Size an inverter
Calculates continuous and peak power, DC currents and the surge ratio. And the item no free calculator shows: idle draw — now from the appliance catalogue for the class the calculator itself determines, instead of a blanket 25 W. In the reference case that is 10 W and thus 10 instead of 25 percent of daily demand. The saving from a night shutdown is translated into module power and module cost using the sun hours at your location.
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
An estimate based on the stated assumptions. The final design must be checked by a qualified professional against the rules that apply where you are.
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Required continuous power | P_cont * headroom / f_temp | 750 W | exact |
| Surge ratio | P_peak / P_cont,req | 1.6 | exact |
| DC current, continuous | P_cont,req / (U_sys * eta) | 69.444 A | exact |
| Idle energy per day | P_idle * h_on | 240 Wh/d | exact |
| Equivalent module power | E_saved / PSH | 77.389 Wp | assumed |
- Formula
P_req = P_cont · headroom / f_temp · I_DC = P_req / (U_sys · η)- Valid for
- Sizing from continuous load, inrush event and headroom. Idle draw comes from the versioned appliance catalogue, selected by the determined device class (next larger; above the largest class no extrapolation). Converting idle losses into module power uses the site’s sun hours in the design month (PVGIS-SARAH3 TMY) — the same quantity as in calculator 2. Reference case: 600 W continuous → 750 W rating, 1 kW class at 10 W idle, 240 Wh/d (10 % of daily demand), 77 Wp equivalent module power.
- Not covered
- Efficiency curve across the load range (the calculator uses two points: typical and peak), reactive power, grid-parallel operation, phase imbalance in multi-phase units, the inverter’s own inrush. The module price stays an input — the market dataset holds net spot prices, not retail prices.
Frequently asked questions
What size inverter do I need for my loads?
The required continuous rating is P_cont · headroom / f_temp — 600 W of load with 25 % headroom means 750 W. The required surge rating is the largest single start-up event plus the running base load, not the sum of all surges; in the reference case a compressor with a surge factor of 5 plus 450 W base load gives a 1200 W peak. Also check temperature derating: many units only guarantee their nameplate rating up to 25 to 40 °C.
How much power does an inverter draw when nothing is switched on?
A 3000 W inverter typically draws 20 to 35 W at idle. Over 24 hours that is 480 to 840 Wh — with a daily demand of 2400 Wh, up to a third of the whole budget just for being switched on. The calculator therefore also quantifies what a night shutdown saves, converted into module watts and module cost.
Is a modified sine wave inverter good enough for my appliances?
For motor, electronic and mixed loads the calculator warns against modified sine output and recommends pure sine. It does not cover reactive power and power factor with inductive loads, the harmonics produced by modified sine waveforms, or parallel and three-phase configurations of several units — it is designed for single-phase operation with resistive to mixed loads.
What does inverter idle really cost me?
More than almost anything else in the system: a 3,000 W unit typically draws 20–35 W at idle. Over 24 hours that is 480–840 Wh — at 2,400 Wh daily demand, 20–35% of total demand, just for being switched on. The calculator converts the saving of a night shutdown directly into module power and euros: for typical van systems it equals 250–500 Wp of solar. A remote switch is often the cheapest “solar upgrade” money can buy.
Why does an undersized inverter shut down instead of simply delivering less?
Because the peak power limit is a hard protective cutoff: at compressor start, 3–7× the rated current stands for 0.5–3 seconds. If the spike exceeds the unit’s peak power, it disconnects — mid-inrush, usually at night when the fridge kicks in. Missizing therefore shows up not as poor efficiency but as sudden shutdown. That exact event is what the surge chart shows; above a surge ratio of 3 a soft starter beats a bigger unit.
Pure or modified sine?
For resistive loads (heating element, incandescent light) modified sine suffices. Motors run hotter, louder and with less starting torque; electronic power supplies and anything with phase control (dimmers, many chargers) can malfunction or fail. For load types “motor” or “electronic” the calculator therefore recommends pure sine and warns on a modified choice. The price difference is small today — the risk is not.
Where does the idle draw come from?
From the appliance catalogue, for the device class this calculator itself determined. Previously a blanket 25 W sat there — the catalogue value of the 3 kW class, applied to every system size. The reference case needs 750 W continuous, which falls into the 1 kW class at 10 W idle: 240 Wh per day instead of 600, so 10 % of daily demand instead of 25 %.
Why is there no interpolation between device classes?
Because the catalogue holds two measured classes, 1 kW and 3 kW, and a straight line between them would be a claim about devices nobody measured. The calculator therefore takes the next larger class; above the largest, its value continues to apply. If you have your unit's datasheet, enter the real value via the switch — that is always the better number.
Which sun hours convert the saving into modules?
Those of your location in the design month, in the plane of your modules — the same quantity as in the module calculator, so both describe the same system. Previously it was a fixed 3.0 hours, and that value was not even in the form. For Berlin at 35 degrees south it is 1.03 hours in December.
Why does the equivalent module power rise although idle draw fell?
Because two corrections pull in opposite directions. Idle fell from 25 to 10 W, but sun hours fell from 3.0 to 1.03 — and they are the divisor. So 67 Wp becomes 77 Wp. That is the more honest figure: sizing for December takes more module area per saved watt-hour than the annual mean suggests.
Why doesn't the calculator take the module price from the market dataset?
Because the dataset holds net spot-market prices and states itself that these are not retail prices — pvXchange cites a factor of 8 to 15 for a turnkey system. A private buyer pays a multiple of the €0.135 per Wp quoted in July 2026 for a single module. The calculator therefore shows the spot value alongside as a reference rather than inventing a retail price from it.