what capacity (wh) do i need for a portable solar power supply? | Insights by ShanPu

Start with a device inventory and time-on-load. Sum each device’s power (W) × hours used per cycle to get total Wh per camping day. Example: LED lights (6W × 6h = 36Wh), phone charging (10W × 2 × 2h = 40Wh), portable fridge (average compressor 45W × 24h = 1080Wh), camp kettle (1500W for 0.2h = 300Wh occasional). Total ≈ 1456Wh/day.

Then apply system factors: inverter efficiency (typical portable inverter: 85–92%; use 90% as baseline), and usable battery fraction (LiFePO4 80–90% usable; lead-acid 40–50%). Practical sizing formula: Required battery capacity (Wh) = (Daily load Wh ÷ usable_fraction) × (1 ÷ inverter_efficiency) × safety_margin. For the example with LiFePO4 at 90% usable and 90% inverter efficiency, battery ≈ (1456 ÷ 0.90) ÷ 0.90 ≈ 1797Wh. Add a 10–20% buffer for extra days or cloudy weather, so target ~2.0–2.2kWh.

Key takeaways: small lighting/charging camps can fit 400–700Wh units; fridge-plus-overnight scenarios routinely require 1,500–3,000Wh depending on compressor duty cycle and temperature. Always confirm continuous inverter rating vs fridge start-up surge.

Use steady-state power and duty cycle rather than nameplate surge power. Measure or check datasheet for average running watts (not starting watts). Example: a 12V compressor fridge may run 40–60W on average; energy = average watts × hours per day. For 48W average × 24h = 1,152Wh/day.

Factor in start-up surge: compressors often draw 2–4× running current for <1s. Ensure inverter continuous and surge ratings accommodate that. Adjust for ambient temperature: hotter ambient increases run time and thus Wh/day (typical increase 10–30% in hot environments). Final required battery capacity uses the same formula: Required Wh = (Fridge Wh/day ÷ usable_fraction) × (1 ÷ inverter_efficiency) × margin.

Practical rule: if you need 24/7 refrigeration for food or medical products, budget 1.2–1.5kWh/day for small efficient fridges and 2kWh+/day for larger or hotter conditions, then size battery accordingly (LiFePO4 3–4kWh to cover multiple days without sun).

List the device electrical draw and required run-time. Common examples: CPAP machines typically draw 30–60W (30W quiet models) — for 8 hours ≈ 240–480Wh. Portable oxygen concentrators vary (100–300W) — for 8 hours they can require 800–2,400Wh. Insulin pump chargers and small monitors are low (<20–50Wh/day).

Because medical devices are critical loads, add redundancy and conservative margins: designers typically specify two nights of autonomy under no-sun conditions and a higher margin for inverter reliability. Use conservative usable_fraction (do not fully discharge batteries daily), select an inverter with clean pure-sine output and medical electromagnetic immunity if required, and plan UPS-style transfer switching if mains/solar alternation is expected.

Example: CPAP 50W × 8h = 400Wh; to supply two nights with 90% DoD battery and 90% inverter efficiency: Battery = (400 × 2 ÷ 0.90) ÷ 0.90 ≈ 987Wh → round up to a 1.2kWh portable station to provide margin and allow charging losses.

Calculate per-device energy: modern laptops often draw 40–80W while charging; assume 60W × 4h = 240Wh per laptop per day for active use and charging. Smartphones are ~10–15Wh per full charge; for safety assume 15Wh × number of phones × charging cycles.

Aggregate example: 3 laptops (3 × 240 = 720Wh) + 6 phones (6 × 15 = 90Wh) = 810Wh/day. Add overhead for USB/DC converter inefficiencies (~5–10%) and AC inverter inefficiency if laptops charge via AC adaptors. If laptops charge via USB-C PD directly from battery inverter or DC-DC, losses are lower.

Budget formula: Battery Wh = (Aggregate device Wh ÷ usable_fraction) × (1 ÷ inverter_efficiency) × safety_margin. Using 80% usable and 90% inverter: (810 ÷ 0.80) ÷ 0.90 ≈ 1125Wh. For business continuity and unexpected loads, choose 1.5× this number; many mobile offices pick 1.5–3kWh units depending on number of devices and days of autonomy desired.

Vanlife sizing depends on day/night split and whether you will recharge daily with solar. Start with daily energy budget: lights 50–100Wh, fridge 400–1200Wh (depends on 12V efficiency and ambient), coffee/induction usage 200–800Wh per brew, devices 100–300Wh. Conservative weekend total often 900–1,800Wh/day for two people.

If you expect to recharge every day from rooftop panels, calculate PV energy: panel watts × peak sun hours (PSH). Example: 400W panels × 4 PSH = 1,600Wh gross; charge controller and battery losses reduce usable to ≈ 1,350–1,450Wh/day. If PV recharge supports daily usage, battery needs only to cover night and low-sun periods (typical night buffer 1× nightly load). If you plan 2–3 days off-grid without sun, multiply daily load accordingly and account for inverter and battery DoD.

Rule of thumb: for a typical van with moderate fridge and charging loads, a 800–1,200Wh battery will cover overnight needs; a full comfortable weekend with cooking and low-sun days commonly needs 2–3kWh of battery capacity plus 400–800W of PV to recharge. Prefer LiFePO4 chemistry for cycle life and higher usable Wh to minimize weight/volume.

Define two numbers: continuous power (W) the inverter must supply and the duration (hours) you need it to run. Continuous Wh demand = continuous W × hours. For multiple loads, sum their steady-state watts. Remember surge ratings for inductive loads; continuous inverter rating must match aggregate steady draw.

Then apply the sizing equation with conservative assumptions for outage scenarios: Battery Wh = (Continuous Wh × hours ÷ usable_fraction) × (1 ÷ inverter_efficiency) × redundancy_margin. Example: powering essential home circuits totaling 800W for 8 hours = 6,400Wh of energy. With LiFePO4 at 90% usable and inverter 90% efficient: Battery ≈ (6400 ÷ 0.90) ÷ 0.90 ≈ 7,901Wh ≈ 8kWh. For multi-day outages, multiply accordingly or consider hybrid genset integration.

Important system notes: continuous inverter rating, surge rating, BMS limits, and thermal derating at high ambient temperatures often drive higher practical battery sizing than pure Wh calculations. For mission-critical outages, plan N+1 redundancy and test under realistic load profiles before deployment.