Let's Encrypt vs ZeroSSL — Which Free SSL Should You Use?

Free SSL has become the baseline standard for modern websites. Let's Encrypt pioneered the space back in 2015, while ZeroSSL has gained traction since 2020 — but how do they actually compare, and which one fits which use case?

This article puts Let's Encrypt head-to-head with ZeroSSL on every dimension developers and sysadmins care about: certificate lifetime, wildcard, rate limits, and ECC support.

What is Let's Encrypt?

Let's Encrypt is a non-profit certificate authority operated by ISRG (Internet Security Research Group), founded by Mozilla, EFF, Cisco, and Akamai. It began issuing free certificates in April 2016 and has now issued over 3 billion certificates.

Its defining trait: built for automation from day one. The ACME protocol it pioneered is now an industry standard, supported by every major hosting panel — DirectAdmin, cPanel, Plesk — and by reverse proxies like Nginx Proxy Manager and Caddy.

What is ZeroSSL?

ZeroSSL is operated by apilayer GmbH (Austria) and issues free certificates backed by Sectigo's root CA (USERTrust RSA / ECC). It rose in popularity because of its easy web UI — you can issue a cert without setting up an ACME client.

ZeroSSL now supports ACME as well and is commonly used as a secondary CA alongside Let's Encrypt to dodge rate limits, or for small projects where automating isn't worth the effort.

Comparison at a glance

Practical differences

1. Both default to 90-day certs

Both CAs issue free 90-day certificates. Let's Encrypt fixes the duration on principle — they advocate for short-lived certs + auto-renew. ZeroSSL lets you pick 90 days (free) or a 1-year cert (paid).

In practice, if you have auto-renew running (e.g., certbot renew in cron), the lifetime doesn't matter — your renewal runs ~30 days before expiry either way.

2. Rate limits hit differently

Let's Encrypt caps you at 50 certificates per Registered Domain per 7 days on public domains — plenty for most sites. But on shared hosting, when many customers' certs share one apex, the combined total can trip the limit.

ZeroSSL has no per-domain rate limit, but free accounts are capped at 3 certificates per 90 days — fine for personal sites, painful for hosting providers.

3. ECC by default = faster, lighter

Since 2024, Let's Encrypt defaults to ECDSA P-256 certs. They're ~8× smaller than RSA-2048 and use far less CPU — better for mobile clients and high-throughput servers.

ZeroSSL supports ECC but still defaults to RSA. You have to pass --ecc in acme.sh or select it in the UI.

4. Hosting panel integration

Every current DirectAdmin / cPanel / Plesk ships with Let's Encrypt as a built-in option — one-click cert issuance. ZeroSSL usually needs you to configure the ACME endpoint manually or use acme.sh from the command line.

Which one should you pick?

• Personal sites / blogs / portfolios — Let's Encrypt via Certbot or your hosting panel. Set once, forget about it.
• Shared hosting with many sites — Let's Encrypt (better suited for bulk auto-renew).
• Small projects, no ACME setup — ZeroSSL UI lets you issue a cert in a browser in minutes.
• Working around Let's Encrypt rate limits — Use ZeroSSL as a fallback in acme.sh with --server zerossl.
• Want a 1-year cert without auto-renew — ZeroSSL Premium (paid).
• Need warranty / OV / EV — Neither offers these. Buy paid SSL from Sectigo, DigiCert, or GeoTrust instead.

Sample command — switch CA in acme.sh

Set the default CA to ZeroSSL:

acme.sh --set-default-ca --server zerossl

Switch back to Let's Encrypt:

acme.sh --set-default-ca --server letsencrypt

Setting Up ACME and ZeroSSL EAB Credentials

Unlike Let's Encrypt — which works out of the box with every ACME client — ZeroSSL requires External Account Binding (EAB): a Kid and HMAC Key pair that you generate from a free ZeroSSL account dashboard before you can issue certificates via ACME.

Steps to register ZeroSSL with acme.sh:

# Register your ZeroSSL EAB credentials first
acme.sh --register-account -m [email protected] \
  --server zerossl \
  --eab-kid <your-kid> \
  --eab-hmac-key <your-hmac-key>

# Issue a new cert from ZeroSSL
acme.sh --issue -d example.com -d www.example.com \
  --webroot /var/www/html \
  --server zerossl

If you skip the EAB step, acme.sh will return an {"error":{"type":"urn:ietf:params:acme:error:externalAccountRequired"}} error. Go back to the ZeroSSL Developer API section in the dashboard to generate your EAB credentials.

ACME Client Compatibility at a Glance

Root CA and Long-Term Browser Trust

An often-overlooked difference between the two CAs is the Root CA chain they use, which affects trust on older browsers and unpatched systems.

Let's Encrypt Root Chain

Let's Encrypt uses its own ISRG Root X1 (RSA 4096) and ISRG Root X2 (ECDSA P-384). Every modern browser and OS trusts these roots fully since 2021. Devices running Android 7.1.1 and earlier had issues when the cross-signed DST Root CA X3 expired in September 2021, but modern Let's Encrypt certificates now use the ISRG Root X1 chain exclusively.

ZeroSSL Root Chain

ZeroSSL certificates are backed by Sectigo's USERTrust RSA root, which has been trusted since 1999. This gives ZeroSSL an advantage on older embedded devices, IoT hardware, and legacy operating systems that have not updated their root certificate stores, since USERTrust has been in those stores far longer than ISRG Root X1.

Running Both CAs in Parallel (Dual-CA Setup)

Advanced operators sometimes run a Dual-CA setup — issuing certificates from both Let's Encrypt and ZeroSSL on the same server, then using a load balancer or failover logic to switch automatically if one CA has an outage.

This approach is useful when:

• Let's Encrypt rate limits are temporarily exhausted — switch to the ZeroSSL cert
• One CA experiences an unexpected outage — traffic continues without interruption
• You want to test renewal workflows without risking production

With acme.sh, issue certs separately per domain and CA, then use a deploy hook to copy each cert to a different path. Configure Nginx to load from the primary path with a fallback to the secondary:

# Issue from Let's Encrypt
acme.sh --issue -d example.com --server letsencrypt \
  --keylength ec-256 -w /var/www/html

# Issue a parallel cert from ZeroSSL (EAB must be registered first)
acme.sh --issue -d example.com --server zerossl \
  --keylength ec-256 -w /var/www/html \
  --cert-file /etc/ssl/zerossl/cert.pem \
  --key-file /etc/ssl/zerossl/key.pem

Certificate Auto-Renewal — Let's Encrypt vs ZeroSSL

One of Let's Encrypt's most compelling advantages is its seamless auto-renewal ecosystem. Every major ACME client — Certbot, acme.sh, Caddy, and built-in hosting panel integrations — provides deploy hooks that automatically install the renewed certificate after each renewal cycle. Once configured, you never need to think about certificate expiry again.

ZeroSSL also supports auto-renewal via ACME, but requires EAB registration first. If you choose the manual web UI path instead, you must download the new certificate and upload it to your server yourself — a manual step that creates real risk of forgetting and letting your certificate expire unnoticed.

Is free SSL enough for a business site?

Free SSL from either CA gives you the same encryption strength as paid SSL — browsers treat all of them as equally valid. The only real differences are warranty (claim coverage if the CA mis-issues) and OV / EV validation that verifies the organization.

For content sites, blogs, portfolios, and most SaaS — free SSL is fine. For banking, finance, and high-trust e-commerce sites that want a trust seal, OV or EV is worth the cost.