Understand your power plan.

This tool prepares a transparent sizing brief for a home, office or small-business backup system. It does not replace a site survey, electrical design or a manufacturer compatibility check.

1. Use electrical input ratings

Preset watts, power factors and startup multipliers are illustrative estimates. Use the equipment nameplate, manufacturer data or measurements. Air-conditioner horsepower and cooling output are not electrical input watts. Startup demand can vary widely, especially for motors and compressors.

Power factor is real watts divided by apparent VA. The planner sums appliance VA separately from watts and specifies both minimum inverter ratings. Its startup VA estimate uses running power factor, so actual starting current and overload duration still need verification.

2. Define the outage

By default, every selected unit operates throughout the chosen 1–24 hour backup period. Every appliance has an enabled hours input in Backup & solar. Type its total operating hours directly; for example, 2 hours for a TV during a 6-hour outage. Use the full-period button to return that appliance to following the overall backup duration. Custom hours remain independent when the backup duration changes, and cannot exceed it. For cycling appliances, those are equivalent hours at the listed input watts. All units of an equipment type share the same schedule; use custom equipment entries for different schedules.

Running load assumes all selected equipment can run together. Managed startup reduces only startup overlap, not continuous load. Select it only when starts can be controlled, including after supply returns.

3. Separate power and storage

Continuous kW = total running watts × (1 + reserve / 100) ÷ 1,000. Continuous kVA = sum of watts × quantity ÷ power factor, multiplied by the same reserve and divided by 1,000.

Equipment energy is watts × quantity × operating hours ÷ 1,000. Battery draw is equipment energy ÷ inverter efficiency + idle watts × backup hours ÷ 1,000. Required usable DC storage is battery draw × reserve factor. Nominal storage is usable DC storage ÷ permitted discharge depth. Percentages use fractions in these formulas.

Defaults: 25% design reserve, 90% inverter efficiency, 90% usable discharge depth and 20 W inverter idle draw. These are editable assumptions for a lithium-based system, not confirmed performance. Use an efficiency allowance that excludes separately entered idle draw. Battery capacity depends on temperature, age and discharge rate. Check BMS current limits as well as kWh.

Continuous and startup power, nominal battery capacity and PV array capacity round upward to the next 0.1 in their respective units. The usable-energy card and breakdown show the calculated requirement to three decimal places. No model or commercial capacity is guaranteed to exist at the displayed size.

4. Treat solar as a daily energy estimate

The model sizes solar to replenish one reserved backup cycle in one solar day, plus extra daytime energy. PV kWp = (required usable DC storage ÷ charging efficiency + extra daytime AC energy ÷ inverter efficiency) ÷ (peak sun hours × PV yield factor).

Default 4 peak sun hours is a placeholder. Use a location-specific, worst-month resource estimate and account for shading. The 75% PV yield assumption covers panel-to-DC-bus losses; 95% battery charging efficiency is separate. Additional daytime energy defaults to zero and must be entered if the system supplies other use while charging.

This is not a weather simulation or a guarantee of solar autonomy. It does not schedule charging, model consecutive cloudy days, account for every battery loss, or verify roof area, PV string voltage, controller current and charging power limits.

Worked example

For a 1,000 W unity-power-factor load over 6 hours, 25% reserve, 90% inverter efficiency, 90% discharge depth and zero idle draw: equipment energy is 6 kWh; required usable DC energy is 8.333 kWh; nominal storage is 9.259 kWh, rounded up to 9.3 kWh. Continuous output is 1.25 kW / 1.25 kVA before upward rounding, or 1.3 kW / 1.3 kVA. Actual startup requirements depend on the equipment.

What your installer should confirm

• Real input watts, running power factor, starting current and surge duration.
• Inverter continuous kW and kVA at the site temperature and its overload curve.
• Battery voltage, compatibility, usable depth, continuous/peak discharge and recharge limits.
• PV resource, shade, roof space, controller limits and time available to recharge.
• Supply phase/voltage, earthing, cable sizing, protective devices and applicable local requirements.

Solar panel quantities

Panel count = the calculated array target in watts divided by the selected panel wattage, rounded upward to a whole panel. For example, a 2 kWp target needs 4 × 550 W panels (2.2 kWp total). The 200–600 W selector offers example nameplate ratings, not specific panel models. Power-station suggestions check the actual rounded array total.

Estimated daily DC-bus energy = array kWp × peak sun hours × PV yield factor. This is before battery charging losses and is not a generation guarantee. Panel dimensions and mounting area require the actual datasheet.

No series/parallel wiring is approved by panel count alone. Confirm cold-corrected open-circuit voltage, operating voltage, short-circuit/operating current, each MPPT input limit, port allocation and cable/connector compatibility. See Victron’s PV input guidance and EcoFlow’s panel compatibility guidance.

Product suggestions

Choose EcoFlow, BLUETTI, both brands or an any-brand shopping brief on the results page. A curated UK/EU shortlist compares normal running watts, battery capacity and any published startup watts with your plan. Required extra batteries are named and counted; they do not increase inverter output. Among qualifying configurations, the tool prioritises the conservative solar input screen, then smaller battery capacity. This is not a price ranking or the whole market.

Where VA ratings or startup duration are unavailable, the suggestion explicitly requires supplier confirmation. A product that meets backup needs but falls short of the solar input screen is labelled accordingly. The screen compares target array nameplate watts with maximum PV input; it does not approve string voltage, current, over-paneling or actual recharge time. Boost/Power Lifting modes are not used as normal output ratings.

Product efficiency, usable depth and idle consumption may differ from the planner assumptions. Confirm these with the supplier and recalculate. Catalogue records were checked on 11 September 2026 and display a review reminder after 180 days. Manufacturer links accompany each suggestion; local availability, warranty and prices are not assumed.

The initial highlighted setup prioritises a passing solar input screen, a published startup-watt figure, fewer extra batteries and smaller total capacity, in that order. This does not rank prices or brand quality. You can choose the alternative; the selected setup is included in your exported plan. Technical specifications are expandable on each card.

Your data

Your equipment and settings are stored in this browser when local storage is available. Shared-device users may see that plan. Start fresh clears the current planner’s equipment and assumptions. Private browsing or blocked storage may prevent saving. No data is automatically uploaded. Download, copy, print and WhatsApp actions are initiated by you.

Reference concepts

Victron: understanding inverter ratings · PVWatts: location-based solar estimates and uncertainty. These explain underlying concepts; they do not certify or endorse Detopsy’s simplified calculation.