Free open-source style Solar, Battery Runtime & Home Energy Calculators (No ads, no lead-gen)

Really useful to see something like this without the usual "enter your email for results" nonsense that most of the commercial calculator sites pull.

I've been piecing together my own spreadsheet for the cabin setup — mostly trying to work out realistic runtime on a 200Ah Fogstar Drift LiFePO4 with a modest 400W Renogy array — and honestly the battery runtime side of things is where I always get unstuck. Peukert's constant, temperature derating, inverter inefficiency... it all compounds and my spreadsheet ends up a mess of half-remembered formulas.

A few questions for anyone who's used these:

• Does the battery calculator account for depth of discharge limits? I keep my LiFePO4 at 80% DoD max and a lot of tools just assume 100%
• Is there any way to factor in seasonal variation for UK irradiance? My winter numbers on the South Coast are depressingly different from the summer peak hours
• Does it handle partial shading scenarios at all, or is that too complex to model simply?

I've also been curious whether tools like this could eventually incorporate Victron MPPT efficiency curves rather than a flat percentage — that would make the solar side considerably more accurate for those of us running SmartSolar controllers.

Would be good to hear from others who've stress-tested these against their actual system logs. Real-world validation is where most calculators fall apart in my experience — especially in a UK climate where "peak sun hours" feels like a fairly generous concept for eight months of the year.

@PartnerProject spreadsheets are great til they get unwieldy 😅 I went down that rabbit hole for my van build and ended up with 47 tabs 🙈

These open-source tools are proper handy for gut-checking your figures before committing to kit. I used a similar one before sizing my Fogstar batteries and it saved me from massively under-speccing.

One thing worth doing — cross-reference the battery runtime calcs against real Victron data if you're running their kit, as some generic calculators use optimistic efficiency assumptions.

Bookmarking this thread for when people ask me the classic "how many solar panels do I need" question 😄

Spreadsheets are fine until you accidentally delete a formula at 2am and spend three hours wondering why your Fogstar 206Ah is apparently powering a small city.

The real trap with any calculator — online or spreadsheet — is garbage-in-garbage-out; most people wildly underestimate their loads (looking at you, inverter standby draw that nobody accounts for).

I'd cross-reference whatever tool you're using against Victron's own sizing guides and then add at least 20% headroom, because theoretical and real-world are very different animals, especially with temperature derating on LFP in a British winter parked on some miserable layby in Wales.

@PartnerProject handy find, cheers for sharing. Been meaning to redo my calcs for the cabin properly — always end up with a rough guess scribbled on paper somewhere.

@FormerCop that 2am deleted formula thing is painfully relatable 😅

One thing I'd want from any calculator is being honest about Peukert losses and not just assuming ideal discharge. Most of the simple ones seem to skip that bit entirely which throws everything off if you're running big loads.

Ha, @FormerCop that's painfully relatable — I've definitely lost an afternoon to a broken cell reference before now!

For what it's worth, I've been running a hybrid approach: use the online calculator to sanity-check the headline figures, then keep a leaner spreadsheet just for the site-specific stuff like our shading losses and the actual measured consumption from our smart plugs. Keeps the complexity manageable without reinventing the wheel each time.

@Fogstar_Fan one thing worth double-checking in any calculator is how it handles winter derating for LiFePO4 — a lot of them assume fairly optimistic temperatures. Here in the Cotswolds that's bitten me before when I've been too optimistic about usable capacity in January.