Vicaya GlobalCalculator · 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.
12 × 450 W at 21% is 2.14 m² per panel — about 26 m² of roof.
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
| 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.
| 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
- Every day in a month is modelled as that month's average day. Cloudiness comes from long-run monthly climate and is applied to all of that month's hours, so the 8,760-hour series holds no individually clear day. Annual energy is a sum and is unaffected; clipping is driven by peaks and is not. Modelled peak AC for a 5.4 kWp array in London reaches 57% of nameplate where a real clear, cool day approaches 85%. Treat the clipping figures here as a floor rather than an estimate — the effect is largest in cloudy climates, where this model reports zero clipping up to a DC:AC ratio of 1.6 and the recommended ratio rises accordingly.
- 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.
Frequently asked questions
What size inverter do I need for my solar panels?
Usually a little smaller than the array’s peak DC rating. The common “1.2 DC:AC ratio” is a sunny-climate rule of thumb, and a cloudy site wants a smaller inverter still. This calculator works out the ratio worth buying where you are, rather than repeating one number.
Use it as a solar inverter sizing calculator: it compares the inverter rating with the panel array so you can see the energy any undersizing would cost.
Can a solar inverter be smaller than the panel array?
Yes, and it usually should be. Panels rarely hit their full rating, so a slightly undersized inverter captures nearly all the energy for less money. The tool shows how far you can go before the energy lost to clipping outweighs the saving on the inverter.
What is the best DC:AC ratio for solar?
There is no single best number — it rises in cloudy climates and falls in sunny ones, because the ratio is a trade-off between inverter cost and clipped energy. Enter your location and the calculator gives the ratio that balances the two for your site.