How to Size a Power Station and Solar for Your Loads

Scroll any portable-power forum and the same question repeats in a dozen forms. One r/SolarDIY poster asks for the “Best Portable Station (~2000Wh) for Small Solar Setup”; another titles their thread “Power requirements for a small system.” Underneath both is the same unanswered need: how big does mine need to be? Most people guess, buy on price or brand, and discover the mismatch during the first real outage.

Sizing is not guesswork — it’s a worksheet. This post walks the full chain: audit your loads, convert to daily watt-hours, gross up for real-world losses, pick a battery, then size a solar array that can actually keep that battery fed. The math is simple arithmetic; the discipline is doing it before you buy instead of after.

Step 1: list every load and its running watts

Write down every device you genuinely need during an outage and its running wattage — the steady draw, not the surge. Get the number from the appliance label, the manual, or ideally a $15 plug-in watt meter, which is far more accurate than spec sheets. Typical figures: LED lights 5–15W each, Wi-Fi router 10–20W, phone charging 10–25W, laptop 30–65W, a modern fridge 80–150W running, a CPAP machine 30–60W, a 5,000 BTU window AC 450–550W, a microwave 1,000–1,200W, a well pump 800–1,200W running.

Separate “always on” loads from “occasional” ones. The router, fridge and a couple of lights might run all day; the microwave runs for five minutes. This distinction matters because a high-wattage device used briefly contributes far less to daily energy than a modest device left on for hours. A 1,200W microwave for 10 minutes is just 200Wh; a 15W router for 24 hours is 360Wh — nearly double, despite drawing 80× less power at any instant.

Step 2: convert to daily watt-hours with duty cycle

For each device, multiply running watts by hours of use per day to get daily watt-hours. For anything thermostatic — fridge, freezer, AC, well pump — multiply again by a duty-cycle factor between 0.3 and 0.5, because those compressors cycle on and off rather than running continuously. A fridge labelled 150W running, used “24 hours,” at 0.4 duty contributes 150 × 24 × 0.4 = 1,440Wh per day, not the 3,600Wh a naive calculation suggests. Skipping this step is the number-one reason people massively overbuy or, worse, underbuy.

Add up every device’s daily Wh to get your total daily energy budget. A realistic essentials-only outage load — fridge, Wi-Fi, phones, a few lights, occasional microwave — lands around 1,800–2,800Wh per day for most households. Add a window AC and you’re at 4,000–6,000Wh. Knowing this single number transforms the buying decision from a brand argument into an engineering match. The duty-cycle and derating logic behind these figures is laid out in full in why nameplate watt-hours never reach your appliances.

Step 3: gross up for losses and pick the battery

Your daily Wh budget is energy delivered to appliances; the battery has to store more than that because the inverter and discharge floor skim 10–15% off the top. Divide your daily budget by 0.88 to find the minimum nameplate capacity for a single day of autonomy with no recharge. A 2,500Wh daily budget needs about 2,840Wh of nameplate — so a 3,000Wh+ unit covers one full day, while a 2,000Wh unit leaves you short by mid-evening.

Then decide how many days of autonomy you want without sun. One day of battery plus daily solar recharge is the efficient sweet spot for most off-grid and backup setups; two-plus days of battery with no solar gets expensive fast, because battery capacity is the priciest watt-hour you can buy. This is why EcoFlow’s expandable DELTA Pro and DELTA Pro Ultra architectures exist — start with a base unit sized to one day, and bolt on expansion batteries only if your real-world usage proves you need them. Cold climates need extra headroom here, since freezing temperatures cut usable capacity, as covered in cold-weather charging limits.

Step 4: size the solar array to daily consumption

Here is the step almost everyone botches: they size the panel array to the station’s maximum input rating, or worse, grab a single 100W panel as an afterthought. The array should be sized to replace your daily watt-hour consumption, with margin for clouds and short winter days. A rule of thumb: a fixed solar panel delivers roughly 4–6 watt-hours per rated watt per clear summer day after derating for heat, angle, wiring and charge-controller losses. So a 400W array yields about 1.6–2.4kWh on a good day.

Match that to your budget. If you consume 2,000Wh daily, a 400–600W array roughly keeps pace in summer; for year-round or cloudy-climate reliability, target 1.5–2× your daily Wh in panel watts — so 3,000–4,000W of panels for a 2kWh daily load is not overkill in winter. Stay within the station’s MPPT input window (DELTA Pro accepts up to 1,600W solar; DELTA Pro Ultra far more), and split large arrays across dual MPPT inputs where available to tolerate partial shading. EcoFlow charge-time tablesgive model-by-model recharge math once you’ve picked an array.

Step 5: sanity-check against a worked example

Take a common prepper-and-RV scenario: fridge (150W, 0.4 duty, 24h) = 1,440Wh; Wi-Fi router (15W, 24h) = 360Wh; 4 LED lights (10W each, 5h) = 200Wh; phone/laptop charging = 300Wh; occasional microwave/coffee = 250Wh. Daily total ≈ 2,550Wh. Grossed up at 0.88, you need about 2,900Wh of battery for one day of autonomy — a DELTA Pro (3,600Wh) covers it comfortably with evening margin, while a 2,000Wh unit would die overnight.

For solar, 2,550Wh of daily consumption wants roughly 600W of panels to break even in summer and 1,000–1,200W to stay ahead in shoulder seasons. That pairing — a DELTA Pro plus ~800W of portable panels — turns a roughly 30-hour battery into genuinely indefinite backup as long as the sun shows up. This is the configuration that survives a multi-day grid failure, and it’s why a battery-plus-solar setup outperforms a fuel generator on long events, as the numbers in battery backup vs generator show. During a blackout, remember that portable panels into a station keep charging even when rooftop grid-tied solar legally cannot — the reason is anti-islanding.