Solar · inverter sizing

What size inverter does this array need?

An inverter smaller than the array it serves is the normal case, not a compromise. This works out how much smaller, by simulating every hour of a year at nine different ratios and pricing what each one clips.

What do kWp and kWh mean?

kWh — kilowatt-hour: A unit of energy, and the thing your electricity bill charges you for. A 1,000-watt heater running for one hour uses one kWh. A typical home uses somewhere between 2,000 and 10,000 kWh a year depending on where it is and how it heats.
kWp — kilowatt-peak: A unit of capacity — how big the solar array is, not how much it makes. It is the output the panels would produce under standard test conditions: bright, cold and perfectly aimed. Real roofs rarely see those conditions, which is why a 4 kWp array does not generate 4 kW for most of the day. One modern panel is roughly 0.4 kWp, so 4 kWp is about ten panels.
Putting them together: kWp is the size of the system; kWh is what it produces over time. The ratio between them — kWh generated per kWp installed, per year — is the honest way to compare locations, because it strips out how big the system happens to be. It runs from roughly 700 in cloudy high latitudes to over 1,800 in sunny deserts.

Location

Where is the array?

Searches use OpenStreetMap. Nothing is stored.

Or pick an example location

Or enter coordinates directly

Latitude

Longitude

Searching or using your location fills these in, so you can always see exactly which point the figures are for.

Climate comes from PVGIS, the European Commission's reference dataset — the same source this model is validated against, for examples and searches alike.

Your panels

Both numbers are on the quote, or on the panel label.

How many panels

Count them off the quote. Type any number — there is no upper limit.

Watts per panel

The number on the label. Most panels sold today are 350 to 500 W.

Your roof

Orientation sets how peaked the generation curve is, and a peak is what clipping feeds on.

Which way does the roof face?

The direction you would face standing on the roof looking down the slope.

How steep is the roof?

Measured up from flat. Most pitched roofs are between 30 and 45 degrees.

What the energy is worth

One blended rate: a unit used at home is worth your import price, an exported one your export price.

Value of a unit p/kWh

Between your export rate and your import rate, weighted by how much you use at home.

Inverter price £

Per kW of AC rating. A bigger inverter costs more; that is the entire trade-off.

The long run

Energy prices rise %

Per year, nominal. Raises the value of the energy a smaller inverter would clip.

Discount rate %

What the money could earn elsewhere. Set to 0 to compare nominal totals only.

Judge over

Inverters are usually warranted for 10 to 12 years and often replaced once.

1.80best DC:AC ratio here

3.00 kWinverter, for 5.40 kWp of panels

0.02%of output clipped at that size

£570what that inverter costs

In London, United Kingdom, the best value is an inverter rated 3.00 kW — a DC:AC ratio of 1.80.

An inverter smaller than the array is not a compromise, it is the normal case. A 5.40 kWp array reaches its nameplate rating only in conditions that barely occur: full sun, cold panels, and the sun square-on to the roof at the same moment. Paying for an inverter that can pass a power level the sun never delivers is paying for headroom that is used for nothing.

What each ratio costs you

Energy lost to the inverter's AC ceiling as the array grows relative to it. The curve stays flat for a long way and then turns sharply — which is why the cheapest sensible inverter sits just before the turn, not comfortably before it.

Every ratio the sweep evaluated. Each row is a full 8,760-interval simulation.
DC:AC	Inverter	Annual output	Clipped	Clipped	Inverter cost	Net present value
1.00	5.40 kW	5,480 kWh	0 kWh	0.00%	£1,026	£16,473
1.10	4.91 kW	5,480 kWh	0 kWh	0.00%	£933	£16,566
1.20	4.50 kW	5,480 kWh	0 kWh	0.00%	£855	£16,644
1.30	4.15 kW	5,480 kWh	0 kWh	0.00%	£789	£16,709
1.40	3.86 kW	5,480 kWh	0 kWh	0.00%	£733	£16,766
1.50	3.60 kW	5,480 kWh	0 kWh	0.00%	£684	£16,815
1.60	3.38 kW	5,480 kWh	0 kWh	0.00%	£641	£16,857
1.80 — best	3.00 kW	5,479 kWh	1 kWh	0.02%	£570	£16,924
2.00	2.70 kW	5,430 kWh	51 kWh	0.92%	£513	£16,824

The rule of thumb is a climate, not a rule

The same 5.40 kWp array, on the same 35° roof, facing the equator, simulated in eight places. Only the sky changes.

A fixed comparison. These eight rows do not move with the controls above — they are held identical on purpose, so the only thing that differs between them is climate.
Site	Latitude	kWh per kWp	Clipped at 1.2	Clipped at 1.5	Clipped at 2.0	Best ratio
Chennai, India	13.1°	1,476	0.00%	0.65%	7.24%	1.4
Denver, United States	39.7°	1,736	0.00%	2.28%	16.24%	1.3
London, United Kingdom	51.5°	1,015	0.00%	0.00%	0.92%	1.8
Nairobi, Kenya	-1.3°	1,399	0.00%	0.00%	3.30%	1.6
Oslo, Norway	59.9°	914	0.00%	0.00%	1.43%	1.8
Phoenix, United States	33.5°	1,910	0.05%	5.63%	20.97%	1.2
Santiago, Chile	-33.5°	1,856	0.05%	5.54%	19.76%	1.2
Tromso, Norway	69.7°	726	0.00%	0.00%	1.10%	1.8

Phoenix, United States wants 1.2 and Tromso, Norway wants 1.8. The reason is not that one place has more sun in total — it is that a clear-sky climate spends more hours near peak, and clipping only ever happens near peak. A maritime climate delivers its annual total through a great many mediocre hours, none of which trouble an inverter. So the cloudier the site, the more of the array's nameplate an inverter can safely be spared.

What this does not model

Inverter efficiency is constant here at 96%. Real inverters are less efficient at very low load, which penalises an oversized inverter that spends its life loafing. Including it would push the recommended ratio higher, not lower — so the figure above is the conservative end of the answer.
No MPPT voltage window, and no string layout. An inverter also has to accept your array's voltage on the coldest morning of the year, and that constraint can override everything on this page. Working it out is string sizing, which this section deliberately does not publish — see below.
No inverter thermal derating. An inverter in a hot loft can throttle below its nameplate, which would show up as clipping this model does not predict.
Prices are a straight rate per kW. Real inverter pricing steps between models, and the cheapest unit above your requirement is the one you can actually buy.

Why there is no string-sizing calculator here

The obvious next question — how many modules per string — is a safety-critical calculation. Getting it wrong in the direction of too many modules puts a cold-morning open-circuit voltage across an inverter that is not rated for it. This section does not publish that tool, because publishing it responsibly needs a review by an engineer credentialled in your jurisdiction and external to whoever built it, and that review does not exist. A disclaimer would not substitute for it, so the tool is absent rather than hedged.