is inside the limit and the product is supplied as a complete tested device. What is not fine is a kit whose inverter exceeds 800 VA, a kit assembled from separately-bought parts, or a substituted plug or lead.

When you compare, read the inverter specification and not the headline wattage. Our buyer's guide to comparing kits sets out the rest of the checks.