The whole method in one line
Daily Wh → divide by efficiency & usable depth to get battery Wh → check surge & continuous output → size solar to your sun hours → add a safety margin. That's it.
1. Add up your daily watt-hours (Wh)
Energy is measured in watt-hours: watts × hours. For each device, multiply its power draw by how long you run it per day, then add them all up.
Daily Wh = Σ (watts × hours)
Example: fridge 150W × 8h = 1200 Wh
lights 30W × 5h = 150 Wh
phone/laptop = 280 Wh
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Total ≈ 1630 Wh/day
A fridge doesn't run continuously — its compressor cycles on and off — so use roughly a third of the hours it's plugged in (≈8h of actual running for a 24h day), or read the yellow EnergyGuide label's kWh/year and divide by 365.
Typical device draws
| Device | Running watts | Notes |
|---|---|---|
| Phone charge | 5–20 W | ~15–20 Wh per full charge |
| Laptop | 30–90 W | Gaming laptops much higher |
| LED light | 5–15 W | Per bulb |
| 12V fridge / cooler | 40–60 W | Cycles ~30–50% of the time |
| Full-size fridge | 100–200 W | Surges 2–3× at startup |
| CPAP | 30–60 W | More with heated humidifier |
| Microwave | 1000–1500 W | Short bursts, high draw |
| Coffee maker / kettle | 1000–1500 W | Big momentary load |
| Space heater / A/C | 1000–1800 W | Will drain any small unit fast |
2. Convert to battery capacity
You can't use 100% of a battery's rated capacity to run AC devices. Two losses stack up:
- Inverter efficiency (~85%) — turning DC into 120V AC wastes ~10–15% as heat.
- Usable depth of discharge — even LiFePO4 lasts longer if you don't drain it to 0% daily. Leaving ~10–15% headroom is a good target.
Battery Wh = Daily Wh ÷ 0.85 (inverter) ÷ 0.85 (usable) × days of backup Example (1 day): 1630 ÷ 0.85 ÷ 0.85 ≈ 2250 Wh needed
So ~1,600 Wh/day of usage really wants a ~2,000 Wh-class station (Delta 2 Max, AC200L, F2000, Explorer 2000). Want to ride out a 3-day outage with no sun? Multiply by 3 → ~6,700 Wh, which points you at an expandable unit or a DIY bank.
💡 Why LiFePO4?
Almost every quality unit now uses LiFePO4 (lithium iron phosphate): 3,000–6,000 cycles (10× older lithium), safer chemistry, and happy sitting partially charged. Older NMC lithium is lighter but wears out faster — for a power station that's charged and drained constantly, LiFePO4 wins.
3. Don't forget surge (the #1 mistake)
Anything with a motor or compressor — fridges, pumps, A/C, power tools — spikes well above its running watts for a split second on startup. Your station must cover both the continuous draw of everything running at once and the momentary surge of the biggest motor starting.
| Load | Startup surge |
|---|---|
| Resistive (heater, kettle, lights) | 1× — no surge |
| Fridge / freezer compressor | 2–3× |
| Water pump / well pump | 3–5× |
| Air conditioner (no soft-start) | 3–6× |
| Power tools (saw, drill) | 2–3× |
A 150W fridge can surge to 600W+. If a station lists 1,800W continuous / 3,600W surge, it can start that fridge while also running other loads. Undersized output is the most common reason a "big enough" battery still trips off.
4. Size your solar to actually recharge
Panels rarely hit their rated watts — angle, heat, clouds, dust and cabling all cost you. Derate to about 70%, then divide by your region's peak sun hours (the equivalent hours of full-strength sun per day):
Solar W = Daily Wh ÷ (0.70 × peak sun hours) Example (4 sun hrs): 1630 ÷ (0.70 × 4) ≈ 580 W of panels
| Region (rough) | Peak sun hours |
|---|---|
| Southwest US (AZ, NV, NM) | 5.5–6 |
| California / Mountain West | 5 |
| Southeast / Gulf & Florida | 4.5–4.7 |
| Midwest / Mid-Atlantic | 4 |
| Pacific NW / Northeast (winter) | 2.5–3.5 |
⚠️ Check the station's max solar input
Every unit caps how many watts (and volts) of solar it will accept. There's no point buying 800W of panels for a unit that only takes 500W — the extra is simply clipped. Match panel wattage and the array's voltage (Voc, which rises in the cold) to the station's input spec.
5. Add a real-world safety margin
The math above is the minimum. In the real world, plan for a cushion:
- +20–30% headroom for cold weather (batteries deliver less when cold), aging (capacity fades over years), and the loads you forget to count.
- Round up to the next real product — it's cheaper to buy one right-sized unit than to discover you need a second.
- Favor expandable units if your needs might grow (home backup especially) — add a battery later instead of replacing the whole system.
- DC beats AC where possible — charging phones/laptops from USB-C or 12V skips the inverter's ~15% loss entirely.
Putting it together: a worked example
Weekend cabin: a full-size fridge, a few LED lights, phone/laptop charging, and a water pump, in a 4-sun-hour region.
- Daily energy: fridge ~1,200 Wh + lights 150 Wh + devices 280 Wh ≈ 1,630 Wh/day.
- Battery: 1,630 ÷ 0.85 ÷ 0.85 ≈ 2,250 Wh → with a 25% margin ≈ 2,800 Wh (a ~3 kWh unit, or 2 kWh + solar).
- Output: pump surge (say 800W running, 3× = 2,400W) → want ≥ 2,000W continuous / 4,000W surge.
- Solar: 1,630 ÷ (0.70 × 4) ≈ 580W of panels to refill daily — check it's within the unit's max input.
Skip the math — let the tool do it.
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