How to figure out how much Solar and inverter is needed to charge a battery

There's a lot of misinformation floating around on this topic so let me lay out the actual methodology.

Start with your load, not your panels.

1. List every device, its wattage, and daily hours of use. Sum it up — that's your daily Wh figure.
2. Divide by your usable battery capacity (typically 80% for LiFePO4, 50% for lead acid) to get your required bank size.
3. Your inverter VA rating needs to comfortably exceed your peak simultaneous load — not average. People consistently undersize here and wonder why their Victron Multiplus trips.
4. Solar sizing: take your daily Wh requirement, divide by your peak sun hours for your UK location (realistically 2.5–3.5 in winter, not the 5hrs the marketing material quotes). Then add 25% for system losses.

In practice for my shepherds hut setup I ran through this properly — 1.8kWh daily load, 4.8kWh Fogstar Drift LiFePO4 bank, 800W of panels, Victron MPPT. Winter charging is still marginal but predictable because the maths was done first.

The inverter question specifically — size it for surge loads. Induction hobs and EV chargers have brutal startup draws. A 2kW continuous rating inverter might only handle 4kW surge for milliseconds.

What are others seeing for their actual winter peak sun hours in different parts of the UK? Would be useful to compile some real figures rather than relying on PVGIS estimates alone.

@SolarNotSure solid start — and the bit most people skip is multiplying that watt-hour total by 1.25-1.3 to account for real-world inefficiencies (inverter losses, cable resistance, battery round-trip efficiency), otherwise you'll size a system that looks perfect on paper but leaves you in the dark by 8pm.

Quick example:

• 500Wh calculated daily load
• × 1.25 = 625Wh actual draw from battery
• ÷ 0.85 (typical MPPT efficiency) = panels need to deliver ~735Wh/day

Then divide by your peak sun hours for your location (UK average is roughly 3-4hrs, not the 5-6hrs those YouTube Americans keep quoting 🙄) to get your panel wattage.

Good thread. Quick question on the efficiency multiplier @Spud mentions — does that 1.25–1.3 figure already account for inverter losses, or is that a separate calculation on top?

Asking because I've been trying to work out sizing for a 200Ah LiFePO4 (Fogstar Drift) and I keep getting confused about where inverter inefficiency fits into the chain. My Victron MultiPlus is rated at ~93% efficiency but I'm not sure whether to apply that to the whole load figure or just the AC loads specifically.

Presumably DC loads bypass the inverter entirely, so the loss only applies to AC stuff?

@JoeTurner no, they're separate things really. The 1.25–1.3 is mainly inverter inefficiency (typically 85-92% on most units). You'd then also want to factor battery round-trip losses on top — Lithium like Fogstar/CATL cells are pretty good, maybe 95-98%, but AGM can be more like 80-85% which really stacks up.

So in practice I just multiply my load total by 1.3 for inverter losses, then divide by 0.9 for battery losses (LiFePO4). Ends up roughly 1.45x your raw consumption figure before you even think about panel sizing or bad weather days.

@MickWebb82 makes a fair point keeping those two separate. The other factor people consistently underestimate is panel derating — your 200W panel rarely delivers 200W in Britain. I work off roughly 70-75% of rated output as a realistic peak figure, and that's on a decent day. On the narrowboat I'd sometimes see my Renogy 200W panel struggling to push 90W under a February overcast.

So the full picture ends up being:

• Daily load (Wh) × 1.25 (system losses)
• ÷ usable sun hours (2-4 for UK, seasonal)
• ÷ 0.75 (panel derating)

That final number is the actual panel wattage you need to spec. Most people do the first step and wonder why their batteries are perpetually half-empty by November.